Perceptions of Risk from Non-Native and Horticultural Plants

Research Collection

Doctoral Thesis

Author(s): Humair Kuhn, Franziska

Publication Date: 2014

Permanent Link: https://doi.org/10.3929/ethz-a-010252721

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ETH Library DISS. ETH NO. 22073

Perceptions of Risk from Non-Native and Horticultural Plants

A thesis submitted to attain the degree of DOCTOR OF SCIENCES of ETH ZURICH (Dr. sc. ETH Zurich)

presented by FRANZISKA HUMAIR KUHN

M.Sc. in Biology, University of Basel, Switzerland

born on November 10, 1968 citizen of Basel (BS), Escholzmatt-Marbach (LU), Waltenschwil (AG)

accepted on the recommendation of

Prof. Dr. Michael Siegrist, examiner Prof. Dr. Peter Edwards, co-examiner Prof. Dr. Petra Lindemann-Matthies, co-examiner PD Dr. Christoph Kueer Schumacher, co-examiner

2014

Summary

1. The life of humans is inextricably linked to biodiversity and functioning ecosystems. Nevertheless, we are in the process of changing our planet to such an extent that many species and species communities are critically endangered. The introduction of new, non-native plant species to established ecosystems is perceived as one of the main threats to global biodiversity: Some of these species may become dominant and lead to novel interactions within ecosystems (plant invasions). Hu- mans are the main driver of plant invasions. In order to better understand the invasion process, not only ecological relationships, but also human motivation be- hind the choice to introduce certain species has to be examined. In a first part, this thesis aims to examine how academic experts and key stakeholders conceptualize and assess invasions by non-native plant species. In a second part, the thesis examines how plant trade contributes to the global distribution of invasive species, as well as the implications of the results for stakeholder participation in species management and risk communication are discussed. 2. Scientific experts are expected to consent on the definition and the use of basic concepts of their disciplines. The first study however showed widespread dissent among environmental experts how to conceptualize and how to evaluate plant invasions. In order to avoid misunderstandings in the communication among experts and between experts and stakeholders, diverging perceptions need to be made transparent. 3. A large-scale survey among Swiss horticulturists revealed that the perceived non-nativeness of a plant was indicative for horticulturists’ perception of environmental risk from this species. Horticulturists’ risk perceptions possibly mirrored prevalent risk communication that emphasizes the non-native origin of problematic species. However, among horticulturists, non-native plants were also perceived as a positive part of Swiss gardening culture. Therefore, risk communication might be more eective if negative environmental eects were emphasized instead of the non-native origin. 4. Consumers’ choice of plant species significantly contribute to plant invasions. Surveillance of online plant auctions on eBay.com showed that invasive woody species were overrepresented in trade compared to non-invasive species, and that also threatened species are included in international plant trade. Monitoring online-trade might prove particularly valuable for an early detection of future invaders. 5. According to Swiss environmental policies, horticulturists have to inform their customers about environmental eects of particular invasive plant species. A survey among horticulturists found positive attitudes towards the regulation. A possible explanation is the inclusion of horticulturists in the legislative procedure and in the implementation of the regulation in practice. 6

Zusammenfassung

1. Das Leben der Menschen wäre ohne biologische Vielfalt und funktionierende Ökosysteme nicht denkbar. Dennoch sind wir im Moment dabei, unseren Planeten dergestalt zu verändern, dass viele Arten und Artengemeinschaften stark bedroht sind. Eine der grössten Gefahren für die globale Biodiversität ist das Einbrin- gen neuer, nicht-einheimischer Pflanzenarten in schon bestehende Lebensgemein- schaften: Einige dieser Pflanzenarten können sich stark ausbreiten und neuartige Interaktionen in Ökosystemen auslösen (pflanzliche Invasionen). Um den Prozess pflanzlicher Invasionen besser verstehen zu können, müssen aber nicht nur die ökologischen Zusammenhänge untersucht werden, sondern auch die Beweggründe der Menschen, gewisse Pflanzen auszuwählen und einzuführen. Der erste Teil dieser Dissertation widmet sich der Frage, in welcher Art wissenschaftliche Ex- perten sowie Mitglieder einer wichtigen Interessensgruppe pflanzliche Invasionen beschreiben und bewerten. Der zweite Teil untersucht, wie der Pflanzenhandel zur weltweiten Verbreitung invasiver Arten beiträgt und welche Schlussfolgerungen aus den Resultaten dieser Studien für die Einbindung von Interessensgruppen ins Artenmanagement, sowie für die Risikokommunikation gezogen werden können. 2. Von wissenschaftlichen Experten wird erwartet, dass sie sich einig sind über De- finition und Gebrauch von Konzepten, welche wichtig für sind für ihr Forschungs- gebiet. Die erste Studie zeigte aber grosse Uneinigkeit unter Umweltexpertinnen und Umweltexperten hinsichtlich der Beschreibung und der Bewertung pflanzlicher Invasionen. Um Missverständnisse zu vermeiden, welche aus unterschiedlichen Wahrnehmungen hervorgehen können, ist eine transparente Kommunikation unter Experten, aber auch im Dialog mit Interessensgruppen deshalb von zentraler Be- deutung. 3. Eine grossangelegte schriftliche Umfrage unter Schweizer Gärtnerinnen und Gärtnern zeigte, dass sie eine Pflanze eher als Gefahr für die Umwelt betrachteten, falls sie die Pflanze als nicht-einheimisch einstuften. Diese Gefahrenwahrnehmung spiegelt möglicherweise die gängige Risikokommunikation wider, welche die nicht- einheimische Herkunft problematischer Pflanzen betont. Da nicht-einheimische Pflanzen in der Hortikultur aber auch als positiver Bestandteil des Gartenbaus angesehen werden, könnte die Risikokommunikation wirksamer sein, wenn sie sich auf negative Umwelteekte nicht-einheimischer Pflanzen konzentrieren würde, anstatt auf ihre fremde Herkunft. 4. Konsumentinnen und Konsumenten tragen mit ihrer Pflanzenauswahl wesent- lich zur Entstehung pflanzlicher Invasionen bei. Eine Untersuchung von Online- Pflanzenverkäufen auf eBay.com zeigte, dass invasive Gehölzpflanzen viel stärker gehandelt wurden, als nicht invasive Pflanzen, dass aber auch gefährdete Arten im internationalen Pflanzenhandel angeboten werden. Das Monitoring vom Online- Handel kann eine wertvolle Möglichkeit darstellen, einfach und schnell Informatio- nen zu möglichen zukünftigen Invasionen zu entdecken. 7

5. Die Schweizerische Umweltgesetzgebung verlangt von Gärtnerninnen und Gärt- nern, dass sie ihre Kundschaft über die Auswirkungen gewisser invasiver Arten in- formieren. Eine Umfrage ergab, dass die Gärtnerinnen und Gärtner diese Regelung grundsätzlich positiv aufgenommen haben. Eine mögliche Erklärung für diese positive Haltung kann der Einschluss der Gärtnerinnen und Gärtner in den Geset- zgebungsprozess und in die praktische Umsetzung des Gesetzes sein. 8

Contents

1 General Introduction 11 1.1 Man made global change ...... 11 1.2 The human dimension of plant invasions ...... 13 1.3 Biological invasions as an environmental risk ...... 14 1.4 Aim of the thesis and chapter overview ...... 18 1.5 References...... 20

2 Expert Perceptions of Invasive Plant Risks 29 2.1 Introduction...... 30 2.2 Methods...... 32 2.3 Results...... 34 2.4 Discussion...... 42 2.5 Acknowledgements ...... 51 2.6 References...... 52

3 Horticulturists’ Risk and Beneﬁt Perceptions 61 3.1 Introduction...... 62 3.2 Methods...... 64 3.3 Results...... 70 3.4 Discussion...... 77 3.5 Acknowledgements ...... 82 3.6 References...... 82

4 Horizon Scanning for Future Plant Invaders 87 4.1 Introduction...... 88 4.2 Material and methods ...... 89 4.3 Results...... 90 4.4 Discussion...... 94 4.5 Acknowledgements ...... 97 4.6 References...... 97

5 Swiss Horticulture and the Invasion Risks 103 5.1 Introduction...... 104 5.2 Methods...... 106 CONTENTS 9

5.3 Resultsanddiscussion ...... 107 5.4 Conclusions ...... 111 5.5 Acknowledgements ...... 111 5.6 References...... 111

6 General Discussion 117 6.1 Diverging framings promote misunderstandings ...... 118 6.2 Eectiveness of environmental policies ...... 119 6.3 Limitations and further studies ...... 120 6.4 Implications for management ...... 120 6.5 References...... 122

7 Acknowledgements 125

8 Curriculum Vitae 127

Appendices 129

A Supplementary Material of Chapter 2 131 A.1 Questionnaire...... 131 A.2 Demographic data ...... 138

B Supplementary Material of Chapter 3 139 B.1 Questionnaire...... 139 B.2 Hierarchical regression analysis ...... 147

C Supplementary Material of Chapter 4 149 C.1 FamiliesoftheGlobalspecieslist ...... 149 C.2 100 most frequently oered species ...... 150 C.3 IUCN’s worst invasive alien species oered ...... 153 C.4 Cumulative frequency distributions of species oered ...... 154 C.5 Regions where non-woody invasives were oered ...... 155 C.6 References ...... 156

1 General Introduction

1.1 Man made global change

Earth’s ecosystems are under pressure. The needs of a growing global population for food, ﬁber, fuels, and medicine have led to transformations and loss of habitats, reshuing of species compositions, unsustainable use and overexploita- tion of resources, climate change, and pollution (Vitousek et al., 1997; Millennium Ecosystem Assessment, 2005; Convention on Biological Diversity, 2010). The impacts of accelerating human activities have become pervasive and substantial to the extent that some scientists are now referring to the Anthropocene, a new era in Earth’s history (Crutzen, 2006). As a consequence of human-mediated global change, more and more new plant species thrive in landscapes where they were not observed shortly before: Globalized trade, travel, and transport are blurring biogeographic barriers, they increase the number of introductions of non-native plants, and pave the way for their dispersal into established species communities. Throughout this thesis, the term non-native refers to plant species that have been introduced by humans after the year 1500 A.D. to an environment a species would not have reached otherwise. The discrimination of plant species introduced before and after 1500 A.D. (archaeophytes vs. neophytes) is a concept particularly used related to plant introductions to Europe. Some archaeophytes arrived in European regions already during the Neolithic period, whereas neophytes were introduced as part of the widespread exchange of plant species since the onset of colonialism after Columbus’ discovery of the Americas around the year 1500 A.D., and notably with increasing globalization.

1.1.1 Biological invasions Human-mediated species movements have reached a level unseen before (Vitousek et al., 1997; McKinney and Lockwood, 1999; Mack, 2000), and the increasing disruption of established species communities by some non-native plant species is of great concern in nature conservation and in environmental policies (Convention on Biological Diversity, 1992; Mack, 2000; Millennium Ecosystem Assessment, 2005). Only a small fraction of introduced species is able to survive in the recipient habitat, to build self-sustaining populations, and eventually to spread from the point of introduction into the environment (the tens rule; Williamson and Fitter, 12 CHAPTER 1. GENERAL INTRODUCTION

1996). In the literature, commonly these introduced, spreading species are called non-native invasive species. However, to date no common concept exists as to how exactly the concept of invasive species should be operationalized. A major point of contention is the question whether the concept of invasive species should also include the valuation of their environmental or economic eects. For example, according to Richardson et al. (2000), negative impacts of an invasive species should not be coupled to the term invasive, whereas other authors (e.g., Mack, 2000), but particularly policy frameworks emphasize the detrimental eects of a species as an inherent characteristic of its invasiveness, i.e., the tendency of a species to become invasive (e.g., Millennium Ecosystem Assessment, 2005). Throughout this thesis, the term invasive refers to species with known negative impacts on biodiversity, health, and/or economy, and that are perceived to be problematic.

1.1.2 Non-native invasive species as a societal problem: Impacts on ecology, human health and economy Biological invasions have developed into a serious societal problem and it is expected that the severity of the problem will increase in the future (Millennium Ecosystem Assessment, 2005; Lodge et al., 2006). In particular, continuing introductions and spread of non-native plant species will result in an increasing homogenization of the Earth’s flora (e.g., Olden and LeRoy Po, 2003; La Sorte et al., 2014). In its latest proposal to tackle the invasive species issue, the European Commis- sion estimates that the total economic costs related to biological invasions amount to 12 billion Euros (16.4 billion U.S. Dollars) per year and it is predicted that these costs will continue to increase (European Commission, 2013). Or, CSIRO, Aus- tralia’s national science agency estimated damage costs resulting from biological invasions of about 7 billion Australian dollar per year (6.5 billion U.S. Dollar; The Commonwealth Scientific and Industrial Research Organisation, 2014). It is dicult however to measure and express the costs of biological invasions in numbers, particularly when non-market values are aected. Such values include for instance the existence value of a species, or cultural values of ecosystems, e.g., a person’s loss of sense of place caused by the replacement of the native flora (Millennium Ecosystem Assessment, 2005). Therefore, the numbers listed above have to be treated with caution. Non-native invasive species may exert a considerable negative impact on the daily lives of people. For example, Eichhornia crassipes (Water Hyacinth) is considered to be one of the 100 world’s worst invasive species (International Union for Conservation of Nature, 2014). E. crassipes is a free floating, fast growing aquatic plant species native to South America. For ornamental purposes, the plant had been globally introduced (Edwards and Musil, 1975; Gopal and Sharma, 1981; Brunel et al., 2010). It poses a particular problem to the people living along the Niger River (West Africa) and along Lake Victoria (East Africa). Due to its fast growth and rapid abundance, the plant may rapidly cover the entire surface of the 1.2. THE HUMAN DIMENSION OF PLANT INVASIONS 13 water, thereby blocking fishing and waterways, shielding sunlight and oxygen from reaching the water column resulting in a change in water quality, creating breeding places for disease vectors, and eventually leading to a loss of biological diversity in the aquatic ecosystems. E. crassipes may also invade aquatic systems in regions with a colder climate. Yet, due to its frost sensitivity, the plant is not able to establish self-sustaining populations there (Wallentinus, 2002). Thus, impacts of aspeciesmaydier between dierent ecosystems (Vilà et al., 2011; Pyöek et al., 2012; Kueer et al., 2013). The societal and ecological impacts of E. crassipes in Africa are examples of particularly severe invasions. Sometimes, however biological invasions are subtle processes such that their eects might only become apparent when the concerned ecosystem has already been aected (Vilà et al., 2011). A frequent feature of invasions is a lag phase between introduction and spread of a species (Essl et al., 2011), and sometimes, lag phases span several human generations. For example, for trees and shrubs, Kowarik (1995) described lag phases of 131 to 170 years between introduction and the beginning of the invasion. The non-immediacy of species invasions may lead to an underestimation of the problem in science and society, and may make it dicult for early detection of and rapid response to biotic invasions, or to raise awareness of the issue (e.g., Brunel et al., 2013).

1.2 The human dimension of plant invasions

Species invasions reflect the socio-economic context at the time when the species were introduced, but also human behavior that paved the way for their establishment and spread (Kueer, 2010; Kueer et al., 2013). Thus, human norms and behavior, are an important driver of species invasions. Probably the most important such human dimension is globalized trade and transport (Hulme, 2009), but human action aecting biological invasions is also influenced by psychology, culture, history, philosophy, ethics, policy, or economy (McNeely, 2001). There are some obvious reasons why plant species are introduced to new geographic areas. Most importantly, plant imports meet the daily needs of an increasing global population. However, the need for food, fiber, fuel, or medicine is not always the primary driver for plant introductions. From early on, plant species were also introduced for ornamental, cultural, or sociological purposes. For instance, already in 1601 Robinia pseudoacacia was introduced as an ornamental tree species from North America to Paris (Wittenberg et al., 2005). Motivated by homesickness, European settlers in North America introduced familiar species from home (Coates, 2006). In colonial days, acclimatization societies aimed to enrich regional faunas and floras by introducing new species (e.g., introductions of Acacia species to Australia; Carruthers et al., 2011). Often botanical gardens served as hubs for the international exchange of ornamental species (e.g., Kew Gardens in England; Mack and Lonsdale, 2001). 14 CHAPTER 1. GENERAL INTRODUCTION

Today, introductions of ornamental plants are still often driven by consumers’ appetite for novel species. Often, consumers prefer plants with particular characteristics that sometimes coincide with properties associated with invasiveness (Dehnen-Schmutz et al., 2007; e.g., Bucharova and Van Kleunen, 2009), for instance rapid growth, high reproduction rate, clonal growth, longer or season- ally shifted flowering periods compared to native species, or high tolerance of anthropogenic site conditions (Rejmánek and Richardson, 1996; Richardson and Rejmánek, 2011). The high number of introductions as well as special care by gardeners further contribute to the invasion success of ornamental plant species (Mack, 2000). Consumers’ demands are supported by a growing, internationalized horticultural industry. Thanks to new and ecient production methods, cheap labor and transportation, or the possibilities to meet consumers’ requests for year- round availability of fruits and vegetables, particularly emerging economies, e.g., in South America or in Africa, profit from new market potentials (Dehnen-Schmutz et al., 2010). Various environmental policy frameworks exist that aim at regulating plant introductions, e.g., cargo inspections for stowaway species, border controls of trav- elers, quarantine measures, the implementation of black lists of unwanted species, or awareness raising campaigns (Swiss Federal Council, 2008; Plant Health Aus- tralia, 2014; U.S. Department of Agriculture, 2014). However, considering the central role of humans in the global reshuing of floras, a deeper understanding of the underlying drivers for human behavior is needed: Why and how are certain plant species introduced, who are the agents of introductions, and what are the factors that might motivate stakeholders to engage in preventive behavior (Carl- ton and Ruiz, 2005; Kowarik and von der Lippe, 2007; Kueer and Hirsch Hadorn, 2008; Pyöek et al., 2010; Kueer et al., 2013)?

1.3 Biological invasions as an environmental risk

Charles Elton’s seminal work The ecology of invasions by animals and plants ini- tialized the discipline of invasion biology (Elton, 1958). Elton deﬁned biological invasions as a societal problem that requires the attention of scientists. Although in the 1970s conservation biology emerged as an independent discipline (Davis, 2006), it took another almost two decades before biological invasions were set on the agenda of environmental policies (Convention on Biological Diversity, 1992). Since the 1990s, more and more articles on invasive species risk assessments were published, and the precautionary principle, that assumes that non-native species are problematic until proven otherwise, became a pillar of invasive species policies (Wittenberg and Cock, 2001; Convention on Biological Diversity, 2002; Simberlo, 2005). Thus, among environmental experts, biological invasions qualify as an environmental risk, just like those related to new technologies (e.g., nuclear power, or nanotechnology). However, contrary to these risks, there is so far only limited knowledge about how invasion risks are perceived by the public or by particular 1.3. BIOLOGICAL INVASIONS AS AN ENVIRONMENTAL RISK 15 stakeholder groups, what the underlying psychological factors inﬂuencing these perceptions are, or which management goals are preferred.

1.3.1 How perceptions of the biological invasions are influenced by key terms and concepts In science and nature conservation circles, there is widespread consensus that non- native invasive plant species exert a variety of negative impacts on biodiversity and ecosystem functioning, and that management action is needed (Convention on Biological Diversity, 2002; Millennium Ecosystem Assessment, 2005). This general assumption that spreading non-native species pose serious environmental threats is however increasingly challenged by various experts. For example, Carey et al. (2012) suggested that some native species should be considered invasive in their native range equally to non-native invasive species. Other authors criticized that knowledge about biotic invasion was biased towards those species with the highest impacts (Pyöek et al., 2008; Vilà et al., 2011). Also, studies on multiple, long-term interactions, reflecting the dynamics of ecosystems as well as the interferences of numerous invaders are mostly lacking (Kueer et al., 2013). Recently, also positive eects of invasive plant species in conservation or ecosystem functioning have been described (e.g., Stromberg et al., 2009; Schlaepfer et al., 2011) and some scientists criticize the general negative portrayal of non-native species (e.g., Davis et al., 2011; Schlaepfer et al., 2012). The prevailing paradigm of non-native species impacting native biodiversity and ecosystems is further challenged by the rapid and pervasive human-mediated environmental change resulting in new and unknown environmental contexts (novel ecosystems; Hobbs et al., 2006). In these situations, the non-native origin of a species might lose its significance, or new traits of non-native species might even be beneficial for the functioning of novel ecosystems. Increasingly, scientists also question the value-laden approach to non-native species. Lately, a group of scientists contested the conservational value of nativeness and required science and conservation to judge species solely on their impact and not on their origin (Davis et al., 2011), but see Simberlo (2011). Others question the use of militaristic or otherwise hyperbolical language used in the literature and in science communication on non-native invasive species (e.g., Larson, 2005; Larson et al., 2013; Kueer and Larson, 2014).

1.3.2 Dierent risks are perceived dierently: lessons from risk psychology When it became obvious that new technologies (e.g., chemical or nuclear inven- tions) have great potential for detrimental impacts on the environment, contro- versies arose between experts and laypeople about the meaning of risk and the acceptance of new technologies (Slovic, 1987). In order to define risks, technological experts referred to probabilistic risk assessments and the magnitude of harm, 16 CHAPTER 1. GENERAL INTRODUCTION whereas laypeople were found to relate to qualitative characteristics of risks. These include for example a person’s familiarity with a risk, the voluntariness of exposure, perceived benefits from or controllability of risks, the potential for and the immediacy of catastrophic consequences, or the level of knowledge about the risk in science and among the public (Fischho et al., 1978; Slovic, 1987). Risk psychology has over the past decades advanced greatly in understanding why dierent laypersons and experts perceive risks dierently. These insights from risk research might also help to explain diverging perceptions towards invasive species, the acceptance of management measures, or how perceptions of risk influence people’s behavior. In the context of climate change, O’Connor et al. (1999) found, for instance, that perceptions of risk increased people’s intentions to address environmental problems. A quantitative representation of dierent risk attitudes is given by the psychometric paradigm which uses two dimensions as primary predictors of people’s risk perceptions: dread risk and unknown risk. The first dimension (dread risk) refers e.g., to the certainty of death in the case of adversity, to the uncontrollability of a risk, or the involuntariness to be exposed to it. The second dimension, (unknown risk) is concerned with the newness of a risk, that it is unknown to those exposed, or that its eects are not immediate (Slovic, 1987). The psychometric paradigm reveals that dierent risks are perceived dierently. For example, Slovic (1987) found that nuclear reactor accidents loaded high on the dread dimension, i.e., laypeople judged them as uncontrollable, catastrophic, or as a high risk to future generations. In contrast, diagnostic x-rays were perceived to be rather con- trollable. Thus, laypeople had diverging attitudes towards dierent applications of the same technology. This could also be observed in other contexts: People perceived more benefits and less risk from medical gene technology applications compared to non-medical ones (Connor and Siegrist, 2010). Or, food packaging with nanoparticles to extend shelf life was more dreaded than medical nanorobots that move freely in the body (Siegrist et al., 2007). In situations of high uncertainty, e.g., when time is limited and people have to assess a risk or its consequences that are unknown to them, people rely on mental shortcuts (heuristics) to make decisions. They rely, for example, on trust in authorities as a basis for their judgments. In the realm of technologies (such as nano- or gene technology), trust refers to the reliance upon the expertise of scientists, technical experts, authorities, or the responsible management (Siegrist et al., 2000, 2005). Besides trust, people also use other heuristics to make decisions under uncertainty, for instance, they may refer to stereotypes or they base their decisions on events they can easily recall (Tversky and Kahneman, 1974). Or, people rely upon their feelings to judge if a technology or a situation is safe or not (aect heuristic; Finucane et al., 2000; Slovic et al., 2002, 2004). Immediate positive aect may then lead to reduced perceptions of risk and vice versa (Alhakami and Slovic, 1994; Finucane et al., 2000). The use of heuristics enables intuitive and fast decision making which may be decisive for survival. However, heuristics 1.3. BIOLOGICAL INVASIONS AS AN ENVIRONMENTAL RISK 17 may also be misguiding, i.e., feelings may be manipulated (e.g., by advertising), or risks may be under- or overestimated, e.g., when people feel more protected, they become less careful. Although the use of heuristics is particularly attributed to laypeople, Kahneman et al. (1982) demonstrated that also experts were prone to many of the same biases when they had to make judgments outside of their expertise.

1.3.3 The perception of invasion risks: current knowledge How people perceive non-native species and their eects significantly depends on how benefits of the species, e.g., economic profit or cultural values, are evaluated. For instance, when introduced to Europe, Robinia pseudoacacia was not only valued as an ornamental, but also for its high quality timber, in reforestation, or for its nectar production. According to DAISIE – Delivering Alien Invasive Species Inventories for Europe (2014), to date, R. pseudoacacia is the most widely planted American tree species throughout Europe, and the tree is still appreciated as a timber tree and a nectar source for honey bees (Binggeli, 2001). At the same time it is considered to exert negative impacts by dominating the native flora and com- peting with native plants for pollinating bees. Conflicts of interests might arise in the future between nature conservation and energy producers, because R. pseudoacacia has potential to be used in biofuel production and might be promoted by this emerging industry (DAISIE – Delivering Alien Invasive Species Inventories for Europe, 2014). Thus, perceptions may change over time, and they may dier between dierent stakeholder groups (e.g., Binggeli, 2001; Starfinger et al., 2003; Stromberg et al., 2009; Kueer, 2013). Risk and benefit perceptions, or attitudes towards management measures related to non-native invasive species vary widely among members of the general public or dierent stakeholder groups (reviewed in Kueer, 2013). For example, tourists in a national reserve in Spain were rather concerned about the ecological impacts of non-native invasive species whereas local residents feared negative social impacts, e.g., loss of sense of place (García-Llorente et al., 2008). Nevertheless, a considerable fraction of local residents supported introductions of non-native species if they generated an economic or recreational benefit. In New Zealand, local Maori tribes were concerned about the use of non-native species for biological pest control, while urban people welcomed new species in their gardens, and some people simply did not seem to care (Veitch and Clout, 2001). Partic- ular invasive plant species also seem to evoke more negative emotions (e.g., Ulex europaeus, Common Gorse; Fal lopia japonica, Japanese knotweed; or Heracleum mantegazzianum, Giant Hogweed) than others, such as Rhododendron ponticum (Common Rhododendron) that is characterized by particularly large and striking flowers (Veitch and Clout, 2001; Bremner and Park, 2007). Public attitudes towards the management of plant species are also informed by various factors. Fischer and van der Wal (2007) found that values held by the public towards nature were decisive for their attitudes towards the management of 18 CHAPTER 1. GENERAL INTRODUCTION an invasive plant species: people who preferred a balanced environment with no species dominating, were supportive of management actions, contrary to people who preferred a nature untouched by humans. In a study by García-Llorente et al. (2011) the willingness to pay for invasive plant management was significantly driven by participants’ interest in nature, knowledge about non-native invasive plant species and their impacts, but also their household income, as well as their sense of place. Also environmental managers hold diering views towards invasive non-native plant species. Andreu et al. (2009) found that senior environmental managers in Spain made decisions about the management of invasive plants based on the situation within their area of responsibility, regardless of national policies.

1.4 Aim of the thesis and chapter overview

Invasions by non-native plant species are a direct consequence of human-mediated species dispersal and pervasive environmental change. Although there is a large body of literature examining many important aspects of plant invasion ecology (e.g., the invasiveness of a species or the invasibility of an ecosystem, Richardson and Rejmánek, 2011), there is little knowledge about how human norms and behavior aect species invasions. However, a thorough understanding of risk and benefit perceptions, values, or concepts held by dierent stakeholder groups towards non-native invasive species is decisive for the design and implementation of management measures, such as preventive policies or adequate risk communication. In a first part, this thesis aims to advance our understanding of how academic experts and key stakeholders conceptualize and assess invasions by non-native plant species. In particular, the thesis examines the influence of key concepts used in ecology and invasion biology, attitudes, and values on perceptions of risk and benefit, and it asks how stakeholders’ willingness to engage in pro-environmental action to mitigate invasion risks can be increased. In a second part, the thesis aims to examine how species selection in plant trade determines the composition of non-native floras and thereby influences invasion risks. The implication of these results for risk communication, stakeholder participation, and horizon scanning of emerging plant invasion risks are further explored.

Chapter 2 Understanding misunderstandings in invasion science: why experts don’t agree on common concepts and risk assessments Recently, cracks in the seeming consensus among environmental experts about the assessment and management of non-native invasive plant species became publicly apparent. In order to examine the multitude of experts’ understandings and valuation of non-native invasive species, we conducted qualitate interviews with academic experts. We found considerable dissent concerning the deﬁnition of basic concepts and the valuation of impacts of non-native invasive species on ecosystem 1.4. AIM OF THE THESIS AND CHAPTER OVERVIEW 19 services, as well as whether biological invasions were considered a priority societal issue. In order to avoid misunderstandings among experts and between experts and stakeholders, diverging understandings need to be made transparent. Risk management and communication should allow for a plurality of alternative deﬁni- tions of key terms such as non-native or invasive.

Chapter 3 Are non-native plants perceived to be more risky? Factors influencing horticulturists’ risk perceptions of ornamental plant species In the biological literature, the non-native origin of a plant species is commonly considered to be an important indicator for a species’ potential to become invasive in a new environment. Through a large-scale quantitative survey, we investigated whether the perceived origin (native vs. non-native) of ornamental plant species also influenced the perception of environmental risks from these plants among horticulturists in Switzerland. Further, we examined horticulturists benefit perceptions towards non-native ornamentals, as well as prevalent intentions to participate in risk mitigation behavior. Our results indicate that the perceived non-native origin of a species significantly increases perceptions of risks, and that risk perceptions from non-native invasive species in general were an important driver for pro-environmental behavior. However, horticulturists also perceived considerable benefits from non-native species, and their familiarity with these species seemed to reduce perceptions of risk. We concluded that benefits from and familiarity with non-native horticultural plants might interfere with risk communication towards horticulturists that emphasizes the non-native origin of a species. We proposed that risk communication should focus on known negative eects of a non-native species rather than on its origin.

Chapter 4 Horizon scanning for future plant invaders. The case of e-commerce Consumers’ choice to buy and cultivate a particular plant species increases its chances to be introduced to new environments and to become invasive. Increasing globalization of horticultural trade and new options for consumers to buy more dierent species (e.g., Internet trade) bear the risk of further spread of recognized invasive plant species and the introduction of new potentially invasive species. We monitored online auctions on eBay.com to document international trade of ornamental plant species, and particularly of known invasive species. We found that the percentage of traded species was markedly higher among invasive species than non-invasive ones, and we found a positive relationship between the global distribution of plant species that were included in online trade and the number or regions where it was documented as an invasive species. We propose to use automated surveillance of online auctions for the implementation of international biosecurity policies: both for the detection of known invaders in horticultural trade and the early discovery of future invaders horizon scanning. 20 CHAPTER 1. GENERAL INTRODUCTION

Chapter 5 Working with the horticultural industry to limit invasion risks: the Swiss experience Horticulturists are a key stakeholder group for the regulation of the transportation of non-native plant species. In a joint eort with the Swiss horticultural industry and Swiss authorities, we assessed horticulturists attitudes towards a new Swiss policy that requires horticulturists to inform their customers about the environmental risks of non-native invasive plant species that they oer. Further, we examined the eectiveness of a particular implementation of this legislation; plant labels that inform customers about invasive species. We found positive attitudes among customers towards the plant label, as well as as positive attitudes among horticulturists towards risk mitigation from plant invasions, and towards a future engagement in plant risk assessments. We discuss that one reason for the positive attitudes of horticulturalists towards invasive species policies might be that they were actively involved as a stakeholder group in the preparation of recent invasive species legislation and their implementation in Switzerland.

Chapter 6 General discussion In the general discussion I summarize the results of the present study as follows: First, I describe the eects of diverging understandings of basic concepts among scientific experts on the dialogue within the expert community, as well as for risk communication to stakeholders. Second, I elaborate on the importance of horizon scanning for the detection of emerging issues in invasion biology. Particularly, I point out the opportunities oered by Internet monitoring for early detection of and rapid response to invasive species. Third, I discuss implications of our findings for risk communication, and participation of key stakeholders in solution finding.

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2 Understanding Misunderstandings in Invasion Science: Why Experts Don’t Agree on Common Concepts and Risk Assessments

Franziska Humair, Peter J. Edwards, Michael Siegrist, Christoph Kueer (2014) In NeoBiota 20:1–30, doi: 10.3897/neobiota.20.6043 Abstract Understanding the diverging opinions of academic experts, stakeholders and the public is important for eective conservation management. This is especially so when a consensus is needed for action to minimize future risks but the knowledge upon which to base this action is uncertain or missing. How to manage non- native, invasive species (NIS) is an interesting case in point: the issue has long been controversial among stakeholders, but publicly visible, major disagreement among experts is recent. To characterize the multitude of experts’ understanding and valuation of non- native, NIS we performed structured qualitative interviews with 26 academic experts, 13 of whom were invasion biologists and 13 landscape experts. Within both groups, thinking varied widely, not only about basic concepts (e.g., non-native, invasive) but also about their valuation of eects of NIS. The divergent opinions among experts, regarding both the overall severity of the problem in Europe and its importance for ecosystem services, contrasted strongly with the apparent consensus that emerges from scientific synthesis articles and policy documents. We postulate that the observed heterogeneity of expert judgments is related to three major factors: (1) diverging conceptual understandings, (2) lack of empirical information and high scientific uncertainties due to complexities and contingencies of invasion processes, and (3) missing deliberation of values. Based on theory from science studies, we interpret the notion of an NIS as a boundary object, i.e., concepts that have a similar but not identical meaning to dierent groups of experts and stakeholders. This interpretative flexibility of a concept can facilitate interaction across diverse groups but bears the risk of introducing misunderstandings. An alternative to seeking consensus on exact definitions and risk assessments would be for invasive species experts to acknowledge uncertainties and engage transparently with stakeholders and the public in deliberations about conflicting opinions, taking the role of honest brokers of policy alternatives rather than of issue advocates. 30 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS

2.1 Introduction

To judge from the biological conservation literature, there is a general consensus that invasions of non-native species per se pose major risks to biodiversity and ecosystem services (Mack et al., 2000; Millennium Ecosystem Assessment, 2005; Mooney et al., 2005; Vilà et al., 2010; Simberlo et al., 2013). However, this view is increasingly being challenged by experts (Davis et al., 2011) and it is debated whether invasive species are a main driver of species extinctions (Gurevitch and Padilla, 2004; Clavero and García-Berthou, 2005). Because ecological as well as other environmental and human-driven processes interact in complex ways, it can be dicult to determine whether invasive species are indeed a driver of environmental change or merely a symptom of some other events (Didham et al., 2005; Kueer et al., 2013). Furthermore, the positive values of non-native species for conservation are increasingly discussed in the literature (Ewel and Putz, 2004; Kueer and Daehler, 2009; Kueer et al., 2010; Goodenough, 2011; Schlaepfer et al., 2011), triggering critical responses (e.g., Vitule et al., 2012; Richardson and Ricciardi, 2013). Then again, native species are sometimes considered to be invasive (Valéry et al., 2009; Carey et al., 2012) in disagreement with standard definitions (Richardson et al., 2011). These conflicting perspectives on invasive organisms and their eects on ecosystems can impede conservation action. This is particularly true if policies build on preventative measures on the grounds that an early response is likely to be more eective than a later cure (Leung et al., 2002; Hulme et al., 2009). Such types of conservation actions rely on a general consensus among experts and stakeholders on the potential future negative impacts of non-native, invasive species (NIS). It is therefore important to understand how perceptions about the eects of biological invasions and the need for management are shaped among stakeholders (aected interest groups) and experts (a person with a high degree of knowledge of a subject that is acknowledged by society, which leads to the attribution of a special role to the person in certain decision-making situations, Mieg, 2009). There has been some work on how stakeholders and the general public perceive the risks and consequences of biological invasions, and the appropriate management options to be taken (Bardsley and Edwards-Jones, 2006; Binimelis et al., 2007b; Bremner and Park, 2007; Fischer and van der Wal, 2007; Garcia-Llorente et al., 2008; Andreu et al., 2009; Selge and Fischer, 2010; Rotherham and Lam- bert, 2011; Selge et al., 2011; Young and Larson, 2011; Gozlan et al., 2013; Kueer, 2013). These studies show that learning about scientific facts related to eects of NIS is just one factor determining attitudes and opinions. Attitudes of stakeholders can also be influenced by the social context (Bremner and Park, 2007; Fischer and van der Wal, 2007; Garcia-Llorente et al., 2008), dierences in value judgments (e.g., emotional connectedness towards a species or towards specific management methods, Fischer and van der Wal, 2007), conflicts of interest (e.g., managers vs. visitors of public parks, Garcia-Llorente et al., 2008), and the various roles 2.1. INTRODUCTION 31 that humans play in promoting invasions (McNeely, 2001; Selge and Fischer, 2010; Rotherham and Lambert, 2011; Selge et al., 2011). Importantly, these studies suggest that stakeholders often dier strongly from experts and among themselves in their attitudes to invasive species and their willingness to participate in management actions (Bardsley and Edwards-Jones, 2006; Andreu et al., 2009).

Less is known about how individual experts or expert communities dier in their perception and assessment of invasion processes, but there are indications that opinions do vary (Young and Larson, 2011) and may be influenced by factors other than scientific facts (Selge et al., 2011). Indeed, it can be expected that in situations where facts and values are highly uncertain, as in the case of biological invasions, expert assessment also becomes highly dynamic and uncertain (Fun- towicz and Ravetz, 1994), and the influence of intuitions, ideologies and values is more pronounced (Fischho et al., 1982; Slovic, 1999). Therefore, it is crucial to understand better how and why experts dier in the understanding and valuation of invasive species and their eects on ecosystem services and biodiversity.

We mapped the understanding of basic concepts commonly used to describe and explain biological invasions, and the ways experts value the risks and eects of biological invasions. To do this, we conducted 26 structured, face-to-face expert interviews. We used a qualitative approach because we were interested in elucidating the interrelated arguments, values and attitudes regarding biological invasions that are dicult to uncover through other methodologies. The experts belonged to two, equally sized groups, one of 13 invasion biologists and the other of 13 landscape experts. Both groups have a professional interest in ecological change, including the spread of non-native species; however, while plant invasions are the main focus of the work of invasion biologists, they are only one issue among many others in the work of landscape experts. By including landscape experts, we control for the convergence of perceptions in a scientiﬁc discipline, in this case invasion biology, that may be driven by an intra-scientiﬁc need to focus research (paradigm, Kuhn, 1962) or the societal expectation for a profession to speak with one voice and act according to certain standards (Mieg, 2009).

The overall aims of this interdisciplinary study (the authors include three biologists / environmental scientists and a risk psychologist) were: (i) to document the variability of the general conceptual understanding and the assessment of biological invasions among invasive species experts, (ii) to identify those aspects where the diversity of understandings and assessments among experts is particularly high and which therefore might account for dissent among invasive species experts, (iii) to investigate whether the consensus among invasion biologists diers from that among other relevant experts, and (iv) to identify possible explanations for any dissent among experts. We found that not only the framing of basic concepts (e.g., non-native or invasive) but also experts’ thinking about the relevance of these concepts as well as the valuation of eects of NIS varied widely. 32 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS

2.2 Methods

We used a qualitative research approach, which is often used in the social sciences to gain a multidimensional understanding of why individuals see the world in a particular way, and to explore the range of dierent thoughts, feelings, and interpretations of meaning of individuals in respect to an issue (Given, 2008).

2.2.1 Study participants The study was based upon face-to-face interviews with 26 academic experts with contrasting expertise in the broad ﬁeld of ecological change. Prior to the interviews, these experts were assigned to two equal groups, one with a research focus on plant invasions (invasion biologists, IB, 9 males) and one with a research focus on ecological change in the landscape in general with plant invasions as one among many possible drivers (landscape experts, LE, 9 males). Landscape experts formed a heterogeneous group, including experts from agricultural and environmental sciences, biology, and geography (see also Table A.1 in Appendix A.2). To avoid contingent dierences in the use of terms such as native versus non-native in particular geographic regions (e.g., USA vs. Europe), we focused on a well-contained group of European experts. In Europe biological invasions have become a major concern for research and management only in the last decades, but currently invasive species research is one of the most active research areas in ecology in particular due to two large European research programs: ALARM (Settele et al., 2005) and DAISIE (DAISIE–Delivering Alien Invasive Species Inventories for Europe, 2009). Given that invasive species are a fresh and very prominent topic, Europe is an ideal study system for understanding the diversity and dynamics of expert thinking. All experts were German speaking and the interviews were conducted in German, and they were all aliated with an academic institution in Switzerland or Southern Germany. The experts were chosen to represent the major research groups at uni- versities as well as applied research institutions in the study area that are working on plant invasions in terrestrial ecosystems. Some study participants were recom- mended by other experts. With the exception of four young scientists (3 IB, 1 LE), at the time of the interviews all participants had a long-standing record of major contributions to the literature on the issue of biological invasions and/or ecological change.

2.2.2 Structure of the interview guideline We performed structured face-to-face interviews including closed and open-ended questions. A strength of this method is that it allows for a direct elicitation of individual understandings and valuations without the bias of social interactions in group settings possibly hampering the expression of extreme views or the recognition of individual uncertainties or lack of knowledge. Our interview guideline (cf. Appendix A.1) was compiled following a literature search and a review of 2.2. METHODS 33 the international scientific literature on biological invasions, and especially plant invasions. We limited ourselves to the perceptions of ecological change caused by plants because a) plant invasions are particularly intensively studied and have driven most of the theoretical debates in invasion science (especially also in Cen- tral Europe), b) plants significantly shape ecosystem processes, and c) the analysis as well as the full acknowledgement of the debate about animal rights inherently linked to the issue of biological invasions by animals would go beyond the scope of this investigation. We focused on concepts that are of particular importance both in the scientific literature on biological invasions and the sociopolitical deliberations about the issue (non-native, invasive, ecosystem services). To finalize the interview guideline, we ran two pilot interviews with a geographer and an environmental scientist, respectively.

The interview guideline consisted of three main parts: in the first part (Q1– Q10), participants were asked about their understanding of key concepts (native, non-native, invasive). The second part focused on the valuation of eects of non-native, invasive plants on ecosystem services (Q11–Q12) (sensu Millennium Ecosystem Assessment, 2005). Experts were presented 12 cards that listed various ecosystem services: two provisioning services (Biodiversity, Food), four regulating services (Climate Regulation, Human Health, Pollination, Protection from Natu- ral Hazards), two supporting services (Primary Production, Soil Formation), and four cultural services (Cultural Heritage, Landscape Aesthetics, Sense of Place, Recreation / Tourism). The study participants then had to assess whether non- native, invasive plants have a predominantly negative, neutral, or positive influence upon the twelve ecosystem services, resulting in twelve separate assessments per expert. Additionally, experts were asked to briefly motivate their decisions. Before participants valued the eects of non-native, invasive plant species on ecosystem services (Q10) they were informed about the definition of an invasive species by the Swiss Commission for Wild Plant Conservation and Swiss federal legislation SR 814.911. However participants were also told that they are free to stick to their own definitions. In the third part, we focused on the invasive species issue as a societal problem. We asked the experts why they considered biological invasions to be a problem (Q13–Q15), confronted them with the problem understanding of the Swiss government (Q16), explored some key dimensions of the problem understanding in more detail (Q17–Q19), asked about the availability of sucient scientific evidence (Q20), and asked for an assessment of the scale of the problem (Q21–Q25). Answers to questions Q20–Q21 and Q23–Q25 were measured on 5-point Likert scales. Additionally the participants were asked to motivate their decisions shortly. The interviews ended with questions seeking information on participants’ current research (Q26) and basic demographic data (Q28, Table A.1). Further, experts were asked if they want to make a concluding statement (Q27). Before the start of the interviews, participants were informed about the general direction of our study, i.e., to elicit the perception of ecological processes related to non-native plants (see interview guideline, Appendix A.1). At the start of 34 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS the interviews, all experts provided verbal consent to audio record and transcribe their answers. Participants were ensured that after transcription, the data would be anonymized. All interviews were led by the first author.

2.2.3 Data analysis All interviews, which mainly lasted for about one hour, were digitally audio recorded and transcribed verbatim in German. The interviews were performed between September and October 2009. In the case of the valuation of eects on ecosystem services (Q11), we were not only interested in the overall assessment (eect evaluated as positive, negative or neutral) by the experts but also in the types of arguments that they used to motivate their valuations. For this analysis, the authors identiﬁed dierent recurring arguments used by the study participants to motivate their valuations. A few questions (Q12, Q13–Q19) were omitted from the analysis because they yielded redundant data only. Questions Q20–Q21 and Q23–Q25 were closed questions, which allowed us to quickly gain information on speciﬁc attitudes experts held towards NIS and biological invasions in the societal context.

2.3 Results

Irrespective of the expert group (invasion biologists or landscape experts) we found diverging framings of key concepts related to biological invasions, varying valuations of eects of non-native, invasive plant species (NIS) on ecosystem services (ES), as well as diering understanding of, and attitudes towards biological invasions as a societal problem. We found a clear dierence between the two expert groups for only a few questions, while within-group variation was generally high.

2.3.1 Understanding of key concepts 1. Non-native origin Experts generally agreed that a non-native plant is a species that arrived in a certain geographic area through movement facilitated by humans. However, expert definitions diered in their temporal and spatial reference, and only some referred to environmental change or human perception. None of the experts mentioned any biological characteristics of a species, e.g., that a non-native species has dierent traits than a native species, as part of the definition of non-nativeness. Landscape experts (LE) oered a greater diversity of definitions than invasion biologists (IB), and tended to discuss more explicitly their diculties in defining a non-native species. For instance, three LE emphasized that the choice of a spatial or temporal reference is a subjective decision: [...] there is no absolute definition of what is native and non-native. [...] what is new, say 10 or 20 years, this I would classify as non-native. [LE8] 2.3. RESULTS 35

But also some IB argued that deﬁnitions are arbitrary:

The definition used is completely arbitrary [...] This means in fact transitions are fluid and [...] if one would comprehend the definition in a broader sense, almost every vascular plant is non-native in Switzerland because they re- migrated after the ice age. [IB8]

Figure 2.1: Contrasting conceptualizations of the non-native origin of a plant species in Central Europe. Invasion biologists mainly referred to the notions of archaeo- and neophytes and thus to the year 1500 A.D. to distinguish non-native from native species (blue boxes). Landscape experts referred to a wide range of dierent time frames, including no mentioning of any reference point in time (brown boxes).

Temporal reference: Experts based their distinction between native and non- native species upon arrival time, using events in human history to define such temporal references. However, while invasion biologists mostly adhered to the same definition, landscape experts varied widely in their temporal reference (Fig. 2.1). Most invasion biologists (IB) defined non-native species as those species introduced after the year 1500 A.D. (i.e., Columbus’ discovery of the Americas), usually adding that this is the accepted or ocial definition in the literature. However, many of them perceived this definition as somewhat arbitrary. Only three landscape experts (LE) referred to the year 1500 A.D., and five LEs did not address the time 36 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS question; the other five LE proposed dates ranging from some unspecified time in the past, to the Neolithic period, the industrial revolution, and the period of globalization (Fig. 2.1).

Spatial reference: In contrast to a temporal reference, most experts used unspe- cific spatial references (e.g., “moved to here” or “moved to where we are”). Only 5 IB and 2 LE specified a spatial reference, basing this upon either human or biogeographical considerations. Thus, some referred to a political unit (such as “not from Switzerland”, “not from Europe”) while others mentioned biogeographical features (“from another continent”, “from a dierent biogeographic area”, or “from an area separated by topographic features (such as mountains or oceans) that hinder dispersal”). The various spatial references varied so widely that, applied to an area such as Switzerland, they would define strongly diering sets of non-native species.

Environmental change: Some experts, both LE and IB, acknowledged that natural or anthropogenic environmental change can aect what is considered to be a nonnative species. Some went on to state explicitly that species dispersing to a new area in response to anthropogenic environmental change (esp. climate change) should also be considered non-native.

Human perception of the non-nativeness of a species: Some LE referred to human perception in their discussion of the deﬁnition of non-nativeness. One LE argued, for instance, that species present for a long-term are sometimes considered to be native by local inhabitants, while another LE speciﬁed that non-native species are those that arrived in an area over a period shorter than a human lifespan.

2. Native species

To deﬁne a native species, most experts used similar considerations as for non- native species, but some experts used “presence since the last glacial period” as their criterion for a native species (3 IB / 2 LE). One invasion biologist pointed out that experts did not agree about whether to regard archaeophytes, i.e., species moved to Europe by humans before 1500 A.D. (Fig. 2.1) as native or as non-native. Thus, as for non-nativeness, nativeness of a species was mainly related to both its geographical distribution and to the period it was present in an area (Fig. 2.1); however, some experts also referred to biological characteristics, mentioning that native species are likely to be adapted to the local conditions (especially climate). Correspondingly, some experts mentioned that a native species evolved in a place, and others raised the possibility that a species can be native to a particular climate zone in mountains. Contrary to criteria used to deﬁne non-nativeness, only one LE discussed environmental change, and no-one considered the possibility that human perception might have a bearing upon a species’ native status. 2.3. RESULTS 37

3. Invasive species After clarifying the terms native and non-native, we asked our participants whether native and non-native plants dier in their behavior. Experts agreed that such biological dierences are not inevitable (9 IB / 9 LE), though some regarded them as likely (3 IB / 7 LE). In this connection, the following characteristics of non-native plants were mentioned: i. novel interactions with or dominance of resident organisms due to a lack of coevolutionary history, ii. adaptation to fast or human-related dispersal, and iii. the possession of novel traits enabling species to occupy an empty niche. In general, however, experts explained that the question was “dicult to answer”, due to “insucient scientiﬁc information”, or “high complexity”, meaning that “evaluations must be done on a case-by-case basis”. Ambiguity among experts became particularly obvious in how they related the non-nativeness and the invasiveness of species: some experts spontaneously valued the behavior of non-native plant species as problematic (4 IB / 5 LE), while others did not mention an invasive behavior at all. We then explicitly asked our participants for a deﬁnition of a non-native, invasive plant species (NIS). Experts generally agreed that an NIS is one that spreads spontaneously and rapidly, and exerts a negative impact on native species, ecosystem processes, the economy, or human health. Two experts (1 IB / 1 LE) stated explicitly that the term invasive does not necessarily imply a negative impact. To them, range expansion alone was a sucient condition for being an invasive plant species.

2.3.2 Valuations of eects on ecosystem services Each expert was then asked to value the influence of NIS in Europe on twelve different ecosystem services (ES) as negative, neutral or positive, resulting in twelve separate assessments per expert. Eight of the ES can be regarded as provisioning, regulating, or supporting services, and four as cultural services. For every ES there were some negative, neutral, or positive assessments, and in general both IB and LE ranged widely in their assessment of the eects of NIS on ES (Fig. 2.2). In total, experts made more neutral valuations (56%) than negative (32%) or positive ones (12%). Our study participants perceived the strongest negative impacts of NIS on ES Biodiversity (56% negative valuations) and ES Cultural Heritage (54% negative valuations). Most favorable eects of NIS were attributed to ES Land- scape Aesthetics with 33% positive valuations. On average, invasion biologists (IB) tended to assess the eects of NIS upon ecosystem services more negatively than landscape experts (LE), especially for ES Cultural Heritage, Sense of Place, Food, and Soil Formation (Fig. 2.3). We were particularly interested in how experts reached their opinions concerning the influence of NIS on ES, and the arguments they used to substantiate them. We identified four issues characterizing various uncertainties that complicate the valuation process: 1. how to deal with a lack of empirical information; 2. how to 38 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS

Figure 2.2: Number of positive, neutral, and negative assessments of the pre- dominant eect of not further speciﬁed non-native, invasive plant species on 12 dierent ecosystem services for 12 invasion biologists (IB1-IB11, IB14, blue dots) and 12 landscape experts (LE1-LE12, brown dots). The perpendicular distance of the parallel lines to the respective corner corresponds to the number of positive, neutral, and negative assessments of a study participant (e.g., LE7: 3 neutral, 5 negative, and 4 positive assessments).

deal with value judgments; 3. what to do when the same species has both positive and negative eects; and 4. how to treat the non-nativeness of a species in value judgments.

1. Decision making with limited information

Experts consistently mentioned a lack of empirical information about eects of NIS on ES. At best experts knew about documented eects of one to a few particular species. Information about eects caused by a broad range of dierent invasive species, however, was mostly missing. We identiﬁed ﬁve ways through which experts coped with this uncertainty: (i) concluding that there were no eects of NIS on ES, (ii) acknowledging the lack of empirical information, (iii) extrapolating 2.3. RESULTS 39

Figure 2.3: Average valuations of the eects of non-native, invasive plant species (NIS) on dierent ecosystem services (ES). On average, invasion biologists (blue dots) assessed NIS eects on ecosystem services (ES) more negatively than landscape experts (brown squares), particularly in the case of cultural ES (asterisks). The overall mean of the evaluations (black triangles) is most negative for ES Bio- diversity and most positive with respect to ES Landscape Aesthetics.

from their knowledge about the eects of particular species, (iv) building on general knowledge about eects of NIS on certain ES, or (v) referring to the frequent overabundance of NIS, that is often stated as a speciﬁc characteristics of NIS in the literature, and deriving predictions about eects from this general pattern. Acknowledged lack of knowledge (i, ii): In almost a third (32%) of all assessments, experts could not recall any eects of NIS on ES. The experts drew one of two conclusions from this lack of knowledge: either that NIS have little or no eect compared with native species, or that important information was missing (either to them as an individual expert or more generally in the literature). Some experts suggested that more research on the subject is needed. But otherwise, with very few exceptions only, experts assessed neutrally in both cases. 40 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS

Extrapolation from information about particular species (iii): Sometimes experts based their assessments on well-known eects associated with particular species, such as Solidago sp., Ambrosia ambrosiifolia, Heracleum mantegazzianum, Rey- noutria sp. and Impatiens glandulifera. In particular, experts recalled the adverse impact on human health of A. ambrosiifolia and H. mantegazzianum, and the destabilizing eects of Reynoutria sp. on soil in general and particularly on stream banks. But experts also emphasized e.g., the attractiveness of ﬂowers of Solidago species. Some experts concluded from these examples that other invasive species may have similar eects, while other experts emphasized that extrapolation is not possible. Extrapolation from general knowledge about NIS (iv): In some cases experts felt conﬁdent to make general statements about NIS eects on ES. These statements were based on generalized knowledge about NIS without reference to particular species. For instance, an expert explained the negative impact of NIS on ES Biodiversity, arguing that “globally NIS were the second most important cause for a decrease of biodiversity” [IB5]. Such generalizations were common for statements about eects on ES Biodiversity: most experts were prepared to assume that NIS have a generally negative eect upon native biodiversity, though few explained the underlying mechanisms or cited empirical evidence. Most experts generalized at least once (9 IB / 10 LE). Reference to overabundance (v): The only general characteristic of NIS that was explicitly mentioned to substantiate general claims about eects of NIS on ES was the fact that invasive species often form dense stands. For all ES except ES Human Health at least one expert recognized overabundance as a reason for negative impact. Overabundance was most frequently stressed in the context of ES Biodiversity, Pollination, and the cultural ES Cultural Heritage and Sense of Place. Overabundance of NIS always led to a negative valuation and was the basis for one third (34%) of all negative valuations.

2. Explicit consideration of values and value judgments

Valuation of an eect entails understanding how an NIS changes an ecosystem property, and then assessing whether this change is positive or negative. Experts rarely explicitly mentioned the second step, and the importance played by values. In particular, experts assumed that the preferred state of non-cultural ES (and associated cultural views or ethical values) was clear and uncontroversial, the only exceptions being for ES Pollination and Food (2 IB). In contrast, for assessments of NIS, inﬂuences on cultural ES, more often value judgments were made explicit. All experts except for three (1 IB / 2 LE) stated at least once either their own feelings or values towards NIS or their eects (9 IB / 9 LE), or they referred to feelings or values of particular stakeholder groups (e.g., tourists, agriculture, general public) (8 IB / 10 LE). Experts remarked for instance that because of positive experiences with a particular NIS their sense of place was “positively connected” 2.3. RESULTS 41 to the presence of that species; on the other side, they spoke of “negative feelings” towards “change in a familiar landscape”, or “getting irritated by monocultures”, and “being distressed” by negative impacts of some NIS on recreation. Interest- ingly, however, explicit consideration of personal values or dierent stakeholder views was less prominent for ES Cultural Heritage. Some experts acknowledged that valuation of stakeholders is rooted in cultural history. LE6 for instance considered NIS eects on ES Cultural Heritage as positive and explained:

It is positive because . . . the fact that plants were moved to particular places is part of our culture, whether we consider it positively or not. [LE6]

LE3 referred to the evolution of perceptions over time:

If one accepts that over time invasive plants [. . . ] become a cultural heritage, then it is not negative. [LE3]

Similarly conifers that were extensively planted in Switzerland during the 19th century were recognized by one LE as a “kind of invasion at the time” [LE2] but were perceived today as enhancing landscape aesthetics. However, only few experts elaborated more generally on the value-laden nature of assessing eects on cultural ES.

3. Ambiguous valuations All experts except 5 LE emphasized the diculty of valuating eects in situations where a species has both positive and negative eects on dierent or even the same ES; or when dierent people value the same eects dierently. LE3, for instance, argued that while invasive species might reduce biodiversity in the short term, they might lead to a higher, new biodiversity in the longer term. Or, LE11 valued the inﬂuence of NIS on ES Landscape Aesthetics neutrally and commented:

Related to landscape aesthetics it’s a matter of taste—here are people who are enthusiastic about dense Solidago stands, but from the point of view of nature conservation, it’s rather negative. [LE1]

4. The role of non-nativeness in valuation of eects on ecosystem services The non-nativeness of a species was used both in value judgments and to explain how the species might aect ecosystem services. It was not always evident whether the non-nativeness of a species was valued in itself or as a reason for expecting some negative impact. Non-nativeness was mentioned in the context of all ES, but was not considered equally important by all participants. Particularly in the context of ES Cultural Heritage, experts described a sense of loss associated with the spread of NIS—for example, “loss of a landscape” they 42 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS had been used to or “loss of identity” as the result of the presence of a non-native species; similarly, experts mentioned their feeling that the “new species did not belong” to their culture. Thus, some experts considered the presence of non-native species in itself as negative for cultural heritage:

Invasive, non-native plants have to be negative, because they are new and not native, and so not part of our heritage. [IB2]

Several experts implied that valuing the non-nativeness of a species depends on knowing which species are non-native and which ones are not. IB9 for instance argued that:

Most people do not have any idea of what is indigenous and what is non- indigenous. Hence they [invasive plant species] do not have any inﬂuence on the recreational factor at all. [IB9]

In other cases, the novelty of non-native species was positively valued - for example, when the species was seen as “an enrichment” of the preexisting ﬂora or a “contribution to the aesthetic value” of a landscape.

2.3.3 Biological invasions as a societal problem To reveal experts’ evaluations of how serious they consider the problem of NIS in Central Europe to be, we asked a series of quantitative questions (Fig. 2.4). All participants recognized NIS as a problem in Central Europe, although most rated it as small to medium at present (Fig. 2.4A). Yet almost all experts expected problems related to NIS to increase in the future (Fig. 2.4B), emphasizing anthropogenic environmental change as a driver of future invasion threats. Especially IBs called for action against NIS through concrete management measures (Fig. 2.4C). Most participants of both expert groups agreed that our causal understanding of why some plants become a problem is inadequate (Fig. 2.4D), arguing, for example, that the complexity of ecosystem processes makes general assessments dicult or even impossible. Interestingly the two expert groups clearly diverged in their assessment on how the problem is recognized by a particular stakeholder group of their choice (Fig. 2.4E). In general, invasion biologists considered that the problem was underestimated by the public and in politics. Landscape experts tended to see the problem as overestimated, particularly due to anxiety and xenophobic feelings among the public.

2.4 Discussion

Our interviews with experts of plant invasions and/or ecological processes in the landscape indicate that their understanding of the phenomenon of non-native plant invasions is diverse and inﬂuenced by individual conceptualizations, beliefs, and 2.4. DISCUSSION 43

Figure 2.4: Experts’ responses to a set of closed questions regarding the invasive species issue as a societal problem (Appendix I, Q20–Q25). Blue boxes: invasion biologists, brown boxes: landscape experts. 44 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS values. While we expected a rather high diversity of perspectives in the heterogeneous group of landscape experts, we were surprised by the lack of consistency in the use of basic concepts amongst invasion biologists, since the research field is well defined, and its leaders have invested much eort in standardizing key concepts (e.g., Colautti and McIsaac, 2004; Valéry et al., 2008; Colautti and Richardson, 2009; Pyöek et al., 2009; Richardson et al., 2011). In fact, the diversity of perceptions within both experts groups was so large that for most issues we examined there was no clear dierence between the groups. Our study was restricted to German-speaking experts of Switzerland and Southern Germany, and an even higher diversity of expert opinions might be expected if we had included a broader geographical range. Studies from other regions indicate that many of the relevant dimensions of expert thinking that we identified for German-speaking Europe might also be relevant elsewhere (Selge et al., 2011; Young and Larson, 2011). We suggest that the heterogeneity of expert judgments observed in our study is related to three major dimensions: (1) diverging understandings of basic concepts, (2) complexities and contingencies of biological invasions, and (3) valuation uncertainties with respect to the qualitative assessment of the eects of non-native, invasive plant species on ecosystems and their services.

2.4.1 Diverging understandings of basic concepts

Central to any understanding of a non-native, invasive species (NIS) are the definitions of non-native and invasive species. Many of our participants accepted, at least in part, conventional definitions widely used in the research field, though the interviews also revealed more diverse thinking. We screened the publications of the participating invasion biologists to check whether the results from the interviews were also reflected in the ways experts used definitions in their scientific publications. We found that authors generally reported a definition of a non-native and invasive species in the introduction or methods section of a publication, but in the rest of the text this definition was rarely strictly applied. For instance, authors might present a definition that distinguishes between non-native and invasive species, but then use the terms interchangeably in the text; or they might compare invasive non-native species with native species without specifying whether or not the native species are also invasive (fast spreading / having a negative impact). This tension between a shared definition and a much broader understanding of key notions is also more generally apparent in the biological invasion literature. Indeed, many of the diculties in operationalizing definitions of non-native and invasive species that we uncovered in this study can be found scattered throughout the literature (Garrott et al., 1993; Shrader-Frechette, 2001; Brown and Sax, 2004; Sago, 2005; Warren, 2007; Davis, 2009; Valéry et al., 2009; Rotherham and Lambert, 2011; Young and Larson, 2011; Webber and Scott, 2012). Given that the definitions of an NIS can be regarded as core elements of the paradigm of the research field (e.g., they are introduced in every textbook), it is 2.4. DISCUSSION 45 surprising that we found such a high diversity of alternative understandings among experts. Even among invasion biologists only two thirds mentioned a common temporal reference for the definition of the non-nativeness of a species (1500 A.D., Fig. 2.1) and only half explicitly stated that non-native species are those moved through human-assisted dispersal. And landscape experts did not agree at all on a common temporal reference. There was also no agreement on other aspects necessary for a non-ambiguous definition of an NIS, such as where a species must come from to count as non-native, and some important aspects were not mentioned at all, in particular how to determine whether a species is fast spreading. This aspect, although basic to the definition of NIS, is dicult to operationalize and several dierent approaches are described in the literature (compare Richardson et al., 2000; Wilson et al., 2009; Sorte et al., 2010; Webber and Scott, 2012). In fact, the dierent answers given by the experts lead to substantially dierent selections of non-native and invasive species. Experts also diered in their understanding of what a native species is, despite the extensive literature on this topic in Central Europe (Schroeder, 1968; Webb, 1985; Kasparek, 2008). In summary, we found uncertainties related to at least eight conceptual dimensions that aect a common understanding of the key notions of a non-native species and an invasive species: (i) minimum and maximum residence time in a new area, (ii) source area, and (iii) dispersal pathway (through human agency or not) of a non-native species; (iv) What counts as human-assisted dispersal? (v) Must an invasive species be fast spreading and/ or have a recognized negative impact? How are (vi) fast spread and (vii) negative impact defined? And, (viii) should the term invasive be reserved for non-native species or also be used for native species?

In many cases, dierences between experts’ definitions reflected dierent ways of framing a socioecological problem. In particular, some experts understood invasions primarily as a biological phenomenon, while others approached it as a sociocultural phenomenon. According to the biological perspective, the non-native origin is important because species introduced into new areas often exhibit dis- tinctive ecological behavior, with respect to both the source population and to the native flora where the species establishes. A non-native species may, for instance, behave dierently from a native species because it is released from its natural enemies (Keane and Crawley, 2002), or has novel traits that are not present in the native flora (Vitousek et al., 1987). In contrast, some landscape experts approached the subject with primarily a sociocultural perspective in mind. Thus, they placed emphasis upon the temporal dynamics of human perception of and cultural attachment to nature and biodiversity, or dierent important historical episodes such as the beginning of industrialization or globalization as the basis for separating native and non-native species (Fig. 2.1). Interestingly, ecological and sociocultural types of reasoning were often intermingled. For instance, while most invasion biologists indicated that they intended to gain an ecological understanding of why non-native species behave dierently from native species, most of them nevertheless referred to a cultural criterion for separating non-native from native 46 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS species, namely the year 1500 A.D. Thus, a cultural definition of the non-nativeness of a species is used in an ecological argument.

2.4.2 The notion of a non-native, invasive species as a boundary object Several authors in the invasion literature have expressed confidence that the problem of conceptual pluralism in research on biological invasions can be overcome by defining key notions more precisely (Colautti and McIsaac, 2004; Pyöek et al., 2004; Valéry et al., 2008; Colautti and Richardson, 2009; Pyöek et al., 2009; Richard- son et al., 2011; Webber and Scott, 2012). However, experience in invasion biology and other fields of ecological research indicate that it is dicult to establish precise definitions that all experts can share (e.g., Shrader-Frechette, 2001; Haila, 2002; Sago, 2005; Hodges, 2008; Moore et al., 2009). The situation is further complicated by the fact that through anthropogenic environmental change, patterns and processes are changing so fast, with the consequence that concepts and research approaches must be continuously adapted (Kueer, 2013). In the case of complex and interdisciplinary problems, therefore, it may not always be possible to reach a consensus on definitions. There is probably no way to avoid a melting pot of diverse terms and perspectives characteristic of an increasingly inter- and transdisciplinary invasion science. It is clearly important for authors to define their key terms in a particular context, but even this may not help much; a better solution may be to classify alternative definitions of concepts that are valid for particular purposes (Hodges, 2008). It may even be that partially ambiguous terms can be beneficial for the research field by facilitating inter- and transdisciplinary dialogue. For this to occur, they must serve as boundary objects, meaning concepts that have a similar but not identical meaning to dierent expert groups (adapted from Star and Griesemer, 1989). Thanks to this fluidity, these concepts can facilitate collaboration between dierent communities because they can be adapted to dierent specialized expert discourses without losing a shared core meaning. Indeed, our data shows that the term non-native, invasive species encompasses a range of dierent meanings that resonate with dierent research interests, an observation that is also reflected by the diversity of perspectives in the literature on invasive species (compare, e.g., Davis, 2009; Richardson et al., 2011; Heger et al., 2013b), Fig. 2.5. For instance, biogeographers are interested in the role of geographic barriers in determining species distributions and richness patterns, and non-native species—defined as species that cross biogeographic barriers—resonate with their interests. Some evo- lutionary biologists and ecologists are interested in how species respond to novel abiotic and biotic conditions and, in turn, how species with novel characteristics can change ecological processes. For them, it is less important whether a species is from another biogeographic area, but it matters whether it introduces some form of ecological novelty to a system. Invasions oer a suitable system for population and community ecologists to study the processes of spread and colonization, but these 2.4. DISCUSSION 47

Figure 2.5: The notion of a non-native, invasive species (NIS) as a boundary object: dierent groups of experts use the same notion with a dierent speciﬁc meaning and purpose in dierent contexts. Thereby the notion of an NIS as an ill-deﬁned concept can help to facilitate collaboration across these diverse experts group (see main text for further explanation).

do not necessarily dier between non-native and native species. Overabundance of some NIS is a feature that they share with some native winners of anthropogenic change, which can be unrelated to biogeographic origin or processes of spread (e.g., Fig. 1 of Rejmánek, 2000). In turn, scientists from the social sciences and human- ities are interested in, for instance, the cultural connotations of the terms invasive and non-native (and associated terms), and in human-nature relationships and how these influence the geographic distribution and human perception of species. In management, NIS are also addressed dierently in contrasting realms, such as transnational biosecurity policies vs. the local management of natural areas. For biosecurity policies the non-native origin of species is central, while origin may be of lesser importance for local ecosystem management. Of course, the pluralistic usage of concepts also bears risks. For instance, the sometimes vigorous conflicts between social and natural scientists related to the invasive species issue (e.g., Simberlo, 2003; Raes, 2011) may have arisen from a failure to recognize that they were using the same term to mean dierent things. Social scientists, accustomed to deliberations about the cultural connotations of terms like alien or nonnative, accuse invasion biologists of being xenophobic (which is a legitimate concern within the narrow boundaries of their specialized debates), though biologists use the term in a very dierent context and usually without any cultural connotations. It is therefore important to carefully reflect on the dierent contexts when using terms such as non-native (or alien, exotic, foreign, etc.) in science or policy. 48 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS

2.4.3 Complexity and contingencies impede proactive action Most participants stressed that risk assessments of biological invasions are made dicult by our lack of basic understanding of the important processes. In more than 50% of the assessments of eects of NIS on ecosystem services, experts decided for a neutral assessment, saying that potentially unknown eects or lack of knowledge prevented them reaching any other conclusion. They also often pointed to the complexity and contingencies of biological invasions, emphasizing that factors such as habitat context and anthropogenic disturbances interact, and that the dynamics and outcomes of invasions can change in time. In particular, they emphasized the diculty of making general statements across many species and contexts, especially when considering the longer-term spatio-temporal dynamics of ecosystems. The only general characteristic regularly used by experts to legitimate their valuations of eects was the overabundance of an invasive species; while in almost all other cases they were forced to extrapolate eects from individual NIS to all invasions. Such extrapolation–from individual cases to invasions in general– is also widely used for predicting the potential eects of NIS in the literature (e.g., Pimentel et al., 2000). We designed our interviews in a way that forced experts to make general statements to mimic their expert role in decision-making processes. Due to the emphasis of current invasion policy upon proactive action, in particular measures to prevent potentially problematic non-native species from being introduced (Leung et al., 2002; Hulme et al., 2009), decisions often have to be taken for species that are not well known or only from other areas. Thus, there may be considerable uncertainty in determining whether or not a non-native species poses a risk (without in-between categories) across all habitats. Many experts in our interviews stated that such extrapolation from a few, often poorly known case examples is problematic, thus echoing a strong critique of invasive species management by philosophers of science (Shrader-Frechette, 2001; Sago, 2005). Alternative approaches to biosecurity that might circumvent this problem include adaptive management processes, participatory methods, or risk assessments that are speciﬁc to particular habitats or introduction pathways (e.g., Kueer and Hirsch Hadorn, 2008; Liu et al., 2011; Hulme, 2012).

2.4.4 Valuation uncertainty influences risk perception and risk assessments The process of risk assessment is also complicated by uncertainties related to the valuation of eects of NIS. In our study we found at least three dierent kinds of valuation uncertainties: (i) ambiguous endpoints for risk assessments, (ii) diering value systems or perceptions, (iii) the role in value judgments of controversial concepts such as non-nativeness. A first valuation uncertainty—ambiguous endpoints of risk assessments—became obvious when our study participants stated that the same invasion can simul- 2.4. DISCUSSION 49 taneously lead to several outcomes—some positive, others negative. For instance, according to the experts Solidago species can provide an important food source for native pollinators and at the same time out-compete native plant species. About one quarter of all neutral assessments reflected such ambiguities.

Second, valuation options may vary among experts and stakeholders depending on their value system and perception. For example, in assessing the contribution of Solidago species for the landscape, some may value the yellow ﬂowering in late summer positively, while others will negatively value the fact that the landscape diers from what they consider to be a natural landscape. Especially for cultural ecosystem services, experts emphasized that valuation depends on the aected stakeholders that are considered.

Third, speciﬁc to the invasive species debate is the use of the native vs. non- native species dichotomy (Fig. 2.1). Not surprisingly, the controversial perception of the importance of a species’ origin for risk assessments added as a third factor to the heterogeneity of expert valuations. Most participants agreed in principle that invasive species should be judged according to their eects on native biodiversity and ecosystems, and not by their origin, i.e., their non-nativeness per se. Many experts therefore stressed the need to distinguish clearly between non-native and invasive species. This corresponds with the way lay people value invasive species, with detrimental impacts being more important than biogeographic origin (Fischer and van der Wal, 2007; Selge and Fischer, 2010). Nevertheless, invasion biologists are regularly criticized for condemning non-native species solely because of their foreign origin (Larson, 2007; Davis et al., 2011). Indeed, despite their intention not to use non-nativeness as a value, many experts revealed an implicit bias against non-native species in their answers to interview questions. This was especially evident when valuing cultural ecosystem services such as Cultural Heritage, Sense of Place, or Landscape Aesthetics, with several experts treating the presence of non-native species negatively because of their non-nativeness. Whether and how non-nativeness is used to make value judgments adds to valuation uncertainty, especially because this criterion is often not explicitly mentioned.

The problems of multiple eects and multiple adequate values are widespread in environmental problem solving, and there is a broad literature on multi-criteria and participatory valuation and decision-making methodologies (Scholz and Ti- etje, 2002). However, such methods have rarely been applied in the context of invasive species (Binimelis et al., 2007a). Indeed, established procedures such as risk screening systems or black lists (Pheloung et al., 1999) do not consider the possibility of the context-dependence of eects and conﬂicting valuation, and new approaches are only now being developed (Randall et al., 2008; Hulme et al., 2009; Liu et al., 2011; Kumschick et al., 2012; Dahlstrom Davidson et al., 2013). Similarly, media communications rarely represent the diversity and uncertainty of valuation of NIS (Chew and Laubichler, 2003; Larson, 2005). 50 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS

2.4.5 Speaking with one voice to the public: an outdated expectation from invasive species experts?

In our study, expert opinion diverged strongly in questions concerning the eects of non-native, invasive plant species (NIS) on individual ecosystem services (Fig. 2.2 and 2.3), and the assessment of the problem posed by these species in Europe (Fig. 2.4). This divergence of opinion revealed in interviews contrasts with the broad consensus on the risks and eects of biological invasions that experts claim in scientific synthesis articles and policy documents (Mack et al., 2000; Millennium Ecosystem Assessment, 2005; DAISIE–Delivering Alien Invasive Species Invento- ries for Europe, 2009; Hulme et al., 2009; Vilà et al., 2010). Indeed, the invasion biologists in our survey have all published articles concerning plant invasions in Central Europe in which they state that biological invasions lead to major biodiversity loss and/or economic costs. In most cases, however, these statements were of a general nature and supported by data from outside of Europe (citing e.g., Pimentel et al., 2000; Mooney et al., 2005). One possible interpretation for this paradoxical situation is to consider the dual nature of an expert as on the one hand a person with a high degree of knowledge of a subject and on the other hand someone with a special role in certain decision-making situations attributed by society (Mieg, 2009). Invasion biologists are faced with the dicult challenge of reconciling high uncertainties in internal expert deliberations (i.e., in their role in producing new knowledge) with their public role as experts, expected to speak with one voice to decision-makers and the public. This situation is even more complicating when a consensus is needed for action to minimize future risks but the knowledge upon which to base this action is uncertain or missing. Indeed, while disagreeing on the current severity of the problem, both invasion biologists and landscape expert tended to agree that the severity of the problem will increase and intervention is urgent (Fig. 2.4). Nevertheless, recent examples of conflicting debates among invasive species experts played out in the media (Davis et al., 2011; Simberlo et al., 2011) suggest that such accord may not be possible in the future. It is questionable whether speaking with one voice is even desirable: in the context of climate change, Curry (2011) argues that pressure to achieve consensus may have led the Intergovern- mental Panel on Climate Change (IPCC) to oversimplify some issues of high uncertainty, thereby hampering the formulation of appropriate policy decisions; and similar concerns have been raised in the context of biodiversity conservation (Turnhout et al., 2012). An alternative to seeking consensus would be for invasion biologists to acknowledge the uncertainties and to engage transparently with stakeholders and the public in deliberations about conflicting opinions (Larson et al., 2013) or diverging management priorities (Bayliss et al., 2013). Here invasion biologists should take the role of “honest brokers of policy alternatives”, taking into account dierent prevalent values and policy preferences, rather than adopting the role of “issue advocates” (Pielke Jr., 2007; Sarewitz, 2011). 2.5. ACKNOWLEDGEMENTS 51

2.4.6 Conclusions

Our results uncovered a high diversity of perspectives within and between two expert groups, invasion biologists and landscape experts, on how to frame and to value biological invasions. Such dissent is in strong contrast with the broad consensus that experts claim in scientific synthesis articles and policy documents (e.g., Millennium Ecosystem Assessment, 2005). We propose that dissent among experts arises for many reasons, and multifarious solutions are therefore needed to improve the situation. First, irreducible uncertainties and contingencies should be acknowledged and taken into account in invasive species research and risk assessments (Hulme, 2012; Jeschke et al., 2012; Heger et al., 2013a; Kueer et al., 2013; Larson et al., 2013). Second, rather than attempting to establish precise definitions for key concepts such as ‘non-native’ or ‘invasive’ that all experts can share, it may be better to recognize explicitly alternative definitions that are valid for particular purposes (Hodges, 2008; Heger et al., 2013a,b). It can be argued that many conflicts about invasive species between social and natural sciences have arisen from a failure to recognize that dierent disciplines may use the same term to mean dierent things. Third, the process of risk assessment is complicated by uncertainties related to the valuation of effects of non-native invasive species. The problems of multiple eects and multiple adequate values are widespread in environmental problem solving, and there is a broad literature on multi-criteria and participatory valuation and decision-making methodologies. Such approaches should be applied in invasive species risk assessment, management, and communication. Lastly, invasion biologists have diering views of the appropriate role for experts in societal decision-making, and especially the degree to which they should advocate particular viewpoints (Young and Larson, 2011). Our results indicate that invasion biologists, rather than claim- ing to represent a consensus when none exists, should engage transparently with stakeholders in deliberations about conflicting opinions and alternative concepts, thereby adopting the role of “honest brokers of policy alternatives” (Pielke Jr., 2007; Sarewitz, 2011).

2.5 Acknowledgements

The authors are grateful to all study participants for their valuable contributions. We also thank Christian Pohl for his help with the analysis of qualitative data, Iris Altenburger and Hans-Heini Vogel for participating in the pre-studies, Ed- ward Connor for proof-reading the English translations of the cited quotations, Sandro Bösch-Pauli for designing Figure 2.2, and two anonymous reviewers for their valuable suggestions to improve the manuscript. 52 CHAPTER 2. EXPERT PERCEPTIONS OF INVASIVE PLANT RISKS

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3 Are Non-Native Plants Perceived to Be More Risky? Factors Inﬂuencing Horticulturists’ Risk Perceptions of Ornamental Plant Species

Franziska Humair, Christoph Kueer, Michael Siegrist (2014) In PLoS ONE 9(7): e102121, doi:10.1371/journal.pone.0102121

Abstract Horticultural trade is recognized as an important vector in promoting the introduction and dispersal of harmful non-native plant species. Understanding horticulturists’ perceptions of biotic invasions is therefore important for eective species risk management. We conducted a large-scale survey among horticulturists in Switzer- land (N = 625) to reveal horticulturists’ risk and benefit perceptions from ornamental plant species, their attitudes towards the regulation of non-native species, as well as the factors decisive for environmental risk perceptions and horticulturists’ willingness to engage in risk mitigation behavior. Our results suggest that perceived familiarity with a plant species had a mitigating eect on risk perceptions, while perceptions of risk increased if a species was perceived to be non-native. However, perceptions of the non-native origin of ornamental plant species were often not congruent with scientific classifications. Horticulturists displayed positive attitudes towards mandatory trade regulations, particularly towards those targeted against known invasive species. Participants also expressed their willingness to engage in risk mitigation behavior. Yet, positive eects of risk perceptions on the willingness to engage in risk mitigation behavior were counteracted by perceptions of benefits from selling non-native ornamental species. Our results indicate that the prevalent practice in risk communication to emphasize the non-native origin of invasive species can be ineective, especially in the case of species of high importance to local industries and people. This is because familiarity with these plants can reduce risk perceptions and be in conflict with scientific concepts of non-nativeness. In these cases, it might be more eective to focus communication on well-documented environmental impacts of harmful species. 62 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC.

3.1 Introduction

Increasing international trade is of major concern in ecology and in environmental management because it enables species dispersal across biogeographic barriers (McNeely, 2006; Hulme, 2009; Brunel et al., 2013). At a global level, frequent and widespread introductions of non-native species have resulted in novel new species compositions and interactions in ecosystems (Hobbs et al., 2013). In the literature it is generally assumed that new species can pose a major risk to biodiversity, ecosystem functioning, or human well-being (Mack et al., 2000; Millennium Ecosystem Assessment, 2005). This is for example the case if introduced plant species (i.e., non-native plant species that arrived in Europe after the year 1500 A.D. thanks to human assistance) become dominant and possibly alter ecosystem functions, or when non-native species are associated with harm to other species or to the economy (Vilà et al., 2010). New and harmful species—i.e., invasive non-native species—are considered to be a leading cause of biodiversity loss (Mil- lennium Ecosystem Assessment, 2005). Research and species management are increasingly concerned with drivers and pathways of species introductions and establishment, and the horticultural industry has been identified as a particularly important vector for the global dispersal of plant species (e.g., Ruiz and Carlton, 2003; Kowarik and von der Lippe, 2007; Hulme, 2009). In order to design eective risk management strategies, it is therefore important to better understand what drives horticulturists’ risk perceptions and what factors influence their willingness to engage in risk mitigation behavior. In this study, we therefore investigated the influence of the perceived non-nativeness of and the perceived familiarity (using the perceived horticultural importance as a proxy) with an ornamental plant species on horticulturists’ perceptions of environmental risks. Further, we were also interested in the roles of risk and benefit perceptions related to risk mitigation behaviors. In its function as a gatekeeper in plant dispersal, the horticultural industry has come into the spotlight of ecology and conservation management (e.g., Reichard and White, 2001; Baskin, 2002; Mack and Erneberg, 2002; Dehnen-Schmutz et al., 2007, 2010; Bradley et al., 2011; Pyöek et al., 2011) because it has been recognized as an important driver of plant invasions. But horticulture is also an important partner in risk mitigation measures such as risk assessments (e.g., Mack, 2005) or consumer information, e.g., by directing consumers towards plants with a low environmental risk (Coats et al., 2011), informing about particular aspects of invasive species (Halford et al., 2013), or instructing consumers to appropriately handle risk species (Humair et al., 2014b). Thus, increasingly researchers, policy makers, and practitioners are interested in horticulturists’ attitudes towards the invasion issue and in their incentives to engage in risk mitigation behavior. Burt et al. (2007) found that awareness of the invasive plant problem and concern for the environment were important factors for horticulturists’ willingness to engage in voluntary action. Another study conducted 3.1. INTRODUCTION 63 in the U.S. found that horticulturists believe that their risk mitigation behavior increases the reputation of horticulture in the public as an environmentally friendly industry (Gagliardi and Brand, 2007). Peters et al. (2006) and Coats et al. (2011) link horticulturists’ interest in voluntary customer information to the perceived responsibility of the horticultural industry for the cost of damage resulting from non-native invasive ornamentals sold in their businesses. These studies described horticulturists’ attitudes, but the psychological determinants that may shape risk perceptions towards non-native plant species were not examined. For the successful integration of stakeholders into risk management strategies or to design adequate strategies to communicate environmental risks, understanding stakeholders’ concerns and their underlying factors is essential (Slovic, 1987; Fis- chho, 1995). However, only little research has been conducted on what shapes risk perceptions in the context of biotic invasions (but see Kueer, 2013). Starfinger et al. (2003) and Stromberg et al. (2009) report that risk perceptions towards non- native plant species have changed over time depending on varying economic needs and values held by dierent stakeholder groups. Fischer and van der Wal (2007) examined attitudes by the public towards the management of a non-native invasive plant species (Lavatera arborea) and found that risk perceptions and support of management measures were connected to environmental values such as ecological balance in nature or naturalness (defined as nature untouched by humans). While those participants that were concerned about the loss of an ecological balance in nature stated a need for action, those who were concerned about naturalness preferred no intervention. In their qualitative studies among lay people, conservation volunteers, and ecologists, Selge and Fischer (2010) and Selge et al. (2011) found that the perceived harmfulness of a species was not necessarily linked to its origin (native vs. non-native). Instead, perceptions of invasion risks from certain species were rather shaped by the notions of negative impacts on nature, economic costs (e.g., for agriculture), available management options, or perceived attractiveness of the invasive species. To some participants it was important to acknowledge the human role in species dispersal and biotic invasions: Natural spread of species was of lesser concern than human-induced dispersal. Risk research has shown that lay people rely on qualitative risk characteristics for assessing the risk associated with a hazard (Fischho et al., 1978; Slovic, 1987). Lay people use the familiarity with a hazard, the controllability of a hazard, the voluntariness to be exposed to it, its dreadfulness, or the severity of consequences to be expected as indicators for the risk. In the psychometric paradigm these qualitative aspects of a hazard could be reduced to the two dimensions Dread Risk and Unknown Risk (Slovic, 1987). The location of the hazard on these two dimensions largely explains why dierent hazards are seen dierently: The more dread is perceived from a hazard and the more unfamiliar a hazard is, the more risk is attributed to the hazard in question. However the method used by Fischho et al. (1978) is based on aggregated data and does not allow to make any statements about individual dierences in risk perceptions (Siegrist et al., 2005). 64 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC.

People dier in their risk perceptions. Some people perceive a lot of risks associated with a specific hazard, whereas other people may not perceive any risks at all. The aect heuristic is one of the mechanisms that has been proposed for explaining individual dierences in risk perceptions (Slovic et al., 2004). It is assumed that the more negative associations a hazard evokes in an individual, the more risky this person perceives the hazard. Familiarity with a hazard may result in a change of aect, i.e., less negative or more positive feelings associated with the hazard. It has been shown that repeated exposure is sucient to create a positive evaluation of a stimulus (Zajonc, 1968). Also, it has been found that familiarity with a hazard may positively contribute to its acceptance; this is when people become knowledgeable about a hazard because it is encountered on a regular basis, such as smoking or driving cars (McDaniels et al., 1996). The mitigating eect of familiarity with a hazard on individual risk perceptions has been discussed in various contexts, e.g., concerning nuclear waste disposal where familiarity with the nuclear industry raises public acceptance of repositories (Chung et al., 2008); in the context of investment decisions where investors prefer familiar financial products that are perceived to be easier to understand and less risky (Wang et al., 2011); or, regarding natural hazards where people familiar with flooding (i.e., those living in floodplains) feel less threatened by flooding events compared to people inexperienced with floods (Baan and Klijn, 2004). The aim of our study was to examine determinants of horticulturists’ perceptions of risks from plant invasions as well as factors influencing horticulturists’ willingness to engage in risk mitigation behavior. For this purpose, a written survey was conducted among members of the Swiss Association of Horticulture (JardinSuisse), the largest horticultural association in Switzerland. Besides negative impacts of a species, risk communication on biotic invasions often also emphasizes the non-native origin of the harmful species in question. We therefore hypothesized that (i) perceived non-nativeness of a species will result in a higher risk perception of this plant. Based on previous findings, we further assumed that (ii) horticulturists’ familiarity with a plant species reduces the perceived risk associated with this plant. A second set of aims for the present research was to examine (iii) horticulturists’ attitudes towards regulation and voluntary actions to mitigate invasion risks. We expected that (iv) perceived risk of non-native plants has a positive and perceived benefit a negative influence on respondents’ willingness to support risk management action against non-native invasive plant species.

3.2 Methods 3.2.1 Ethics Statement This study was based upon written questionnaires with members of the Swiss Association of Horticulture. Participants were ensured that the data would be anonymized. According to the directives of ETH Zurich, opinion surveys do not need to be approved by the ethics commission. 3.2. METHODS 65

3.2.2 Procedure and Participants

In fall 2012, a postal questionnaire was sent out to 1331 businesses associated with the Swiss Association of Horticulture (JardinSuisse), and operating in German- speaking parts of Switzerland. Mailing addresses had been provided by JardinSu- isse. For each company, the person mainly responsible for the plant assortment was asked to complete the questionnaire. The survey was accompanied by a letter and a prepaid return envelope. Participants were informed that the study aimed to investigate how non-native plants were perceived in Switzerland and what sig- niﬁcance they had for horticulture. One month after the questionnaire had been mailed, a reminder was sent out to non-responders enclosing another copy of the questionnaire. Data collection lasted in total for two months.

3.2.3 Questionnaire

In the first part of the questionnaire, our study participants were asked to make four judgments about 18 ornamental plant species (see paragraph below and Table 3.4). We asked the participants to i. rate the importance of each species for landscape design in Switzerland, ii. classify the origin of each species as native or non-native, iii. express the perceived environmental threat of each plant species (threat / no threat), and iv. rate on a six-point scale the importance of each species for their own business (see also the copy of the questionnaire in Appendix B.1). The plant species appeared in a random order, however, all participants received the same questionnaire. A second set of items was used to explore risk and benefit perceptions from ornamental plant species. Further, we examined participants’ attitudes towards risk mitigation behavior aimed at reducing the invasion risk of non-native ornamental plant species (e.g., increasing prices for species known to be invasive in Switzer- land). Participants’ general perception of risks emerging from non-native invasive plant species in Switzerland was measured on a 6-point scale, (Table 3.1, item 1). Perceived economic and cultural benefits of non-native plant species for the horticultural business in Switzerland were measured using eight items, e.g., “For my business, non-native plants are economically important”, (Table 3.1, items 2–9). Participants’ attitudes towards risk mitigation behavior were examined using four items related to regulations of import and trade of non-native plants, (Table 3.2, items 10–13), and five items measuring the willingness to voluntarily promote the sale of native plant species (Table 3.2, items 14–18). In the last part of the questionnaire, participants were asked socio-demographic questions about their age, gender, educational level, business sector, main source of income, average number of full-time employees, and their function in the company (see also Table 3.3 and the questionnaire in Appendix B.1). Anonymized data from this survey will be provided upon request by the first author. 66 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC.

Table 3.1: Perceptions of risk and beneﬁt.

Scales and Items M, SD General Risk Perception 1 What is your general estimate of the size of the problem 3.99, 1.34 cause by invasive non-native plants in Switzerland? Beneﬁts Cronbach’s – (scale mean): 0.84, 3.74 2 For my business, non-native plants are economically 3.70, 1.74 important. 3 It is easier to sell exotic plants than native species. 3.51, 1.53 4 The choice of plants has to be complemented by non- 4.77, 1.43 native plants because native ones do not possess all of the properties desired by the customers. 5 New species and cultivars may improve biodiversity. 3.40, 1.60 6 If I don’t sell a non-native plant because of 3.66, 1.75 the risk it poses to the environment, my customers may buy it in another store. 7 To me, it is important to frequently oer my customers new 3.79, 1.60 species and cultivars. 8 The cultural value of a horticultural landscape is essentially 2.97, 1.54 dependent on new species and cultivars. 9 Non-native plants belong to our gardening culture. 4.12, 1.59

Note: Numbers in Tables 3.1 and 3.2 vary from 608 to 622 because of missing data. Rating scales for item 1 went from 1=‘very small problem’ to 6=‘very big problem’, for items 2–13 from 1=‘not agree at all’ to 6=‘totally agree’, and for items 14–18 from 1=‘I cannot imagine at all’ to ’I can imagine very well’. Some of the survey questions were adapted from Peters et al. (2006) and Coats et al. (2011). 3.2. METHODS 67

Table 3.2: Attitudes towards the willingness to engage in risk mitigation behavior and factor loadings from principal component analysis.

Scales and Items M, SD (Factor loadings) Willingness to Engage in Risk Mitigation Behavior Cronbach’s – (scale mean): 0.84, 3.74 10 As long as one cannot rule out the possibility that a non- 3.36, 1.83 native plant will become invasive in Switzerland, a ban (0.72) on the import of this plant should be imposed. 11* In Switzerland, trade of non-native plants does not 2.96, 1.85 require legal regulations. (0.64) 12* There should be no restrictions on importation of non-native 3.70, 1.82 plants into Switzerland, as long as there is no evidence (0.65) that these plants potentially become invasive in Switzerland. 13 No plants should be allowed for import into Switzerland that 4.50, 1.68 have been shown to have been invasive in another country. (0.57) 14 Increase prices for plants that are included in the Black list 3.00, 1.93 (i.e., plants that had been listed by the Swiss Commission (0.48) of Wild Plant Conservation to be invasive non-native plants in Switzerland, and that cause negative impacts in the context of biodiversity, health, and/or economy). 15 Remove plants from my stock that are listed 4.64, 1.56 on the Black-list. (0.68) 16 Ban the sale of any non-native plant, until it has been 3.90, 1.73 shown that it does not pose a danger to humans or the (0.76) environment. 17 Inform customers about invasive non-native 5.31, 1.03 plant species. (0.62) 18 Promote the sale of native plants. 4.73, 1.40 (0.52)

Note: Numbers in Tables 3.1 and 3.2 vary from 608 to 622 because of missing data. Rating scales for item 1 went from 1=‘very small problem’ to 6=‘very big problem’, for items 2–13 from 1=‘not agree at all’ to 6=‘totally agree’, and for items 14–18 from 1=‘I cannot imagine at all’ to ’I can imagine very well’. *Item was recoded prior to principal component analysis. Some of the survey questions were adapted from Peters et al. (2006) and Coats et al. (2011). 68 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC.

Table 3.3: Socio-demographic characteristics of the analysis sample (N = 625).

Variable N % Gender Male 576 93 Female 41 7 Educational Level Primary / lower secondary school 22 4 Upper secondary vocational school / 297 49 Upper secondary university preparation school College / university 174 28 Special training (e.g., gardener in chief) 119 19 Business sector Private consumer business 395 66 Wholesale market 30 5 Mixed clientele 172 29 Main Source of Income Horticulture, landscape architecture & gardening 219 36 Horticulture & landscape architecture 133 22 Gardening 86 14 Potted plants and cut ﬂowers 77 13 Tree nurseries 43 7 Various combinations of horticultural sectors 47 8 Average number of full-time employees per year 1–5 280 45 6–15 194 31 16–30 79 13 > 30 65 11 Function in the company General manager / brand manager 538 89 Head of department 39 6 Other position, e.g., in administration 31 5 Age (range 23–96 years) M SD In years 47 11 3.2. METHODS 69

3.2.4 Plants selected for this study The list of plant species used in this study was compiled from a catalogue of ornamental plant species available in Switzerland provided by the Swiss Association of Horticulture (JardinSuisse, 2012). We used this catalogue because we assumed that most participants would be familiar with these species. Species were chosen based on their status of origin in Switzerland (Landolt et al., 2010), as well as based on plant risk assessments supervised by Swiss authorities (Black-list and Watch- list) (Info Flora, 2014). Because we were particularly interested in perceptions related to non-native plant species—notably those with known or presumed negative impacts on biodiversity, health, and/or the economy—our selection consisted primarily of species from the Black-list (invasive, 4 species) and the Watch-list (potentially invasive, 8 species). Three non-native species were chosen that at the time of the study were not considered to pose any threat to the environment. Hence, our choice of species reﬂected a range of environmental risks from very invasive to non-invasive. Also, the 3 native species used in this study were not regarded to be environmentally risky by Swiss authorities and served as a control, see Table 3.4. All of the species chosen were part of the basic training for prospective gardeners in Switzerland supported by JardinSuisse (JardinSuisse, 2012). Further, these species were particularly promoted by the industry as ornamentals, or in the case of the spiny shrub Prunus spinosa as a natural protection of economically important plants against game browsing. Further, the native species Ilex aquifolium had been proposed by JardinSuisse as an alternative to Buxus species that in Switzerland are threatened by the non-native invasive box tree moth (Cydalima perspectalis). In Switzerland, over 600 non-native plant species have been recorded (c. 20% of the total ﬂora in Switzerland, Conedera and Schoenenberger, 2014). Thereof, around 10% are considered to have negative eects, see also (Info Flora, 2014) or (Annex 2; Swiss Federal Council, 2008). Thus, non-native and in particular, non-native invasive species used in this study are overrepresented compared to the actual situation in Switzerland.

3.2.5 Statistical Analysis Fisher’s exact tests were performed using the statistics software R, version 2.15.2 (http://www.r-project.org), all other statistical analyses were performed using SPSS Version 20 (SPSS Inc., Chicago, IL, USA). We applied a multiple linear regression analysis using the forced entry method to evaluate to what extent perceived risk and perceived benefits, as well as socio- demographic variables influenced horticulturists’ willingness to engage in risk mitigation behavior. Socio-demographic variables were recoded into dummy variables: Gender classified as male =0, female =1; Age, categorized into three age classes of 21–40, 41–60 (reference category), and over 60 years at survey; Position in Busi- ness, classified as general manager or branch manager, or other (including heads 70 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC. of department, case handlers, trainees, or administrative stu); Main Source of In- come, with six categories: 1=horticulture, landscape architecture, and gardening (reference category), 2=horticulture and landscape architecture, 3=gardening, 4=potted plants and cut flowers, 5=tree nurseries, and 6=others (including dierent combinations of Categories 1–5 or supply services); Business Sector, categorized as wholesale market, private consumer business (reference category), or mixed clientele; Number of Full Time Employees, classified according to the Swiss horticultural market structure into the categories 1–5 (reference category), 6–15, 16–30, and over 30 employees.

3.3 Results

3.3.1 Respondents The questionnaire was sent to 1331 members of the Swiss Association of Horticul- ture (JardinSuisse) in the German-speaking parts of Switzerland. The response rate was 47% (N = 625). Most respondents were male. This might reﬂect a general gender bias in the industry. The demographic characteristics of the study participants are described in Table 3.3.

3.3.2 Associations between Perceived Origin and Perceived Environmental Threat Participants were presented with a list of 18 ornamental plant species and were asked to classify these plants as native or non-native. Irrespective of their true origin, all plants were classified as non-native by some and as native by other participants. The fraction among participants not following the classification of origin given in the literature accounted for between 6% and 44% (M = 20%) in the case of non-native species, and between 3% and 12% (M =6%) in the case of native species (Figure 3.1). In order to analyze the relationship between perceived origin and perceived environmental threat, for each plant we divided the participants into two groups, depending on whether they classified a given plant as native or as non-native. For both groups, we then calculated the percentage of participants that perceived the plant as an environmental threat (Table 3.5). We found for all plants a strong association between perceived origin and perceived environmental threat: independent of the true origin of the species, the fraction of participants who perceived a plant to be risky was larger among those that classified the plant as non-native than those that classified it as native (p =0.05 0.001, Figure 3.2). ≠ 3.3.3 Perceived Importance and Perceived Environmental Threat The more important the plant was perceived to be for landscape design or for a participant’s business, the less risky the plant was evaluated (Table 3.5). These 3.3. RESULTS 71

Non-native Native

Lonicera japonica, N = 609

Prunus laurocerasus, N = 607

Buddleja davidii, N = 607

Robinia pseudoaccacia, N = 606 Non-native, risky

Paulownia tomentosa, N = 608

Trachycarpus fortunei, N = 609

Viburnum rhytidophyllum, N = 608

Cornus sericea, N = 602

Mahonia aquifolium, N = 609

Lonicera henryi, N = 602

Sedum spurium, N = 609 Non-native, potentially risky Lupinus polyphyllus, N = 602

Fallopia baldschuanica, N = 604

Wisteria sp., N = 606

Non-native Syringa sp., N = 603

Ilex aquifolium, N = 611

Euonymus europaeus, N = 609 Native Prunus spinosa, N = 612

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Figure 3.1: Perceived origin of plants used in this study. correlations were rather small for the three native plants and two of the three non- native plants not considered to pose an environmental threat according to Swiss authorities.

3.3.4 Risk and Beneﬁt Perceptions

In average, our study participants rated the overall risk from non-native invasive plant species in Switzerland as medium (Table 3.1, item 1). We asked our study participants a set of questions referring to potential economic and cultural beneﬁts generated by non-native plant species (Table 3.1, items 2–9). The internal reliability of the beneﬁt scale was good (Cronbach’s – = .84, M =3.74, SD = .53). The large majority felt it was necessary to complement the product range of the horticultural industry with non-native plant species because native species were missing characteristics requested by customers. Although horticulturists tended to accept non-native plant species as a part of horticulture, only a minority felt that new 72 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC. species or varieties were essential for the cultural value of a garden. Participants diverged in their responses to three questions: whether non-native plant species were of economic importance for their businesses, whether non-native plants were easier to sell than native ones, and whether new species and varieties were a valuable addition to local biodiversity. In addition, horticulturists did not feel a particular need to regularly present their customers with new species or cultivars, nor did they seem to fear business competition if they would cease to sell a non-native plant species judged harmful to the environment.

3.3.5 Willingness to Engage in Risk Mitigation Behavior To identify meaningful dimensions describing participants’ willingness to engage in risk mitigation behavior, a principal component analysis (PCA) was performed on the items measuring attitudes towards import and trade regulations (Table 3.2, items 10–13), as well as on the items describing voluntary actions (Table 3.2, items 14–18). The ﬁrst factor accounted for 40% of the total variance in the measures. The second factor however had an eigenvalue only marginally greater than one and it explained only 12% of the variance in the data. The scree-plot further suggested that a one-component solution should be favored. We therefore decided for a one- factor solution and the items described above were combined to a scale labeled Willingness to Engage in Risk Mitigation Behavior. The internal consistency of this scale was determined using Cronbach’s alpha (– = .80, M =4.08, response scales ranged from 1–6). Most study participants supported an embargo on import of plant species that were invasive in another country, while only a minority was in favor of a regulatory- free trade of non-native plant species in Switzerland. However participants were rather reluctant to support preventive measures as long as the invasiveness of a plant was not demonstrated conclusively and the means of the ratings were slightly below and above the midpoint of the 6-point scale, respectively. We found particularly positive attitudes towards customer information, towards the promotion of native plant species, and towards the removal of problematic invasive plant species from the assortment. The options to renounce sale of any non-native plant species as a precautionary measure, or to increase prices for problematic non-native ornamentals were least welcomed.

3.3.6 Multiple Linear Regression We used linear regression analysis to evaluate to what extent perceived risk and perceived beneﬁts, as well as socio-demographic variables—Gender, Age, Position in Business, Main Source of Income, Business Sector, and Number of Full Time Employees—inﬂuence Willingness to Engage in Risk Mitigation Behavior. Socio- demographic variables were recoded as dummy variables (Table 3.6). In a multiple linear regression were all predictors where entered at once, our model explains 46% percent of the variance (Table 3.6). The variable describing 3.3. RESULTS 73

Table 3.4: Description of the ornamental plant species used in this study. Species are ordered according to their assessed impact. In the questionnaire, these plant species appeared in a random order.

Plant Common Assessed English Name Impact Status of Origin: Non-Native 1 Buddleja davidii Butterﬂy bush Cause damage 2 Lonicera japonica Japanese honeysuckle Cause damage 3 Prunus laurocerasus Cherry laurel Cause damage 4 Robinia pseudoacacia Black locust Cause damage 5 Cornus sericea Red-osier dogwood Potential damage 6 Lupinus polyphyllus Russell lupin Potential damage 7 Viburnum rhytidophyllum Leatherleaf viburnum Potential damage 8 Mahonia aquifolium Oregon grape Potential damage 9 Paulownia tomentosa Princess tree Potential damage 10 Sedum spurium Creeping sedum Potential damage 11 Trachycarpus fortunei Chinese windmill palm Potential damage 12 Lonicera henryi Henry’s honeysuckle Potential damage 13 Fallopia baldschuanica* Russian vine None 14 Syringa sp. Lilac None 15 Wisteria sp. Wisteria None Status of Origin: Native 16 Ilex aquifolium English holly None 17 Prunus spinosa Blackthorn None 18 Euonymus europaeus European spindle tree None

Note: Status of origin for all species is taken from Flora indicativa (Landolt et al., 2010). Assessed impact is based on whether the species is listed on the Black-List, or on the Watch-List (Info Flora, 2014). *At the time of our study, the use of F. baldschuanica in horticulture was allowed by Swiss authorities, although the species belongs to the Fallopia-complex, that was included in the list of prohibited plant species (Swiss Federal Council, 2008), but not in the Black- or the Watch-List. 74 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC.

Table 3.5: Descriptives of variable perceived environmental threat if not handled correctly and point-biserial correlations between perceived importance and perceived environmental threat. unmseuropaeus Euonymus spinosa Prunus aquifolium Ilex sp. Wisteria sp. Syringa baldschuanica Fallopia fortunei Trachycarpus sericea Cornus henryi Lonicera aquifolium Mahonia tomentosa Paulownia polyphyllus Lupinus rhytidophyllum Virburnum spurium Correlations: Sedum japonica Lonicera pseudoaccacia Robinia laurocerasus Correlations: Prunus davidii Participants of Buddleja % name Plant 6 22 15 11 13 68 42 58 52 33 34 45 46 37 vs. 54 vs. Switzerland 69 in 70 Importance 85 Business Own Design Landscape Importance Threat Environmental Perceived who ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ Threat ≠ ≠ ≠ ≠ ≠ Environmental Threat Environmental 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ...... 11 19 23 14 16 35 22 24 26 27 28 29 32 34 27 28 29 35 ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ...... 16 22 23 17 17 36 18 15 27 29 21 26 30 25 28 28 24 26 ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** ** **

Note: **p<.01; Rating scales for perceived importance went from 1=‘absolutely unimportant’ to 6=‘very important’; perceived environmental threat: 0=‘no threat’ and 1=‘threat’; N varies between 569 and 610, reﬂecting missing values. 3.3. RESULTS 75

% of respondents who perceived environmental threat given that they perceived a plant to be native % of respondents who perceived environmental threat given that they perceived a plant to be non-native

N = 109 Buddleja davidii*** N = 498

N = 196 Robinia pseudoaccacia*** N = 410

N = 101 Prunus laurocerasus*** N = 506

Non-native, risky N = 39 Lonicera japonica** N = 570

N = 117 Cornus sericea*** N = 485

N = 136 Lonicera henryi*** N = 466

N = 196 Lupinus polyphyllus*** N = 406

N = 88 Viburnum rhytidophyllum*** N = 520

N = 61 Trachycarpus fortunei N = 548

N = 145 Sedum spurium*** N = 464

Non-native, potentially risky N = 121 Mahonia aquifolium*** N = 488

N = 40 Paulownia tomentosa* N = 568

N = 82 Fallopia baldschuanica*** N = 522

N = 268 Syringa sp.** N = 335

Non-native N = 103 Wisteria sp.*** N = 503

N = 541 Ilex aquifolium*** N = 70

N = 593 Prunus spinosa N = 19 Native N = 582 Euonymus europaeus N = 27

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Figure 3.2: The percentages of participants who perceived a plant as an environmental threat depending on the perceived origin of the plant. Note:*p<.05, **p<.01, ***p<.001, Fisher’s exact test, one-tailed. N corresponds to the number of study participants who classiﬁed a plant as either non-native or native. 76 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC.

Table 3.6: Results of multiple regression analysis with willingness to engage in risk mitigating behavior as dependent variable.

Independent Variable B SE — Socio-Demographic Variables Gender 0.08 0.14 0.02 Age 21–40 years 0.01 0.08 0.00 ≠ 41–60 years† ––– over 60 years 0.03 0.11 0.01 Position in Business 0.04 0.11 0.01 Main Source of Income Horticulture, landscape architecture, & gardening† ––– Horticulture and landscape architecture 0.24 0.09 0.09** Gardening 0.19 0.11 0.06 Potted plants and cut ﬂowers 0.12 0.11 0.04 ≠ ≠ Tree nursery 0.31 0.15 0.08* ≠ ≠ Others (comb. of several sources of income) 0.09 0.13 0.02 ≠ ≠ Business Sector Wholesale market 0.20 0.16 0.04 ≠ ≠ Private counsumer business† ––– Mixed clientele 0.04 0.08 0.02 Number of Full Time Employees 1–5 employees† ––– 6–15 employees 0.02 0.08 0.01 16–30 employees 0.07 0.11 0.02 ≠ ≠ Over 30 employees 0.19 0.12 0.06 ≠ ≠ Psychological Variables Beneﬁt 0.34 0.04 0.35*** ≠ ≠ Risk Perception 0.30 0.03 0.39***

Note: †Reference category. *p<0.05, **p<0.01, ***p<0.001, N = 541, R2 =0.48, adjusted R2 =0.46 Dummy variable gender was coded 0=male, 1=female. Position in Business was coded as 0=other (e.g., head of department / administration), 1=general manager or branch manager. 3.4. DISCUSSION 77 general risk perception (— =0.39, p<.001) and the benefit scale (— = .35, ≠ p<.001) significantly influenced horticulturists’ willingness to engage. Our study participants displayed high willingness to engage when risk perception related to non-native invasive plant species was high and when the perceived cultural or economic benefit emerging from non-native plants was perceived to be low. We also observed a weak dependence of the willingness to engage in risk mitigation behavior on the predictor variable Main Source of Income. However, only in two cases this weak dependency was statistically significant. All other socio-demographic variables did not significantly contribute to the participants’ willingness to engage. In order to test how much variance was explained by the psychological variables Benefit and Risk Perception in addition to the socio-demographic variables, a hierarchical regression was conducted (Table B.1 in Appendix B.2). In the first step, socio-demographic variables were included (R2 =0.16). In the second step, we included psychological variables describing risk perceptions and perceptions of benefit. The additional inclusion of psychological variables caused R2 to increase significantly by 32% (adjusted R2 =0.46). This analysis suggests that the psychological variables are more important than socio-demographic ones.

3.4 Discussion

3.4.1 General Discussion

Our data suggest that horticulturists’ perceptions of environmental risks emerging from ornamental plant species were significantly influenced by the perceived origin (native vs. non-native) of the study species, and the perceived importance for landscape design in Switzerland and for participants’ businesses. Horticulturists did not unanimously follow the classifications of a plant as native or non-native as given in the literature and used by Swiss authorities. Further many of our study participants accepted regulations of trade and declared a willingness to engage in various voluntary actions to mitigate invasion risks from non-native ornamentals. Thus, our data suggest that horticulturists seemed to be aware of the invasion issue and of the necessity to take preventive measures. Yet, participants perceived only medium environmental risks emerging from non-native invasive plant species in general. Voluntary actions to mitigate invasion risks from non-native ornamentals were significantly influenced by risk and benefit perceptions. Our data were derived from a large-scale survey among members of the Swiss Association of Hor- ticulture that represents most horticulturists in Switzerland. To the best of our knowledge, no previous large-scale quantitative study has reported factors underlying horticulturists’ perceptions of risk and attitudes towards the willingness to engage in risk mitigation behavior. 78 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC.

3.4.2 Non-Nativeness as Risk Factor

The perceived origin of an ornamental plant species significantly influenced horticulturists’ perceptions of environmental risks: Participants who perceived a plant species to be non-native were significantly more likely to perceive this species to be risky, compared to those participants who perceived the same species to be native. Participants might have perceived the non-nativeness of a species as a negative stereotype that characterizes non-native plant species in general as environmentally risky. Due to the correlational nature of our analysis, we can however not exclude the possibility that the perception of origin was influenced by the perceived riskiness of a plant species instead of the opposite. Our finding that the perceived non-nativeness of a plant species might serve as a subjective risk metric among horticulturists sheds new light on the perceptions of risks among stakeholders in the management of plant species. Results from previous studies indicate that to non-academic stakeholders the origin of a species does not seem to be relevant for their risk perception; for them documented negative eects on the new environment matter (Selge and Fischer, 2010; Selge et al., 2011). These results come from studies that elicited risk perceptions in focus groups and by using direct measures. This means that participants explicitly elaborated on the importance of the origin of a species for their perceptions of environmental risks. Using open group discussions is a particularly useful instrument to explore attitudes and beliefs towards an object in-depth instead of quantitatively analyzing attitude scores. However in some cases, participants in non-anonymous group discussions might be prompted to reason in a socially desirable way (Joinson, 1999). In the studies cited above, participants might have felt compelled to deny that their perceptions of risks were influenced by the non-nativeness of the species in question. In our study however, we indirectly measured associations of perceived origin and perceived environmental threat. In our questionnaire, we first asked the participants to classify a species as either native or non-native. Subsequently the participants were asked to evaluate the species’ environmental threat. The ratings for these two separate questions were then used to calculate the impact of the perceived origin on perceived environmental threat. The notion of a species’ origin as subjective risk metric is particularly notewor- thy given our finding that our participants did not unanimously follow the classification of origin used by Swiss authorities. Note that the latter finding might also (partially) reflect a bias caused by the unequal size of the groups of non-native (15 species), and native plant species (3 species). For example, if participants assumed that there was an equal probability for a species to be non-native or to be native, a tendency towards assessing non-native species as native might be expected in the case of such an unbalanced sample. Given the importance of perceived origin for risk perception, diverging perceptions of which plant species is native and which is non-native might result in diverging risk perceptions and ultimately in diverging views on the necessity of 3.4. DISCUSSION 79 management measures, such as, e.g., a precautionary ban on import for novel ornamental plant species.

3.4.3 Familiarity or Beneﬁt Mitigating Perceptions of Risk

Horticulturists’ perceptions of environmental risk from a plant species were also significantly associated with perceptions of the importance of the species for landscape design in Switzerland and their businesses. This might be because either benefit from or familiarity with an ornamental plant species mitigated risk perceptions; both eects are well documented through risk perception studies performed in a wide range of contexts. On the one hand, there might be an inverse relationship between risk and benefit perceptions that was also shown in the contexts of technological risks, e.g., aviation, electric power, X-ray technology (Fischho et al., 1978; Slovic et al., 1991). Considering that horticulturists have more experiences with plant species that are important for their businesses or the industry, perceived importance might alternatively be interpreted as a proxy for horticulturists’ familiarity with a species. This interpretation is supported by our findings that for most species there were positive associations of perceived native origin and perceived importance (p<0.01, r [0.15, 0.34]). Indeed, often it is not the | | œ native origin, but the novelty and the “exotic” origin of horticultural plants that are used as important selling arguments (Dehnen-Schmutz et al., 2010). Familiar- ity might also explain why our participants tended to perceive non-native plants as native (Figure 3.1). The apparent risk mitigation eect of familiarity we have observed is in line with previous research on the influence of perceived familiarity on risk perception from activities or technologies where it was found that perceived familiarity with a hazard significantly increased risk tolerance (Fischho et al., 1978). Such an eect of familiarity might be explained by an induced positive feeling through repeated exposure to the same stimulus (Mere Exposure Eect; Zajonc, 1968). Particularly when time and/or information are limited, aective evaluation influenced by positive feelings may then serve as a heuristic in judgments of risks or benefits and replace rational analysis of the available information. In summary, our data suggest that work experience with an ornamental plant species aects horticulturists’ risk awareness either through increased perceived benefit or a familiarity eect.

3.4.4 Willingness to Engage in Risk Mitigation Behavior

Our results suggest that risk and benefit perceptions largely explain the variance in the reported willingness to engage in risk mitigation behavior. Both variables had a comparable degree of importance in the model, yet eects were opposite in direction. Horticulturists’ willingness to engage was reduced if they perceived benefits from non-native plant species, i.e., economic or cultural values, or specific plant characteristics thought to be important to customers. General perceptions 80 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC. of risk from non-native invasive plant species seemed to increase horticulturists’ reported willingness to engage. We found particularly high approval of some voluntary actions; namely customer information, promotion of native plant species, and the removal of known harmful non-native plant species from the stock. Participants also agreed on regulations of trade of non-native plant species in general, albeit it seemed that voluntary actions were preferred over government-controlled regulations. Particularly unpopular was the option to voluntarily increase sales prices for non-native ornamentals known to be invasive. Equally in the studies of Gagliardi and Brand (2007) and Barbier et al. (2013), market-based management measures such as taxes or fees were the most undesired ones. Besides a general dislike for taxes or fees, Barbier et al. (2013) also discuss horticulturists’ concerns over scientific uncertainties that might make it dicult to design and implement adequate market-based measures. In general, our results are in line with the findings of other authors, who reported horticulturists’ expressed willingness to participate in risk mitigation processes, e.g., in a voluntary code of conduct (Peters et al., 2006; Gagliardi and Brand, 2007; Coats et al., 2011; Vanderhoeven et al., 2011; Barbier et al., 2013). In a study among horticulturists in the U.S., Burt et al. (2007) found environmental concern, the cultivation of the reputation of an environmentally responsible business, and customer demand among the most influential incentives to engage in risk mitigation behavior. Interestingly, our data indicate that among horticulturists there is no clear support for precautionary measures targeted at non-native species in general. In ecology and environmental management, it is widely assumed that precautionary measures are likely more eective than a later cure (Mack et al., 2000; Leung et al., 2002). However, whereas a ban on import for non-native ornamentals with known invasive potential was well received by our study participants, the precautionary ban on import or sale of non-native species despite missing knowledge about its invasive potential did not receive much support. Gagliardi and Brand (2007) found ambiguous attitudes towards the ban on invasive ornamental plants among nursery and landscape industry members in the U.S. state Connecticut: Far more study participants opposed the ban on economically important than unimportant plant species. Thus, also among Swiss horticulturists, expected loss of economic benefits from the sale of non-native species might have fueled refusal of preventive measures.

3.4.5 Limitations and Further Research Some limitations of the presented results have to be discussed. We cannot exclude that estimations of risks and reported perceived importance of ornamental plant species might reflect a self-presentation bias. That is, some horticulturists might have had the tendency to present their businesses in the best light and thus to report lower perceptions of risk and of importance of a study plant than are actually experienced. Or, some horticulturists might have strategically under- 3.4. DISCUSSION 81 estimated plant risks in order to prevent further governmental regulations. Also, perceptions of risk from non-native invasive plant species in general were measured using only a single-item and therefore the reliability of this measure might be low. Usually, single-items also explain less variance. However, we found that general risk perceptions had a substantial influence on the regression model. Thus, using a single-item might have led to an underestimation of the influence of the general risk perceived. In order to increase eectiveness in risk communication and to facilitate con- certed risk management, future research may continue to reveal horticulturists’ attitudes and perceptions towards non-native plant species and to examine further factors determining horticulturists’ willingness to engage in risk mitigation behavior, or these factors framing subjective risk perceptions. Further, longitudinal studies among horticulturists would allow observing if their perceptions of environmental risks related to non-native invasive plant species change over time, what role experience with plant invasions may play in the formation of risk perceptions, and if risk communication and other management strategies are successful (Siegrist, 2013).

3.4.6 Implications for Management Our finding that the perceived non-nativeness of an ornamental plant species is associated with increased environmental risk perception seems to support the prevalent practice in risk communication that emphasizes the non-native origin of harmful species. However, according to our data such a communication strategy might bear the risk of being ineective. Among our study participants, the perceptions of which ornamental plant species are native (or non-native) were often not congruent with the classifications proposed by experts in the literature and adopted by Swiss authorities. Even among academic experts there exists a high diversity of alternative concepts of the non-native origin of a plant species in Europe (Hu- mair et al., 2014a). Such divergent perceptions of a key concept might hinder consensus building and therefore weaken risk communication. It might be possible to better explain to non-experts the scientific understanding of the non-native biogeographic origin of a species, but such clarification of the underlying science might not suce. When a socially deeply rooted metaphor such as ‘non-native’ is employed in science communication, its understanding can be influenced by other legitimate factors such as—in this case—broader concepts of nativeness or familiarity (Humair et al., 2014a). Indeed, if our interpretation is correct (see above) the familiarity of horticulturists with a plant might be the reason why perceptions of environmental risks from particular ornamental plant species diered between horticulturists and academic experts or authorities; especially in the case of ornamental plants of high importance to the horticultural industry. Horticulturists’ familiarity with a non-native species might lead to a cognitive conflict with the expert definition of the non-native origin of a species: a species that is characterized as ‘foreign’ by experts is ‘familiar’ to horticulturalists due to their daily 82 CHAPTER 3. HORTICULTURISTS’ RISK AND BENEFIT PERC. work. Thus, while the non-native origin of a species can be an important scientific concept for understanding why a species is harmful, it can be problematic in risk communication, especially in the case of those species that are long-established and well-known in a country and therefore familiar to the public or particular stakeholder groups. As an alternative it might in these cases be more eective to focus communication on well-documented environmental impacts of harmful species (e.g., Fischer and van der Wal, 2007). The communication choices that are most intuitive to involved scientists—for instance fear-based stereotypes in invasive species or in climate change communication—are not always the most eective ones (Kueer and Larson, 2014).

3.5 Acknowledgements

We are grateful to the Swiss Association of Horticulture (JardinSuisse) for providing us with contact details of its members. The authors would also like to thank Andres Altwegg, Heinz Kunz, and Erwin Meier (all JardinSuisse), Daniel Fischer (Oce of Waste, Water, Energy and Air, Canton of Zurich, Switzerland), and An- drea de Micheli (invasive species expert) for providing data about the horticultural industry and environmental policy issues in Switzerland, as well as for participating in pre-studies. We are also grateful to Edward Connor for proof-reading the English translations of the survey questions, as well as to the members of Swiss Association of Horticulture for their participation in our survey. We would like to thank the handling editor and several anonymous reviewers for very helpful comments on an earlier version of the manuscript.

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4 Horizon Scanning for Future Plant Invaders. The Case of E-Commerce

Franziska Humair, Luc Humair, Fabian Kuhn, Christoph Kueer (2014) In Review

Abstract Biological invasions are a major concern in conservation; especially because global transport of species still rapidly increases. Conservationists hope to prevent future invasions by anticipating and targeting the introduction of potentially invasive species through species risk assessments and international trade regulations. Among many introduction pathways of non-native species, horticulture is increasingly recognized as a particularly important driver of plant invasions. In recent decades, the horticultural industry experienced global expansion and structural change through the emergence of new distribution channels including Internet trade (e-commerce). We surveyed global plant trade on eBay.com. A high number of recognized invasive plant species are frequently and worldwide traded on the Internet. Invasive species were highly overrepresented in plant e–commerce with–depending on plant family–up to 85% traded compared to a few percentages for non-invasive species. Families with more trade had a higher proportion of invasive species, and for woody species there was a signiﬁcant positive relationship between the number of regions from where a species was sold and the number of regions where it was invasive. Our results indicate that biosecurity is not yet eective enough in regulating online plant trade. Also, automated monitoring of e-commerce might in the future assist prevention; especially for addressing emerging trade connectivity that is not bound to national borders, and for horizon scanning for future risks: new source areas and plant species that are integrated into international trade. 88 CHAPTER 4. HORIZON SCANNING FOR FUTURE PLANT INVADERS

4.1 Introduction

Biological invasions are a major concern in conservation; especially because global transport of non-native species still rapidly increases (Hulme, 2009; McGeoch et al., 2010). Conservationists hope to prevent future invasions by anticipating and targeting the introduction of potentially invasive species through species risk assessments and international trade regulations. It appears that in geographically isolated countries such as Australia and New Zealand strict biosecurity measures indeed reduced the rate of non-native species introductions (Simberlo et al., 2013). But in most countries including those of the European Union introduction rates of new non-native species are still increasing (DAISIE – Delivering Alien Invasive Species Inventories for Europe, 2009; Simberlo et al., 2013). An early anticipation of future invaders is crucial for the success of preventive measures. For this purpose, risk assessment systems help to predict invasion risks of particular species based on knowledge gained from experiences with their invasive behavior in the past (Hulme, 2012; Kumschick and Richardson, 2013). However, in a time of rapid environmental and economic change, new species are integrated into global trade on a daily basis, and for such species no experiences regarding possible invasive behavior in non-native places exist (Kueer, 2010). Further, invasion opportunities change rapidly due to global change (e.g., climate change) possibly favoring dierent functional groups of non-native species in the near future (Walther et al., 2009; Bradley et al., 2010). In mountains, it is for instance expected that invasion risks will greatly increase in the near future with dierent non-native species posing a higher risks than those currently present at high elevations (Kueer et al., 2013a). Woody plant species are also considered to be of increasing importance as invasive species (Richardson and Rejmánek, 2011). Thus, insights gained from past invasions might not necessarily be appli- cable to prevent future invasions (ghost of invasion past; Kueer, 2010; Kueer et al., 2013b). This problem is ampliﬁed by time lags between introduction and invasion (invasion debt; Essl et al., 2011). In order to anticipate future invaders, or human activities that might increase invasion risks, it is therefore essential to monitor current introductions and scan them for emerging risk species (horizon scanning; Sutherland et al., 2014). Plant species that are of recent economic importance might turn out to be the invaders of tomorrow (e.g., biofuels; Richardson and Rejmánek, 2011), and more generally socio-economic changes, such as changes in consumer behavior, economic growth of a country or structural changes in relevant industries might lead to novel non-native ﬂoras and dierent invasion risks (McNeely, 2001; Dehnen-Schmutz et al., 2010; Sharma et al., 2010; Essl et al., 2011; Gallardo and Aldridge, 2013; Kueer, 2013). Among many introduction pathways of non-native species, horticulture is increasingly recognized as a particularly important driver of plant invasions (e.g., Reichard and White, 2001; Dehnen-Schmutz et al., 2010; Humair et al., 2014). In recent decades, the horticultural industry experienced global expansion and struc- 4.2. MATERIAL AND METHODS 89 tural change through growing demand for horticultural products, falling trade barriers, and improved production methods (Dehnen-Schmutz et al., 2010). Among others, there is a trend towards fewer but larger growers, and new distribution channels are emerging; in particular Internet trade (e-commerce) (Dehnen- Schmutz et al., 2010). E-commerce has considerable advantages for sellers and customers, but is a concern for biosecurity (Derraik and Phillips, 2010; Martin and Coetzee, 2011; Wilson et al., 2011; Kikillus et al., 2012). Internet sellers can directly approach a global clientele at low costs. Thereby, e-commerce not only further accelerates the global interchange of live plants or propagules, it also bears the risk of bypassing border controls, or other plant trade regulation (Maki and Galatowitsch, 2004; Giltrap et al., 2009). In this study we monitored global e-commerce of plants via eBay.com to address three main questions. How important is e-commerce as a pathway of recognized invasive species? Is there an empirical relationship between the intensity of e- commerce of particular plant species and the invasiveness of these species? And, what is the potential of automated monitoring of e-commerce as a biosecurity tool? Here we were particularly interested in whether it is possible to identify emerging potentially invasive species before they are widely distributed (horizon scanning).

4.2 Material and methods

4.2.1 Data sources

We automatically tracked Internet oers of plant species or their propagules in the category Flowers, Trees & Plants on eBay.com. We used these auctions as eBay is one of the world’s largest online marketplaces. The U.S. site at eBay.com was chosen because U.S. eBay sellers are frequently engaged in international trade (Lendle et al., 2013). Taxa used for our queries were extracted from two species lists:

(i) Global Flora (‘Global’): complete global species lists of 23 ﬂowering plant families that together represent c. 3/5 of the world spermatophyte ﬂora (assuming a total of 250’000 species, Kadereit and Bresinsky, 2013). We used data from the Kew World Checklist of Selected Plant Families (KEW Royal Botanic Gardens, 2013) for 21 families and completed it with data for Asteraceae (Global Compositae Checklist, Flann, 2009) and Leguminosae (ILDIS World Database of Legumes, Roskov et al., 2005) (Tab. C.1). We restricted the analysis to 23 families for reasons of data quality (we included only data from families for which review of the data was completed) and global comparability (all selected families have a global distribution). All families except two (Begoniaceae, Orchidaceae) included species listed on the Invasive Species List (below), and they mostly harbored a high number of invasive species (Daehler, 1998). 90 CHAPTER 4. HORIZON SCANNING FOR FUTURE PLANT INVADERS

(ii) Invasive Species List (‘Invasives’): a global list of invasive spermatophytes based on the data (species names including synonyms, invasive ranges, life forms, human uses) from Weber (2003) and Rejmánek and Richardson (2013). Because these datasets over-represent woody species, we performed some analyses sepa- rately for non-woody and woody species. In total, we searched for 153’394 dierent species, using Latin species names, and including synonyms for genus and species for invasive species. After initial tests, we did not use vernacular names to avoid false-positive search results.

4.2.2 Data collection and analysis We searched for species oers on eBay.com at 7 p.m. CET on 50 days between February and April 2014 (February 2–February 19, February 26–March 21, March 31–April 6, and April 8). Search hits were treated as valid if the species name was found in the header of an auction site. Although the oers were placed on an U.S. website, species were sold from numerous countries. We determined the location(s) of a species based on the information given in the item location ﬁeld of each oer. Item locations were assigned to 13 geographic regions following Richardson and Rejmánek (2011), except that the Russian Federation was treated as a single region (Tab. 4.1). To test the validity of our data we inspected two random oers per search day. Out of 100 random oers, only one was a false positive result caused by an erroneous product labeling. Statistical analyses were performed using SPSS Version 20 (SPSS Inc., Chicago, IL, USA).

4.3 Results

4.3.1 Number of species oered on eBay.com In total 2706 species were oered on eBay.com during the 50 days of our search (Global: N = 2121, 1.4% of Global species list; Invasives: N = 510, 39.7% of In- vasives species list). A major proportion of the 100 most frequently oered species were ornamentals (64%), sometimes in combination with other uses (e.g., medicine, stimulant) (Tab. C.2). Forty-one of the most oered species were invasives; most invasives were woody species (73%) and ornamentals (75%). Out of the 35 plant species on the “100 of the world’s worst invasive alien species” list (International Union for Conservation of Nature, 2014), 13 were oered on eBay.com during our search period (Tab. C.3). The average number of oers per day diered considerably between species from the Global Flora and the Invasive Species List (Global: M = 317.5, SD = 3456.1; Invasives: M = 508.4, SD = 664.0). Cumulative frequency curves (Fig. C.1) indicate that a longer search period would yield substantially more species oered on eBay.com. 4.3. RESULTS 91 0 0 ). Bold numbers in the 0 = 2424 N 0 0 88 (18) ered for sale in the region is indicated. 183 (25) -diagonal entries denote species shared between two regions; in 33 ered for sale in one particular region; and in brackets the number of 0 283 (40) 0 ered for sale in this region is given. O 139 (5) Africa AfricaNorth. South. America Amer. Central0 North Amer.0 Asia South00 Austral- Europe Middle2(1) Russ. 87(8) Carri- 19(8) asia00 0 Indian 18(13) 111(3)12(6)440(127)8(5) 411(122)468(150) Pacific 2(1) 1 50(2)3(1) 4(1)000 8(4) 1039(333)18(12) 8(6) 241(14)17(8) 2(1) 186(77) 494(147) East 400 0 Fed. 5(4)04(2)2(2)7(5)0 beanIs. 11290000 110 2(2)39(12)015(8)8(5)17(8)30 OceanIs. Islands 2216 00 0 3(1)02 46(18) 1(1)0140 19(7) 24(10) 46(18) =3 =412 =31 =1837 =24 =698 =920 =1711 =60 =13 =42 =4 =35 N N N N N N N N N N N N N Africa Northern Africa Southern America Central America North America South Asia Australasia Europe Middle East Russian Federation Caribbean Islands Indian Ocean Islands Pacific, Islands Average(Global list) Average(Invasives list) 0.85 31.31 0.31 4.54 2.08 2.15 170.31 4.15 51.69 85.77 2.85 97.08 27.46 176.92 31.08 11.23 51.69 1.38 6.54 4.23 1.00 0.77 2.69 6.38 0.15 0.00 Table 4.1: Numbers ofdiagonal species refer of to the the Globalinvasive species number and of Invasives exclusively lists species o thatbrackets: that are number were common of exclusively to shared o pairs invasives. of In regions the ( first column the number of species o 92 CHAPTER 4. HORIZON SCANNING FOR FUTURE PLANT INVADERS

All species Invasive species

N = 1798 All 15 families***, N = 110498 N = 206 N = 191 Asteraceae***, N = 29469 N = 17 N = 265 Fabaceae***, N = 19995 N = 70 N = 25 Rubiaceae***, N = 13515 N = 4 N = 70 Poaceae***, N = 11260 N = 16 N = 103 Euphorbiaceae***, N = 6493 N = 6 N = 75 Lamiaceae***, N = 6256 N = 9 N = 157 Myrtaceae***, N = 5850 N = 32 N = 53 Araceae**, N = 3278 N = 2 N = 253 Asparagaceae***, N = 2876 N = 8 N = 115 Arecaceae***, N = 2510 N = 17 Amaryllidaceae, N = 2199 n = 155 No invasive species offered N = 254 Iridaceae***, N = 2006 N = 12 N = 38 Apocynaceae***, N = 1786 N = 6 N = 31 Zingiberaceae***, N = 1565 N = 3 N = 13 Araliaceae***, N = 1440 N = 4 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Fraction of species offered

Figure 4.1: Percentage of species oered on eBay.com in the 15 families with most species in plant trade. The families are ordered based on total number of species. The proportion of species in trade was signiﬁcantly higher among invasives than all species (one-tailed Fisher’s exact test, **: p<0.01, ***: p<0.001).

4.3.2 Overrepresentation of invasive species in e-commerce Compared to the Global list, invasive species were signiﬁcantly overrepresented in plant auctions on eBay.com–both across all families and for particular families (Fig. 4.1). There was a weak positive correlation between the percentage of invasive species per family and the number of oers for all species per family (rs =0.41,p> 0.05, Spearman’s rank correlation coecient). Only a small fraction of species were oered from some families with many invasives (i.e., Poaceae: 0.6%, Asteraceae: 0.7%), compared to other families (e.g., Iridaceae, 12.7%).

4.3.3 Geographic distribution of plant oers Oers were sold from 65 countries or overseas territories, mostly from the U.S. (N = 1822), Australia (N = 919), United Kingdom (N = 863), Germany (N = 472), Italy (N = 430), and South Africa (N = 412) (Fig. 4.2); related to the following 4.3. RESULTS 93

Figure 4.2: Map of the 65 countries (or overseas territories) from where species were oered on eBay.com with percentage of invasive species among oered species indicated in color: grey indicates no invasives, yellow to red indicate an increasing percentage of invasive species. The total number of species located in a country is indicated if it was at least 200. geographic regions: North America (N = 1837) oered most species, followed by Europe (N = 1711), Australasia (N = 920), Asia (N = 698), and Africa southern (de facto South Africa) (N = 412). Particularly many invasive species were found in the U.S. (N = 413, 81% of all globally oered invasives), U.K. (N = 231, 45%), and Australia (N = 196, 38%), while we found remarkably few invasive species in South Africa (N = 20, 4%). Most species from the Global list were sold from one (32%, N = 680) or two regions (42%, N = 892), whereas most invasive species were found in two (43%, N = 221) or three regions (23%, N = 116). Only few species were sold from a higher number of regions, and most of them were invasives (Tabs. 4.1 and C.2). We found the highest similarity of traded species between Europe and North America (43% in common, Tab. 4.1). These two regions shared also many species with Australasia (c. 20%) and Asia (c. 18%). Africa southern shared particularly many species with Europe (59%), but at most 21% with other regions. The remaining pairs of regions had only 2% or less of their plants in common. Some species were sold from only one region. The fraction of such species was particularly high in Australasia (20%) and Africa southern (34%, Tab. 4.1). We found a signiﬁcant positive relationship between the number of regions where a particular species was sold from and the number of regions where it was invasive for woody species (r = 0.7, p < 0.01, Fig. 4.3) but not for non-woody species (Fig. C.2). 94 CHAPTER 4. HORIZON SCANNING FOR FUTURE PLANT INVADERS

Figure 4.3: Signiﬁcant relationship between the number of regions where an invasive woody species is oered on eBay.com (x-axis) and the number of regions where it is known to be invasive according to Richardson and Rejmánek (2011) and Weber (2003) (y-axis) (N = 387; r =0.7, p<0.01). The red line shows the average number of regions where species oered in a particular number of regions are invasive.

4.4 Discussion

4.4.1 Unabated e-commerce of invasive species We show that international horticultural e-commerce of recognized invasive species is apparently not yet eectively regulated. About 40% of the studied invasive species—including 13 listed on IUCN’s list of the world’s most invasive plant species (Tab. C.3)—were oered on eBay.com (Fig. 4.1); many of them daily (Fig. C.1) and from numerous countries and dierent world regions (Fig. 4.2). Most sellers oered to ship plants to most countries worldwide. The results of this study are based on a conservative search strategy. Therefore, most likely our data represents a substantial underestimation of true e-commerce trade of invasive plants. According to cumulative frequency curves, the numbers of species found in this study have not reached saturation and a longer search period would probably yield many more invasive species oered on eBay.com (Fig. C.1). Possibly, dierent species are oered in other seasons of the year, or some sellers might use vernacular names or synonyms not covered by our search. Further, although eBay.com is one of the largest online markets, it represents only a fraction of global horticultural trade and overrepresents sellers located in the U.S. More invasive species are likely 4.4. DISCUSSION 95 oered through other eBay domains, other online marketplaces (e.g., Amazon), or on the websites of individual horticultural companies. With the global spread of e-commerce, the complexity of international plant trade increases. Online trade oers new market opportunities to sellers and fa- cilitates shopping for consumers. Therefore e-commerce is expected to considerably contribute to the dispersal of invasive species (e.g., Walters et al., 2006; Papavlasopoulou et al., 2014) and is of major biosecurity concern (e.g., Australian Government, Department of Agriculture, 2014). Particularly, import channels or suppliers that are not in the jurisdiction of a regulatory body might circumvent national biosecurity regulations (Giltrap et al., 2009), and ever-new species are easier available and faster include in trade. Further, direct shipment from international sellers to private buyers via ordinary mail may hamper invasive species border control. As a consequence, increasingly non-experts might be involved in plant trade who tend to be ignorant of biosecurity regulations or do not correctly identify their products using the scientiﬁc name (Walters et al., 2006; Giltrap et al., 2009). Preventive measures depend on collaborations with professional sellers but such liaison might become more dicult because of the diversiﬁcation and globalization of e-commerce.

4.4.2 Invasive species are overrepresented in the international trade Invasive species were highly overrepresented in plant auctions on eBay.com. De- pending on the plant family, up to 85% of the invasive species were on sale, compared to typically only a few percent of all species (Fig. 4.1). This high fraction is even more remarkable when considering that plant trade—and particularly e-commerce—is only one introduction pathway of non-native species. Indeed, families with many weedy, i.e., ruderal or early successional species that are often introduced involuntarily to new regions—such as Asteraceae or Poaceae—had a lower proportion of oered invasive species, while families with a high proportion of horticultural species had also a particularly high fraction of traded invasive species (e.g., Myrtaceae or Verbenaceae). Invasive species also tend to be traded from more regions (2 or 3) than non-invasive species (1 or 2 regions). Indeed, we found a weak correlation between the fractions of invasive and traded species per family, and the more widely an invasive woody species was traded the wider was its invasive range (Fig. 4.3). One reason for the overrepresentation of invasive species in e-commerce might be that inclusion in international trade increases the likeli- hood of becoming invasive (Dehnen-Schmutz et al., 2007). Alternatively, it might also be that commonness in trade might increase the use of plants and thereby their invasiveness. It has been shown that increased familiarity with widespread plant species—such as invasive ones—increase people’s positive attitudes towards and decrease perceptions of associated environmental risks (Humair et al., 2014), possibly driving consumers toward buying those species. We did not find the same distribution pattern for invasive non-woody species, possibly because many non- 96 CHAPTER 4. HORIZON SCANNING FOR FUTURE PLANT INVADERS woody species have only recently been included in global trade and are still in the process of spreading, or because many herbaceous invasive species are weedy and do not depend on plant trade as a vector. In contrast to invasive species, only a small fraction of the global flora—c. 1% of the species—was oered on eBay.com (Fig. 4.1; and even fewer species were oered frequently and in dierent countries (Tab. C.2). We thus document a global plant- related socioculture that is highly homogenized and favors some species while it neglects many others. Such socioeconomic homogenization of floras is well known from agricultural crops and forestry where very few species and varieties make up a major fraction of global food production (Food and Agriculture Organization of the United Nations, 2010).

4.4.3 The need for more ecient biosecurity policies We demonstrate that despite major eorts, many recognized invasive species are still daily traded through the Internet. Many invasives were oered in political entities that consider themselves leaders of invasive species prevention: Australia or the U.S. (Plant Health Australia, 2014; U.S. Department of Agriculture, 2014). Given the resources that are spent on preventive measures at local, national and international levels, this might indicate a need to strengthen biosecurity policies. Surveillance of online trade can reveal real-time information on which invasive species are traded and from where, and such surveillance might be used as a new tool to survey trade networks that become more diverse and complex. The classical model of biosecurity assumes that non-native species are transported from a country in the native range to a country in the non-native range, and that this transport can eectively be intercepted at the national boundaries of the recipient country. Our data however documents frequent oers of invasive species from the non-native range of the species as well as the possibility of trade of invasive species within national or regional boundaries. Automated online surveillance— coordinated among national and regional (e.g., EU) regulatory bodies—can help to deal with emerging trade connectivity that is not bound to national borders.

4.4.4 Horizon scanning for emerging new risk species Monitoring Internet trade can help to identify emergent risk species early on when they are integrated into horticultural trade (Gibson et al., 2011). Indeed, recent changes in plant trade might not yet be manifested in invasion patterns (Kueer, 2010). For instance, the horticultural market in Africa and more generally in tropical countries is rapidly expanding since the 1990s only (Dehnen-Schmutz et al., 2010). In our study, Africa southern oered 412 species but to date, only few of these species are recognized as invasive ones. This might be particularly problematic because the region also seems to harbor a distinct traded ﬂora: A third of the species found in South Africa were exclusively sold from there, and some of them might become invasive in the future (Essl et al., 2011). Indeed, it has been 4.5. ACKNOWLEDGEMENTS 97 observed that increasingly species native to tropical countries are introduced in trade (Dehnen-Schmutz et al., 2010). Therefore, it must be expected that plant families and item locations such as African countries that are not yet widely integrated into international trade are currently contributing new potentially invasive species to international plant trade. Thus, data on emerging new sources of plant trade can help to implement targeted preventive measures and information campaigns; especially in regions such as in Africa where problem awareness and management capacity is low but horticultural trade is rapidly growing. A straightforward horizon scanning approach might be to focus on new species integrated into trade from plant genera and families that are known to have a high proportion of invasive species. We found for instance 28 Acacia species in trade that are not included on the invasive species list used in our study, (e.g., Acacia simplex, native to New Caledonia and the only species sold from there). However, it must also be kept in mind that the traits of future invaders might dier from those of known ones (Kueer, 2013). Analyzing social media data might give further information on changing preferences for particular plants or their traits, as well as on early signs of the novel invasive behavior of species.

4.5 Acknowledgements

We thank Ewald Weber for sharing electronic ﬁles containing data about invasive species (Weber, 2003), Jialin Zhang for compiling the Global species list, and Antonio Carzaniga for providing IT infrastructure.

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5 Working With the Horticultural Industry to Limit Invasion Risks: The Swiss Experience

Franziska Humair, Michael Siegrist, Christoph Kueer (2014) In Bulletin OEPP/EPPO Bulletin 44(2):232–238, doi: 10.1111/epp.12113

Abstract Building relationships with stakeholders involved in species introductions is increasingly considered as an eective means to mitigate invasion risks. The horticultural industry is of particular importance to the regulation of alien plants. This paper presents the initial experiences with a recently implemented environmental policy called ‘Duty to Inform’ which aims to limit the risk of plant invasions through alien horticultural plants in Switzerland. The Duty to Inform requires horticulturalists to inform their customers about risks of black-listed plants and watch list plants. In a joint eort with the Swiss horticultural industry and Swiss authorities the authors examined customers’ acceptance of plant labels as a means to supply information, and horticulturalists’ attitudes towards the Duty to Inform regulation, and their willingness to act pro-environmentally. Plant labels were found to be an eective means to inform customers about plant invasion risks and appropriate handling of risk species. The results indicate that among Swiss horticulturalists the awareness of the invasive species problem and the willingness to act is high. Horticulturalists had positive attitudes towards the Duty to Inform regulation and other actions to reduce invasion risks. The authors suggest that these positive attitudes might stem from an active participation of horticulturalists in the development of invasive species regulations in Switzerland. 104 CHAPTER 5. SWISS HORTICULTURE AND THE INVASION RISKS

5.1 Introduction

Global travel, transport, and trade are important drivers of species invasions. Human-mediated dispersal allows some species to bypass biogeographic barriers (Mack and Lonsdale, 2001; Ruiz and Carlton, 2003; McNeely, 2006; Hulme, 2009; Brunel et al., 2013). Long-distance dispersal by humans is therefore of central importance for the invasion process. Some of these new species eventually become invasive, i.e., they establish, spread rapidly, become abundant, and have a detrimental impact on biodiversity and ecosystem functions (invasive alien species; Vitousek et al., 1997; Vilà et al., 2010). Dispersal pathways have recently attracted substantial attention from invasive species scientists and managers (Kueer and Hirsch Hadorn, 2008; Ruiz and Carlton, 2003). This research showed that introduction pathways determine which species are introduced, at what frequency and magnitude (propagule pressure), and whether species proﬁt from cultivation eorts (e.g., breeding or acclimatization, Mack et al., 2000; Carlton and Ruiz, 2005; Kowarik and von der Lippe, 2007; Wilson et al., 2009; Pyöek et al., 2011; Kueer, 2013). To be eective, invasive species management requires a thorough understanding of these pathways of species introductions, and the options for management to reduce the risks of introducing potentially invasive species through dierent pathways (Hulme, 2009; Ruiz and Carlton, 2003). According to OEPP/EPPO (2009), of all the alien plant species that have become invasive in Europe, the vast majority (nearly 80%) was deliberately im- ported for horticulture, agriculture, or forestry. Similar results are reported by Reichard and White (2001) for North America, and Groves (1997) for Australia. A particularly important vector of the deliberate global dispersal of plant species is the horticultural industry (Reichard and White, 2001; Mack and Erneberg, 2002; Dehnen-Schmutz et al., 2007; Drew et al., 2010), and this industry is expected to increase substantially in the future. Horticulture is an industry of increasing importance for global commerce, particularly for emerging economies and partly due to reduced trade barriers and improved propagation techniques (for a review, see Dehnen-Schmutz et al., 2010). According to a report from the United Nations Conference on Trade and Development published in 2012, horticulture accounted for over 20 percent of world agricultural exports (United Nations Conference on Trade and Development, 2012). Horticultural marketing strategies as well as consumer behaviour may promote the establishment and invasion success of some alien ornamentals. Customers’ demand for particular plant traits such as ease of propagation, pest and disease resistance, or a high germination rate may lead to the preferential introduction of species with a particularly high invasiveness (Dehnen-Schmutz et al., 2007; Chrobock et al., 2011). It is well established that availability of plants in nurseries or planting frequency is positively correlated with invasive success of alien ornamentals (Dehnen-Schmutz et al., 2007; Bucharova and Van Kleunen, 2009; Pemberton and Liu, 2009). 5.1. INTRODUCTION 105

5.1.1 Importance of stakeholders in invasive species management Policy makers increasingly involve stakeholders in invasive species management. Such participatory processes are not only meant to enable informed decisions, they may also increase acceptance and support for management measures (e.g., Renn, 2006; Kueer and Hirsch Hadorn, 2008). However, the understanding of the invasive species problem, its valuation, and the definition of appropriate management goals can diverge substantially between dierent expert and stakeholder groups (Selge et al., 2011; Young and Larson, 2011; Kueer, 2013; Humair et al., 2014a). A survey among Swiss horticulturalists for instance indicated that their perceptions of the environmental risk of particular ornamental plant species only partly corresponded with expert risk assessments used by the Swiss authorities (Humair et al., 2014b). In particular, horticulturalists’ risk perceptions were significantly influenced by their classification of a plant’s origin (native vs. non-indigenous), which often diered from expert opinions, but also by their familiarity with an ornamental plant species through their daily work. In order to ensure the support of horticulturalists for invasive species policies, it is therefore important to acknowledge their perceptions of the issue and dierent management actions. Published studies suggest that horticulturalists are generally in favour of measures aiming to reduce the invasion risks stemming from the horticultural trade such as consumer education, labelling of potentially invasive plants, restricting the sale of known invasive plants, or the promotion of non-invasive or native species as an alternative to invasive ones (e.g., Peters et al., 2006; Burt et al., 2007; Gagliardi and Brand, 2007; Coats et al., 2011; Vanderhoeven et al., 2011; Humair et al., 2014b). However, Barbier et al. (2013) documented opposition against market-based instruments such as license fees or import taxes incurred on businesses selling alien plant species.

5.1.2 The invasive species issue in Switzerland The invasive species issue has only relatively recently attracted substantial attention by policy makers in Switzerland (Weber, 2000; Wittenberg et al., 2005). As a result, in the course of the revision of the Ordinance on the Handling of Organisms in the Environment (Release Ordinance), the Swiss Federal Council adopted new legislations regulating the handling of alien plant species (Swiss Federal Council, 2008). Over 600 alien plant species have been recorded in Switzerland (c. 20% of the total ﬂora in Switzerland, Conedera and Schoenenberger, 2014). According to the Release Ordinance, 8 alien species and the species of 3 genera are currently not allowed to be traded, e.g., Ambrosia artemisiifolia (Release Ordinance; Swiss Federal Council, 2008). Although trade of all other alien plants is allowed in Switzerland, the Duty to Inform regulation of the Release Ordinance requires horticulturalists to inform customers about environmental impacts and the need for careful handling (and disposal) of 12 plant species where damage is known 106 CHAPTER 5. SWISS HORTICULTURE AND THE INVASION RISKS

(Black-List plants), and 22 further species that are possibly harmful (Watch-List plants), (Info Flora, 2014). The formulation of the Duty to Inform regulation built on a participatory process involving authorities, scientiﬁc experts, as well as representatives of Swiss horticulture. Based on a choice of dierent communication means (e.g., videos, posters, or information provided via electronic devices) it was eventually decided to use labels that are directly attached to problematic alien plant species to inform customers in garden centres. The use of such plant labels as a way to implement the Duty to Inform regulation was tested in a pilot project (see below).

5.2 Methods

This paper reports on two surveys of consumers and practitioners in the Swiss horticulture sector, respectively. Both studies were performed in close collaboration with Swiss environmental authorities, i.e., representatives of the Federal Oce for the Environment and members of state environmental authorities, as well as representatives of Swiss horticultural industry, in particular those of the Swiss Association of Horticulture (JardinSuisse). The consumer survey focused on the eectiveness of plant labels as a means to implement the Duty to Inform. The survey among Swiss horticulturalists aimed to reveal factors inﬂuencing their risk and beneﬁt perceptions concerning alien ornamental plant species, as well as their willingness to participate in environmentally-friendly behaviour.

5.2.1 Implementation of the Duty to Inform (plant labelling) – a customer survey The objective of the ﬁrst study was to test the eectiveness of a plant label to inform walk-in customers about environmental eects and about the adequate handling of alien ornamental plant species with a potential to become invasive (e.g., seed removal or proper disposal), Figure 5.1. A written questionnaire was used that was distributed by the sta of two businesses associated with JardinSuisse (one garden centre and one tree nursery) to walk-in customers after their purchase of study plants. The study was performed between spring and autumn 2012. The plant label contained information on (i) unwanted environmental eects of the labelled plant species, (ii) how to minimize the risk of spread of the plant (restricting planting to urban areas; adequate tending such as cutting back plants, or seed / fruit removal), and (iii) proper disposal of cuttings (through green or normal waste). Four species from the black list (Buddleja davidii, Lonicera japonica, Prunus laurocerasus, Rubus armeniacus) and one Watch-list plant (Lonicera henryi) served as study species. The authors were in particular interested whether customers (i) noticed the label before plant purchase, (ii) understood the information on the label, and (iii) intended to follow the instructions about the responsible handling of the purchased plants. 5.3. RESULTS AND DISCUSSION 107

5.2.2 Survey among horticulturalists The second study explored the attitudes of horticulturalists towards (i) the Duty to Inform regulation, and (ii) their willingness to voluntarily engage in plant risk assessments. A written questionnaire was mailed to 1331 member businesses (garden centres, tree nurseries, gardening and landscaping companies, landscape design) of JardinSuisse in the German speaking part of Switzerland. The questionnaire was directed to those professionals in each company that were responsible for the choice of plant species on oer to customers. Further questions about horticulturalists’ understanding of the biogeographic origin (native vs. non-indigenous), economic importance and environmental threat of particular ornamental native and alien plant species, horticulturalists’ general risk and beneﬁt perceptions of alien plant species, and their willingness to act pro-environmentally are discussed in Humair et al. (2014b). The study was conducted in autumn 2012.

5.3 Results and discussion

5.3.1 Implementation of the Duty to Inform regulation (plant labelling) Current Swiss environmental law requires horticulturalists to educate their customers about problematic alien ornamental plant species (Duty to Inform). In the context of the implementation of this environmental policy, the authors examined customer responses to plant labels about environmental impacts of selected ornamental species, as well as their correct handling to prevent plant invasions, see Figure 5.1. Customers who had purchased one or several study species were asked by the sta of two horticultural businesses to participate in the study. Forty customers agreed to participate and 39 customers completed the survey (34 in the garden centre / 5 in the tree nursery). The chosen sampling strategy might have led to some bias because customers who noticed the label but did not buy the plant could not be considered in this study. Furthermore, among respondents there might have been an overrepresentation of those customers who are generally prepared to make informed purchasing decisions or have a particular interest in the issue. Eighty-two percent of all respondents indicated that they had noticed the label before plant purchase, and 89% stated that the handling instructions were clear. Among those participants who read the label before purchase, 79% declared that they are willing to follow the instructions, others indicated that they already knew about the correct handling beforehand, or respondents did not answer the question. Customers’ positive attitudes towards the label, as well as their self-reported willingness to adapt their behaviour in order to mitigate invasion risks indicate that plant labels might be an eective means to implement the Duty to Inform regulation. Previous studies suggested that horticulturalists also consider plant labelling an adequate means for customer education (Peters et al., 2006; Gagliardi 108 CHAPTER 5. SWISS HORTICULTURE AND THE INVASION RISKS

Figure 5.1: The label template that has been prepared by Swiss environmental authorities for Swiss horticulturalists to inform customers about environmental eects and the handling of Black- and Watch-List plants. The template is available in four languages (German, French, Italian, English). Swiss horticulturalists are free to change the layout of the template (colour, form), and to choose the languages. and Brand, 2007; Coats et al., 2011; Halford et al., 2011); and it has been shown that customers are less likely to buy plants labelled as invasive (Reichard and White, 2001; Yue et al., 2011).

5.3.2 Survey among horticulturalists The large-scale survey of Swiss horticulturalists explored their attitudes towards the Duty to Inform regulation, as well as their willingness to voluntarily engage in plant risk assessments. The response rate was 47% (N = 625), (for more information, see Humair et al., 2014b).

5.3.3 Horticulturalists’ attitudes towards the Duty to Inform The authors observed a positive attitude towards the Duty to Inform regulation (Figure 5.2). The vast majority of our participants considered this new environmental policy to be reasonable (83%) and practical (70%). These ﬁndings are in line with participants’ endorsement of other industry regulations, as discussed in Humair et al. (2014b). For example, 60% of the participants explained that no plants should be allowed for import into Switzerland that have shown an invasive behaviour in another country. Half of all participants explicitly stated that legal regulation of the trade of alien plants was necessary. 5.3. RESULTS AND DISCUSSION 109

Figure 5.2: Attitudes towards the Duty to Inform regulation were measured in Switzerland using a six-point Likert scale and concatenated to a three-point scale by merging the two levels of disagreement, neutral attitudes, and agreement, respectively. The sum of absolute numbers (indicated in the bars) varies between 607 and 612 due to missing data.

Participants’ acceptance of environmental policies targeting alien invasive species indicates that most Swiss horticulturalists are aware of the problem and concerned about it. A reason for the high awareness—despite the relatively recent recognition of the problem in Switzerland—might be that horticulturalists were involved as an important stakeholder group in the process of formulating the new legislation (e.g., the Duty to Inform as part of the Release Ordinance; Swiss Fed- eral Council, 2008)). Also studies from other countries documented awareness and concern among professionals in the horticulture sector (e.g., Peters et al., 2006; Coats et al., 2011; Barbier et al., 2013; Halford et al., 2013). However, Halford et al. (2013) for instance reported that not all horticulturalists that considered invasive species to be an important issue did personally feel concerned about the issue. The data gathered during this study also suggests that expected beneﬁts from the Duty to Inform regulation were an important incentive for engagement in invasive species risk management: sixty percent of the study participants anticipated an improvement of the public image of the horticultural industry through their participation in customer information. Promoting the business image as an incentive for engagement in invasive species risk mitigation measures was previously reported by other authors (e.g., Burt et al., 2007; Gagliardi and Brand, 2007; Halford et al., 2013). As a consequence of the Duty to Inform regulation, 61% of the study respon- 110 CHAPTER 5. SWISS HORTICULTURE AND THE INVASION RISKS dents expected changes in the range of plant species oered for sale. In particular, 48% believed that more and more species will be banned from sale. Yet, perhaps surprisingly, only 20% also expected losses of sales due to the new environmental policy. This result suggests that those plant species that at the time of the study were subjected to the Duty to Inform regulation (Black-List / Watch-List species) were perceived to have relatively little market importance. Further, given that all Swiss horticulturalists are equally aected by the new regulation, the regulation might be more acceptable because it does not lead to a competitive advantage of those businesses that would without regulation sell alien plant species regardless of their invasiveness. Indeed, Peters et al. (2006) report that in the Midwest region of the US, where at the time of their study no government regulations were in place, horticulturalists felt that potential competitive disadvantages reduced their willingness to stop selling invasive species; although they still expressed their willingness to engage in voluntary customer education about invasive plant species.

5.3.4 Willingness to engage in invasive species risk management

A majority of the study participants (65%) found it desirable to play a more influential role in not further specified risk assessments of alien plant species. Horticul- turalists’ willingness to voluntarily act pro-environmentally also became apparent through their positive attitudes towards other voluntary actions such as the removal of plants from their stock that are listed on the Black-List, or customer information about alien invasive plant species in general (Humair et al., 2014b). The results from this study are in line with other studies that found high levels of support among horticulturalists for various voluntary measures to reduce plant invasion risks. In particular, horticulturalists have been shown to feel responsible for educating customers, the public, or themselves about invasive plant species, e.g., Peters et al. (2006) or Coats et al. (2011) in the USA, or Vanderhoeven et al. (2011) in Belgium. To what extent these results from the USA, Belgium, and Switzerland are relevant to other countries and cultural settings would have to be determined in future studies. However, the authors do not know if the expressed willingness to engage in risk mitigation will indeed result in action. For example, horticulturalists’ willingness to engage in pro-environmental actions targeted against alien species might be counteracted by perceived economic and cultural benefits from an assortment that includes alien ornamentals (Humair et al., 2014b). Horticulturalists’ familiarity with an ornamental plant species might also reduce the perception of a risk (Hu- mair et al., 2014b). This is known as the Risk Perception Paradox in the literature (Wachinger et al., 2013): although risks are understood, some individuals do not change their behaviour because (i) perceived benefits exceed perceived risks, (ii) responsibility to act is denied, or (iii) lack of resources impedes action. 5.4. CONCLUSIONS 111

5.4 Conclusions

The data presented above suggests that Swiss horticulturalists are aware of environmental risks related to alien ornamental plant species and of the necessity of management measures to address the invasive species issue: study participants expressed positive attitudes towards mandatory as well as voluntary measures aimed at mitigating plant invasion risks. However, risk communication by authorities or science must take into account that horticulturalists’ risk perceptions are also inﬂuenced by other factors than ocial risk assessments such as Black-Lists: for instance perceived beneﬁts from these plants, alternative concepts of a plant’s origin (native vs. non-indigenous), or their familiarity with an ornamental plant species through their daily work (Humair et al., 2014b). To account for these additional dimensions, eective risk communication might rather emphasize unfamiliar characteristics of a new plant species than the non-indigenous biogeographic origin. If the species is already familiar to horticulturalists, risk communication might prove to be most eective if it focuses on known negative impacts without emphasizing that the species is alien, which might not resonate well with a perceived familiarity of the species.

5.5 Acknowledgements

The authors would like to thank the members of the Swiss Association of Hor- ticulture; particularly A. Altwegg, H. Kunz, and E. Meier, D. Fischer (Oce of Waste, Water, Energy and Air, Canton of Zurich, Switzerland), and A. de Micheli (invasive plant species expert) for providing data and for their assistance in the study.

5.6 References

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6 General Discussion

Biological invasions by non-native species are influenced by humans, and their attitudes and behavior. Current invasions often mirror past behavior resulting from a culture that assumed that the introduction of new species was necessary in order to sustain the livelihood of people, or to satisfy people’s desire for novelty. In the last decades, however, with the rise of invasion biology, norms have changed, at least in science: The introduction of species is tolerated as long as the species are useful for human well-being, and as long as their behavior in the new habitat is not perceived as problematic. However, these new perceptions, defined by the scientific community, did not slow down international trade of species. In contrary, increasing globalization continuously opened up new market opportunities, and species introductions have reached a level unseen before. As a consequence, the number and extent of species invasions are increasing. In such a context where those who set the norms—invasive species experts—dier from those who drive change— various stakeholders involved in species trade and introductions—finding solutions is particularly complex: the priority of invasion biology and nature conservation to protect native biodiversity from detrimental impacts of invasive non-native species conflicts with the interests of consumers and industries such as horticulture. In this thesis, I explored how the complexity of these issues is further aggravated by an apparent fading of the importance of the current paradigm within the scientific community. As I show in Chapter 2, academic experts are in considerable disagreement about how to conceptualize biological invasions and their management. Further, I also found that horticulturalists held their individual interpretations of concepts commonly used in invasion biology (Chapter 3), I uncovered a lack of stringent environmental policies preventing future invasions (Chapter 4), and I report on a possible solution in such a unruly situation: key stakeholders should be explicitly involved in the development of solutions, as it was the case with horticulturalists that contributed to the formulation and implementation of invasive species legislation in Switzerland (Chapter 5). 118 CHAPTER 6. GENERAL DISCUSSION

6.1 Diverging framings promote misunderstandings and hamper risk communication

There is a large body of literature demonstrating that introduced, i.e., non-native species dier in their eects on the recipient habitat, and that these eects are strongly dependent on the ecological context (Vilà et al., 2011; Kueer et al., 2013). That is, the same species may have dierent eects within the same ecosystem, or dier in its eects on dierent ecosystems. Likewise, also human perceptions towards non-native species are context-dependent, depending on the aected ecosystems or socioeconomic situations. Such an acknowledgement of context-dependency is however rare in the literature on biological invasions, that emphasizes general rules governing negative impacts from non-native species on ecosystems and human well-being. However, the results of this thesis suggest that such prevailing expert opinions are changing. As described in Chapter 2, experts’ attitudes towards invasive species, and the valuation of their eects on dierent ecosystems were highly diverse. The data suggests that this diversity of thinking may largely be explained by a lack of reliable knowledge about the drivers of invasions (Hulme, 2012). Experts also relied largely on their individual values and attitudes to evaluate environmental risks or societal problems emerging from non-native invasive species. Overall, the study revealed considerable dissent among experts concerning the definition of key concepts used in ecology and invasion biology. Particularly, the basic concept of non-nativeness was individually framed and was of ambiguous importance for the evaluation of environmental risk. The non-native origin of a species is an important concept in invasion biology. In the literature, non-nativeness is often used to explain why certain species become invasive, e.g., when natural enemies are missing in the new area (Keane and Crawley, 2002), or when non-native species have novel traits that are not present in the native flora (Vitousek et al., 1987). Thus, risk assessments often take the non- native origin of a species as a baseline for environmental threat, (e.g., Millennium Ecosystem Assessment, 2005; Simberlo et al., 2013). However, the perceived importance of non-nativeness for environmental risk seems to be fading and our results suggest that a critical assessment of the current paradigm among environmental experts is more widespread than could be assumed based on expert statements in the literature. Ambiguous interpretations of shared concepts as well as diverging perceptions of impacts have high potential to create misunderstandings within the scientific community, but also to hamper risk communication to the public, to stakeholders, or to policy makers. Our findings of missing consensus among environmental experts are particularly relevant given that experts are increasingly expected to contribute to the solving of societal problems (Kueer and Hirsch Hadorn, 2008). In situations of uncertainty, as it is the case in the realm of biological invasions, it is dicult or impossible to separate facts from values, and to avoid conceptual pluralism (Funtowicz and Ravetz, 1994). 6.2. EFFECTIVENESS OF ENVIRONMENTAL POLICIES 119

How to deal with this situation is a challenging issue and heavily debated among invasion scientists (Larson et al., 2013; Simberlo et al., 2013). A possibility to reduce tension is an open debate about values and interests at stake. Further, it might be helpful to allow for a more fluid use of key concepts—an approach however that requires authors to clearly and openly define how they use a concept in a given situation. Further, clear definitions of key concepts, such as the non-nativeness of a species, are important because these concepts are an important basis for environmental policies (e.g., black-listing of problematic non-native species), or in risk communication to stakeholders and the public. Current risk communication emphasizing the non-nativeness of problematic species might also aect horticulturists’ perceptions of environmental risk from non-native ornamental plant species, as described in Chapter 3 of this thesis. The revealed importance of non-nativeness for the formation of risk perceptions among horticulturists is particularly relevant considering the crucial role, non-native species play in the context of horticulture. Non-native ornamental plants have high economic and cultural values, and were perceived to be an inte- gral part of Swiss gardening culture by the study participants. Thus, risk communication directed to horticulturists and based on the non-nativeness of well known and probably economically important species might induce considerable tension, possibly resulting in horticulturists’ resistance to environmental policies. In order to avoid ineective risk communication, science and authorities should be aware that people dier in their risk perceptions, and that the design of adequate communication strategies requires a thorough examination of perceptions, values and attitudes towards an issue held by the target audience.

6.2 Eectiveness of environmental policies and the importance of horizon scanning

Those engaged in environmental policies against invasion risks face the problem that their eorts might be torpedoed by free market policies. Such a conﬂict of interests is particularly important in regions such as the U.S., Australia, or Europe. These regions report considerable economic damage from species invasions. While they invest heavily in biosecurity regulations, they also promote further globalization of trade. By monitoring online plant auctions on eBay.com (Chapter 4), we found that from these regions a particularly high fractions of known invasive species is traded. Thus, the data indicate that existing environmental policies are not ecient enough to prevent further global dispersal of known invasive plant species. E-commerce might be the upcoming introduction pathway for future invasive plant species, particularly considering growing Internet penetration in emerging markets that are increasingly engaged in international plant trade (Dehnen- Schmutz et al., 2010). However, e-commerce might prove to be an easy to ac- 120 CHAPTER 6. GENERAL DISCUSSION cess and aordable means for horizon scanning for emerging invasion risks. For instance, the surveillance of e-commerce might help to detect changes in global species movements or consumer preferences, the presence of invasive species within a country, or loopholes in environmental policies. It might also be particularly interesting to follow and to analyse discussions in Internet forums with relevance to species invasions, such as information exchange among garden aﬁcionados, invasive species experts, or people interested in conservation or landscape restoration. Fre- quently upcoming topics regarding particular species or their behavior might be indicative for emerging invasions, or for upcoming consumer preference in particular plants traits, eventually leading to increasing introductions of the respective species. The surveillance of social media might prove to be particularly valuable for the early detection of new invasions.

6.3 Limitations and further studies

Some limitations of the present thesis should be mentioned. First, the studies that examined perceptions toward plant invasions and stakeholders’ willingness to engage in pro-environmental action (Chapter 2, 3, and 5) were restricted to two groups of participants, academic experts and horticulturists. In order to gain a broader understanding of prevalent perceptions towards the issue, it might be valuable to include further stakeholder groups, e.g., foresters, ocers at environmental agencies, or members of nature conservation, but also the general public. Second, these studies reﬂect only the situation in Switzerland. However, the recent debate among member states of the European Community regarding the implementation of a strategy for the management of non-native invasive species clearly showed dierences in the understanding and the perception of the invasion issue between countries (European Commission, 2013). In order to further international collaboration in invasive species management, it might therefore be relevant to compare perceptions towards the invasion issue between European countries, as well as in other countries with a particularly high proportion of invasive species (e.g., the U.S. or Australia). Third, long-term studies among stakeholders and the public might help to detect changes in risk and beneﬁt perceptions from non-native invasive species. These insights might be helpful to continuously adapt established environmental policies to the actual societal situation.

6.4 Implications for management

Biological invasions are a societal issue characterized by uncertain facts, values that are in dispute, high stakes, and urgently needed decisions (sensu Funtowicz and Ravetz, 1994). I found in this thesis that experts’ and stakeholders’ attitudes towards non-native plant species, perceptions of risk and benefit from these species, as well as the valuation of their eects on the environment were very diverse, and driven by a multitude of dierent factors. Our findings make a strong case for a 6.4. IMPLICATIONS FOR MANAGEMENT 121 necessity for deliberation of interests and values among experts, stakeholders, and the public in order to find widely accepted and supported invasive species management strategies (Kueer and Hirsch Hadorn, 2008). These solutions preferred by aected stakeholders might dier from those held by environmental experts. The prevalence of diverging perceptions and attitudes as found in this thesis also indicates that risk communication must carefully choose an adequate language that is adapted to dierent target audiences (Kueer and Larson, 2014). This im- plies however training of academic experts involved in risk communication but also authorities, and journalists. It might for example be inecient to emphasize the non-nativeness of an invasive species when the audience perceives this species to be particularly beneficial. Or, when non-native species have become so familiar such that the audience considers the species to be native. A particular diculty for risk communication in species management stems from the often observed delays between introduction of a non-native species, colonization of a habitat, and negative eects of the invading species on the habitat. It might for instance be dicult for a person to understand why his/her current behavior, i.e., such as the promotion of species introductions has the potential to induce plant invasions in the future. Further, eects of non-native invasive species are multifarious. Nevertheless, often risk communication only refers to some few negative flagship species and it might not be straightforward to make a connection between the invasive nature of these species with those a person has in his/her garden. Therefore, not only immediate eects of few species should be communicated, but also ecological and particularly societal factors that lead to species invasions. This kind of deeper understanding of biological invasions might already be incorporated in early education, e.g., through workshops for children or teacher training programs. The results of our studies with horticulturists in Switzerland also indicate how eective participatory processes that involve key stakheholders might be for the acceptance of new regulations (Chapter 5). However, our findings also indicate that in order to make progress towards more eectiveness in the management of invasive species, it is crucial to thoroughly examine and take into account the psychological factors that are informing perceptions towards the issue. For instance, perceptions of risk from non-native invasive plant species were clearly informing horticulturists’ willingness to engage in risk mitigation behavior, whereas benefit perceptions from non-native ornamental species had the contrary eect (Chapter 3). Thus, in order to better understand the ecological and societal complexity inherent to species invasions it is inevitable to tackle the issue from various disciplinary angles. This thesis makes such an interdisciplinary eort and we hope that our findings will prove to be valuable in species management and risk communication. 122 CHAPTER 6. GENERAL DISCUSSION

6.5 References

Dehnen-Schmutz, K., Holdenrieder, O., Jeger, M. J., and Pautasso, M. (2010). Structural change in the international horticultural industry: some implications for plant health. Scientia Horticulturae, 125(1):1–15. European Commission (2013). Proposal on the prevention and management of the introduction and spread of invasive alien species. http://ec.europa.eu/environment/nature/invasivealien/index_en.htm. Ac- cessed 01/23/2014. Funtowicz, S. O. and Ravetz, J. R. (1994). Science for the post-normal age. Futures, 25(7):739–755. Hulme, P. E. (2012). Weed risk assessment: a way forward or a waste of time? Journal of Applied Ecology, 49(1):10–19. Keane, R. M. and Crawley, M. J. (2002). Exotic plant invasions and the enemy release hypothesis. Trends in Ecology and Evolution, 17(4):164–170. Kueer, C. and Hirsch Hadorn, G. (2008). How to achieve eectiveness in problem- oriented landscape research: the example of research on biotic invasions. Living Reviews in Landscape Research, 2:1–49. http://www.livingreviews.org/lrlr-2008- 2. Kueer, C. and Larson, B. M. H. (2014). Responsible use of language in scientiﬁc writing and science communication. BioScience, 64(8):1–6. Kueer, C., Pyöek, P., and Richardson, D. M. (2013). Integrative invasion science: model systems, multi-site studies, focused meta-analysis and invasion syndromes. New Phytologist, 200(3):615–633. Larson, B. M. H., Kueer, C., and ZiF Working Group on Ecological Novelty (2013). Managing invasive species amidst high uncertainty and novelty. Trends in Ecology and Evolution, 28(5):255–256. Millennium Ecosystem Assessment (2005). Ecosystems and human well-being : synthesis. Technical report, Island Press, Washington, DC, USA. Simberlo, D., Martin, J.-L., Genovesi, P., Maris, V., Wardle, D. A., Aronson, J., Courchamp, F., Galil, B., García-Berthou, E., Pascal, M., Pyöek, P., Sousa, R., Tabacchi, E., and Vilà, M. (2013). Impacts of biological invasions: what’s what and the way forward. Trends in Ecology & Evolution, 28(1):58–66. Vilà, M., Espinar, J. L., Hejda, M., Hulme, P. E., Jaroöík, V., Maron, J. L., Pergl, J., Schaner, U., Sun, Y., and Pyöek, P. (2011). Ecological impacts of invasive alien plants: a meta-analysis of their eects on species, communities and ecosystems. Ecology Letters, 14(7):702–708. 6.5. REFERENCES 123

Vitousek, P., Walker, L. R., Whiteaker, L. D., Mueller-Dombois, D., and Matson, P. A. (1987). Biological invasion by Myrica faya alters ecosystem development in hawaii. Science, 238(4828):802–804.

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7 Acknowledgements

Invasive plants? Uh, that’s the bad guys, right? In the last few years, I was often asked this question. How easy it would have been to simply answer by YES! However, life taught us that simple is for beginners. So, I normally ended up in explaining that in the context of biological invasions the valuation of plant species as good or bad was a matter of perception, that every coin had two sides (also the ecological one), and that at the end of the day it was not the plants consciously deciding to leave their places of origin and to invade new habitats—but it was humans who introduced them to new ecosystems. By this, most probably, I rather confused people instead of helping them to understand and usually they asked something like . . . um, so, what are they, then? There is not the one conclusive definition of an invasive plant species or of biological invasions in general but there are many ways how to describe the phenomenon. I am grateful to everybody who helped me stepping outside the box of sweeping judgments where invasive species are simply described to be the bad guys. During my PhD-studies, I have learned that—as a general rule—biological invasions are a consequence of human driven global and pervasive environmental change. Thus in order to understand the phenomenon of biological invasions, we need to understand how nature works, why and how new species interact with long- established species—but also what are the motivations to introduce certain species to new habitats, what drives humans to promote their spread, or why some people perceive a species to be an environmental risk (some even call for actions against certain species), while others welcome the same species as a precious addition—or just don’t care. As a biologist, and thus kind of an invasive species myself, I was given the chance by Michael Siegrist to learn about the psychological aspects driving human environmental behavior. I am very grateful for his guidance throughout this thesis and for providing me the exciting opportunity to get insight into another discipline. Thanks also to the members of the CB group for all the support I received. Some of them, in particular Melly Connor and Thomas Brunner, I regularly annoyed with my admittedly sometimes dummy questions about statistics, consumer behavior, or risk perceptions—thanks a lot for helping me out! In return, I might organize relocation assistance or collect leaves of Rubus americanus for 6-legged ”children”? I wish the CB-PhDs all the best and a lot of power for the remaining time until their defenses, and the members of the CB group in general much success in their 126 CHAPTER 7. ACKNOWLEDGEMENTS dierent research endeavors. Particularly, I keep my fingers crossed for Tamara Bucher—keep believing in yourself, I am sure you’ll achieve your goals! Many thanks also to “Mr. Ed” for translating and/or proof-reading my questionnaires, to the IT-crowd Sandro Bösch and Markus Schaner for always promptly helping me out in adversity. Special thanks go to Maria Rey who impressed me with her friendliness, calmness, and her talent to keep track of all the many things that were brought to her attention during a single ordinary workday. A person particularly involved in this thesis was Christoph Kueer. Christoph gave me food for thought, sorted my ideas, and worked through each single un- finished version of our papers. During the last six years, he not only received far more than 1000 e-mails from me to answer. Regularly, he also had to explain to his wife why there was always something he had to do for me. Sorry Eva and thanks for your patience! He is all yours again . . . Thanks Christoph for your time and the energy you invested in this project, I am very grateful! My special thanks go to Peter Edwards who always found a slot to squeeze me in and to give me valuable advice—despite his tight schedule. Sorry for not finishing my thesis in time, however, that way I got the opportunity to broadcast my defense life to Singapore—which sounds pretty cool to me! I also want to express my great thanks to Petra Lindemann-Matthies who was so kind to instantly accept my request to be my external referee without knowing me in person. I got to know you as a very warm person and I am happy that we found common ground so quickly. Last but not least, I am deeply grateful to my family for supporting me during all those years: to my mom Anneliese who made it possible for me to pursue my goals, my sons Luc (who actually also became a co-author) and David who tolerate that their mom sometimes is . . . say . . . dierent, to my brother Fritz who hopefully is following my example (if I can, you can too). Finally, I am deeply indebted to my husband Fabian who is also my best friend. He made so many contributions to this project that it is dicult to list them all. Thank you for everything you have done for me!

Fabian’s Contributions – Random Selection

• becoming a co-author • getting late night pizzas • being happy for me • having beer with me • consulting on LATEX • providing Erdsnumber3 • downloading TV series • making available duplex printers • encouraging me • replacing remote colleagues • enduring my laments • spell-checking manuscripts • funding my studies • working night shifts out of solidarity 127

8 Curriculum Vitae

Franziska Humair Kuhn

Born November 10, 1968, Basel (Switzerland)

Education

2008–2014 Ph.D. Candidate, ETH Zurich, Switzerland Perceptions of Risk from Non-Native and Horticultural Plants 1999–2004 Studies in Integrative Biology University of Basel, Switzerland and ETH Zurich Thesis: RBR1 – Arabidopsis thaliana: in vivo and in silico Plant Biotechnology, Prof. Wilhelm Gruissem, ETH Zurich 1984–1987 High School, Oberwil (BL, Switzerland); Matura B (Latin)

Experience

National Coordinator Switzerland 2011–2012 International Fascination of Plants Day 2012

Communications Ocer / Public Relations 2010–2012 Swiss Plant Science Web

Research and Teaching Assistant 2005–2007 Conservation Biology, University of Basel, Switzerland

Freelance Journalist 2006–2009 Swiss Public Radio SRF 2 (Wissenschaft / Kontext), SRF 4 News 1999–2009 Basellandschaftliche Zeitung, Liestal, Switzerland 1992–1995 Basellandschaftliche Zeitung, Liestal, Switzerland

Editor / Presenter 1992–1999 Radio Basilisk, Basel, Switzerland 1990–1992 Radio Studio B, Dornach, Switzerland

129

Appendices

131

A Supplementary Material of Chapter 2

A.1 Questionnaire

Interview guideline: perception / valuation of ecosystem change related to non-native, invasive plants

Aims of the study: In the context of my PhD Thesis I am interested in the perception of ecological processes that may be related to non-native plants. I am particularly interested in how dierent stakeholders perceive these processes. To address these questions, I conduct interviews and analyze them qualitatively. To ensure the correct analysis of your responses, I would like to record our interview. The sound recordings are assigned for internal use only, they will be anonymized and will not be passed to third parties. Do you agree to a sound recording? I imagine you still have questions regarding the project. In order to avoid inﬂuencing your answers, I am happy to answer your questions at the end of our conversation. In order to avoid inﬂuencing other potential experts, I ask you to keep the content of our conversation secret.

QUALITATIVE QUESTIONS

Clariﬁcation of terms used by participants

Getting started with landscape experts: ∆ Research and practice are talking about non-native plants. . . 132 APPENDIX A. SUPPLEMENTARY MATERIAL OF CHAPTER 2

Getting started with invasion biologists: ∆ You have already done research on ecological processes, also related to non- native plants. . .

Q1 – non-native Could you please explain to me what you understand by a non-native plant?

If previously the term originally was used: Q2 – originally

What do you understand by the term originally?

If previously the term natural was used: Q3 – natural

What do you understand by the term natural?

Q4 – native What do you understand by the term native?

If previously the terms archaeophyte / neophyte were used: Q5 – archaeophyte / neophyte

What do you understand by the term:

• archaeophyte? • neophyte?

Q6 – ecological behavior Do native and non-native plants dier in their behavior?

• Please motivate your answer shortly!

If previously the term invasive was used: Q7 – invasive

What do you understand by a non-native, invasive plant?

If previously the term invasive was NOT used: Q8 – invasive

There is a term “non-native, invasive plant”. What do you understand by this term?

Q9 – Question for the understanding 1 Are all non-native plants invasive? A.1. QUESTIONNAIRE 133

Q10 – Question for the understanding 2 Is it possible that here native plants can be invasive too? Clariﬁcation of terms used by the interviewer You told me what you understand by non-native, native, and non-native, invasive plants. If I am talking about these terms, I mean the following.

• Invasive species: “Swiss Commission for Wild Plant Conservation” Species that spread extensively and quickly so that – they suppress other species that are characteristic for a speciﬁc habitat. • Non-native species: After Swiss federal legislation (SR 814.911) – do not occur naturally in Switzerland or the remaining EFTA countries, nor in EU member states (without overseas territories), nor in agriculture, nor in a domesticated form in horticulture of these countries, – do not originate from populations from the countries mentioned above.

During our remaining conversation I will refer to these terms while talking about non-native, invasive plants. I do not refer to Switzerland only but to Central Europe. However you can stick to your deﬁnitions.

Q11 – Ecosystem Services We clarified what you understand by non-native plants, and non-native, invasive plants. Now I would like to focus on the influence of non-native, invasive plants on ecosystem services. By the term “ecosystem services” I mean • The benefits for humans, provided by an ecosystem. These benefits comprise: – products emerging from an ecosystem, e.g., food – regulations of the ecosystem processes, e.g., climate regulation – cultural services, e.g., natural environments and recreational areas

If I talk about ecosystems, I mean also those ecosystems that were shaped by humans, e.g., cultivation areas.

Q11 – Task I noted some of these ecosystem services on cards and I ask you to tag all ecosystem services with • a plus sign, where you think non-native, invasive plants have a predominantly positive inﬂuence upon a particular ecosystem service. 134 APPENDIX A. SUPPLEMENTARY MATERIAL OF CHAPTER 2

• a minus sign, where you think non-native, invasive plants have a predominantly negative inﬂuence upon a particular ecosystem service. • for all other cases, please mark a zero

Please motivate your answer shortly! ∆ Q12 – Importance of NIS inﬂuences What is in your opinion the most important

• positive inﬂuence, non-native, invasive plants have on ecosystem services? • negative inﬂuence, non-native, invasive plants have on ecosystem services?

Q13 – Emergence of the problem We were talking about dierent problems that are assigned to non-native, invasive plants. Could you please explain to me, why do these plants become problem plants?

Q14 – Importance of explanations Could you explain to me, which one of the reasons you mentioned is the most important to you? Please motivate your answer shortly! ∆ If previously only plant properties were mentioned: Q15 – plant properties for explanation

Did I understand you correctly, the reason for non-native plants to become invasive (in the context of those that are becoming problem plants), has to be assigned to speciﬁc properties of the plant alone? Please motivate your answer shortly! ∆ Q16 – Explanations of the Swiss Federal Oce for the Environment The Swiss federal oce for the environment (FOEN) writes the following in its “Inventory of alien species and their threat to biodiversity and economy in Switzer- land”, 2006: There are three major categories of factors that determine the ability of a species to become invasive:

• intrinsic factors or species traits • extrinsic factors or relationships between the species • abiotic and biotic factors; and the human dimension, incorporating the importance of species to humans. A.1. QUESTIONNAIRE 135

Could you please explain to me what the FOEN meant with the last point: what role do humans play in non-native plants becoming invasive?

Q17 – Ranking of spatial disctinction Invasions are spatial dispersal processes. What spatial dimension related to plant invasions in your opinion is the most most important?

• global • regional • local

Q18 – Importance of origin for ecological change In expert discourse as well as in public discourse about ecological change, very often the focus lays on invasive plants that are non-native. Do you think the origin of a plant is important to explain ecological change? Please motivate your answer shortly! ∆

Q19 – Importance of origin for valuation Do you think the origin of a plant is important to valuate ecological change? Please motivate your answer shortly! ∆

Q20 – Available knowledge Is there enough knowledge about why a plant can become a problem species?

• no, rather no, medium, rather yes, yes

Please motivate your answer shortly! ∆

Q21 – Size of the problem We were talking about positive and negative inﬂuences of non-native plants on ecosystem services, but also about how these plants directly aect humans. How big do you think the problem caused by non-native, invasive plant species is in Central Europe?

• very small, small, medium, big, very big

Please motivate your answer shortly! ∆ 136 APPENDIX A. SUPPLEMENTARY MATERIAL OF CHAPTER 2

Q22 – Problem comparison If you compare the Situation in Central Europe with other areas, do you think we do have a problem here or not?

Q23 – Problem development Does the problematic related to non-native, invasive plant species in the future

• decrease, slightly decrease, stay the same, slightly increase, increase?

Please motivate your answer shortly! ∆

Q24 – Valuations Is the problematic related to non-native, invasive plants

• underestimated, rather underestimated, neutrally valued, rather overestimated, overestimated?

Please motivate your answer shortly! ∆

Q25 – Need for intervention How urgent is need for intervention for Switzerland / Central Europe regarding the management of non- native, invasive plants?

• not urgent, not really urgent, medium, rather urgent, urgent

Please motivate your answer shortly! ∆

CONCLUDING / DEMOGRAPHIC QUESTIONS

Q26 – Own research Do you do own research on non-native, invasive species?

• yes / no • If yes: – What is the percentage of your daily work that is related to research on non-native, invasive species? – What part of your research has a connection with what we discussed about non-native, invasive plants? A.1. QUESTIONNAIRE 137

Q27 – Comments / concluding remarks Do you have a comment or a concluding remark concerning the issue, something that is important to you, however we did not discuss?

Q27 – Demography • age • gender • grown-up in (rural / urban) • domiciled in (rural / urban) • current position • educational background • focus diploma / Master thesis • focus PhD thesis • current research focus • additional education 138 APPENDIX A. SUPPLEMENTARY MATERIAL OF CHAPTER 2

A.2 Demographic data

Table A.1: Socio-demography of the study participants.

Socio-demographic data Invasion Landscape Biologists Experts N = 13 N = 13 Age (years) Range 30–67 31–64 Mean 42.7 48.1 SD 11.5 10.2 Sex (number) Male 99 Female 44 Grown-up in Rural 74 Urban 69 Domiciled in Rural 34 Urban 10 9 Current Position Professor 47 Group Leader 22 Scientiﬁc Collaborator 52 PhD Student 21 Head of Scientiﬁc Network 1 Educational Background Agricultural Sciences 22 Biology 10 4 Environmental Sciences 1 Geograpy 5 Landscape Architecture 11 Type of Research Invasion Biology (Science) 10 Invasion Biology (Management) 3 Landscape Research (Ecology) 6 Landscape Research 7 (Social Sciences) 139

B Supplementary Material of Chapter 3

B.1 Questionnaire

Questionnaire S1. Written questionnaire with members of the Swiss Association of Horticulture, 2012.

In our survey, we often use the term non-native plant. Please try to answer the following question as spontaneously as possible.

1. For a moment, think about non-native plants. What are the first three words or images that spontaneously come to your mind?

Word/image 1: ______

Word/image 2: ______

Word/image 3: ______

2. You may now evaluate the words or images you just noted using a scale. Please indicate for every word or image what feeling you have.

very negative very positive -3 -2 -1 0 1 2 3 Word/image 1: Word/image 2: Word/image 3:

140 APPENDIX B. SUPPLEMENTARY MATERIAL OF CHAPTER 3

JardinSuisse has published the catalogue „Plants for our gardens“. This catalogue also lists the plants that are listed in questions 1 – 4 of this paragraph.

1. In your opinion, how important are the following plants for landscape design in Switzerland? Please check what best applies to you. absolutely very unimportant important 1 2 3 4 5 6 Wisteria (Wisteria sp.) Oregon grape (Mahonia aquifolium) Red-osier dogwood (Cornus sericea) Cherry laurel (Prunus laurocerasus) European spindle tree (Euonymus europaeus) Henry’s honeysuckle (Lonicera henryi) Blackthorn (Prunus spinosa) Russell lupin (Lupinus polyphyllus) Creeping sedum (Sedum spurium) Japanese honeysuckle (Lonicera japonica) Princess tree (Paulownia tomentosa) Lilac (Syringa sp.) Black locust (Robinia pseudoacacia) Russian vine (Fallopia baldschuanica / Fallopia aubertii) English holly (Ilex aquifolium) Butterfly bush (Buddleja davidii) Leatherleaf viburnum (Viburnum rhytidophyllum) Chinese windmill palm (Trachycarpus fortunei)

2. In your opinion, do the plants listed below belong to the native flora or are they non-native?

native non-native 1 2

Wisteria (Wisteria sp.) Oregon grape (Mahonia aquifolium) Red-osier dogwood (Cornus sericea) Cherry laurel (Prunus laurocerasus) European spindle tree (Euonymus europaeus) Henry’s honeysuckle (Lonicera henryi) Blackthorn (Prunus spinosa) Russell lupin (Lupinus polyphyllus) Creeping sedum (Sedum spurium) Japanese honeysuckle (Lonicera japonica) Princess tree (Paulownia tomentosa) Lilac (Syringa sp.) Black locust (Robinia pseudoacacia) Russian vine (Fallopia baldschuanica / Fallopia aubertii) English holly (Ilex aquifolium) Butterfly bush (Buddleja davidii) Leatherleaf viburnum (Viburnum rhytidophyllum) Chinese windmill palm (Trachycarpus fortunei) B.1. QUESTIONNAIRE 141

3. Is there any threat that the following plants spread and have negative effects on the environment if they are not handled correctly?

no threat threat 1 2

4. How important are the following plants for your own business?

absolutely very unimportant important 1 2 3 4 5 6 Wisteria (Wisteria sp.) Oregon grape (Mahonia aquifolium) Red-osier dogwood (Cornus sericea) Cherry laurel (Prunus laurocerasus) European spindle tree (Euonymus europaeus) Henry’s honeysuckle (Lonicera henryi) Blackthorn (Prunus spinosa) Russell lupin (Lupinus polyphyllus) Creeping sedum (Sedum spurium) Japanese honeysuckle (Lonicera japonica) Princess tree (Paulownia tomentosa) Lilac (Syringa sp.) Black locust (Robinia pseudoacacia) Russian vine (Fallopia baldschuanica / Fallopia aubertii) English holly (Ilex aquifolium) Butterfly bush (Buddleja davidii) Leatherleaf viburnum (Viburnum rhytidophyllum) Chinese windmill palm (Trachycarpus fortunei)

142 APPENDIX B. SUPPLEMENTARY MATERIAL OF CHAPTER 3

In the following, we would like to ask you some general questions about so-called non-native plants.

Definition according to the Swiss Commission of Wild Plant Conservation, since January 2012 Info Flora

1. Many garden centers sell non-native plants. We are interested in why these plants are included in their offers. Please specify to what extent you agree with the following statements.

not agree totally at all agree 1 2 3 4 5 6 a For my business, non-native plants are economically important. b It is easier to sell exotic plants than native species. c The choice of plants has to be complemented by non-native plants because native ones do not possess all of the properties desired by the customers. d New species and cultivars may improve biodiversity. e If I do not sell a non-native plant because of the risk it poses to the environment, my customers may buy it in another store. f To me, it is important to regularly offer my customers new species and cultivars. g The cultural value of a horticultural landscape is essentially dependent on new species and cultivars. h Non-native plants cope particularly well with environmental change. i Non-native plants belong to our gardening culture.

B.1. QUESTIONNAIRE 143

In the following paragraph, we will ask you questions about so-called invasive non-native plants.

By non-native plants, we mean plants that have been introduced to Switzerland by humans, deliberately or undeliberately, that originate from foreign regions, mostly different continents, and that are established here in the wild. Non-native plants are called invasive if it is known or if it has to be assumed that they spread at uncontrollable rates and that they produce such a high stand density that biodiversity or its sustainable use may be compromised, or that humans, animals, or the environment may be threatened.

Definition according to the Release Ordinance (SR 814.911 Art. 3f) and to the Swiss Commission of Wild Plant Conservation, since January 2012 Info Flora

1. Many garden centers offer non-native plants that may become invasive if not handled correctly, that is, they may have negative impacts on humans, animals, or the environment. We would like to learn more why these plants are sold in Switzerland. Please check what best applies to you.

not agree totally at all agree 1 2 3 4 5 6 a It is hard to remove some invasive non-native plants because there are no appropriate alternative plant species. b If I know that a non-native plant is a problem somewhere, I do not sell it. c Invasive plants may have characteristics such as fast growth or easy propagation that make them an interesting garden plant.

2. What is your general estimate of the size of the problem caused by invasive non-native plants in Switzerland?

very small very big problem problem 1 2 3 4 5 6

144 APPENDIX B. SUPPLEMENTARY MATERIAL OF CHAPTER 3

The following part of our survey refers to the legal regulations that should be enacted by Switzerland to regulate import, trade, and the proper handling of non-native plants.

In your opinion, how should import and trade of non-native species be regulated? Please indicate how much you agree with the following statements.

not agree totally at all agree 1 2 3 4 5 6 a As long as one cannot rule out the possibility that a non-native plant will become invasive in Switzerland, a ban on the import of this plant should be imposed. b In Switzerland, trade of non-native plants does not require legal regulations. c There should be no restrictions on importation of non-native plants into Switzerland, as long as there is no evidence that these plants potentially become invasive in Switzerland. d No plants should be allowed for import into Switzerland that have been shown to have been invasive in another country.

The green industry is required to inform its customers about characteristics of plants and their effects on organisms and the environment. Further, the green industry has to instruct its customers about the appropriate handling of plants (e.g., proper disposal). These are the requirements of the Federal Act on the Protection of the Environment as well as the Release Ordinance (Duty to Inform Regulation).

How much do you agree with the following statements regarding the Duty to Inform Regulation? Please check what best applies to you.

do not agree agree 1 2 3 4 5 6 a The Duty to Inform regulation will lead to a loss of sales of economically important plants. b The Duty to Inform regulation will have a positive influence on the public image of the horticultural industry. c The Duty to Inform regulation will result in an increasing number of banned species that are not allowed to be sold. d The Duty to Inform regulation will result in a replacement of the sale of alien plants by the sale of plants that are considered unproblematic by legislature (alternative plants). e The Duty to Inform regulation is reasonable for a (not further specified) selection of plants. f The Duty to Inform regulation is practical for a (not further specified) selection of plants. B.1. QUESTIONNAIRE 145

We now propose a number of concrete measures in the handling of non-native plants. These measures go beyond the legal regulations.

We are interested to learn if you can imagine voluntarily participating in one or in several of the following measures. Please check what best applies to you.

I cannot I can imagine imagine at all very well 1 2 3 4 5 6 a Increase prices for plants that are included in the Black list (i.e. plants that had been listed by the Swiss Commission of Wild Plant Conservation to be invasive non-native plants in Switzerland, and that cause negative impacts in the context of biodiversity, health, and/or economy) b Remove plants from my stock that are listed on the Black-list. c Ban the sale of any non-native plant, until it has been shown that it does not pose a danger to humans or the environment. d Inform customers about invasive non-native plant species. e Promote the sale of native plants.

Different actors assess the risk whether a non-native plant will cause damage in Switzerland, or whether it has the potential to cause damage.

1. Until to date, to what extent has the green industry been included in creating lists with invasive plants, with plants that have the potential to become invasive, or with plants that are (no longer) allowed to be traded?

too little accurately excessively 1 2 3

2. In your opinion, what influence should the green industry have in future risk assessments of non-native plants compared to the prevalent practice?

less the same more 1 2 3

146 APPENDIX B. SUPPLEMENTARY MATERIAL OF CHAPTER 3

Finally, we ask you to provide some information about your business as well as about yourself.

1. In what business sector are you 2. In what sector of the green industry are you mainly working? Please mainly working? differentiate between the main source of income and the additional income.

main source of additional income income a wholesale a horticulture / landscape market architecture b private b landscaping / gardening consumer c supply services business d potted plants and cut flowers c mixed clientele e tree nursery

3. What is the number of employees that on 4. What is your function in your company? average have been working in your company for the past three years? (Counted up to full-time jobs during peak season)?

a 1 - 5 full-time jobs a general management b 6 - 15 full-time jobs b branch management c 16 - 30 full-time jobs c head of department d more than 30 full-time jobs d case handling

e training in the horticultural sector f internship g administration h other......

5. What is your gender? 6. What is your year of birth?

a male 1 9 b female

7. What is currently your highest level of education?

a primary school b lower secondary school c upper secondary vocational school d upper secondary university preparation school e college / university f others......

B.2. HIERARCHICAL REGRESSION ANALYSIS 147

B.2 Hierarchical regression analysis: Willingness in risk mitigation behavior

The following Table B.1 shows the results of the hierarchical regression analysis with Willingness to Engage in Risk Mitigation Behavior as dependent variable. In the first model including socio-demographic variables only, the model explained 16% (R2 =0.16) of the variance, F (14, 526) = 7.03, p<0.001. Four variables denoting the main source of income (Horticulture and landscape architecture (— = 0.10), Potted plants and cut flowers (— = 0.10), Tree nursery (— = 0.23), ≠ ≠ and Others (combination of several sources of income) (— = 0.15)), as well as ≠ the variable Over 30 employees (— = 0.12) referring to the number of full time ≠ employees per year showed a significant influence on horticulturists willingness to engage in risk mitigation behavior. The additional inclusion of the psychological variables describing risk perceptions (— =0.39) and perceptions of benefit (— = 0.35) caused R2 to increase significantly by 32% F (2, 524) = 29.62 , p<0.001, ≠ (F = 158.353, adjusted R2 =0.46). Only the contributions of the variables Horticulture and landscape architecture (— =0.09) and Tree nursery (— = 0.08) ≠ remained significant, and particularly the proportion of variance accounted for Tree nursery decreased considerably. 148 APPENDIX B. SUPPLEMENTARY MATERIAL OF CHAPTER 3

Table B.1: Hierarchical regression analysis with Willingness to Engage in Risk Mitigation Behavior as dependent variable. needn Variable Independent Age Gender Variables Socio-Demographic ikPerception Risk Beneﬁt Variables Psychological Employees Time Full of Number Sector Business Income of Source Main Business in Position vr3 employees 30 over employees 16–30 employees 6–15 employees 1–5 clientele Mixed business consumer Private market Wholesale income) of sources several of (combination Others nursery ﬂowersTree cut and plants Potted Gardening architecture landscape and gardening Horticulture & architecture, landscape Horticulture, years 60 over years 41–60 years 21–40 † † † † ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 B ––– – –– – –– . . . . . – –– . – –– . – –– . – –– . . . . – –– . – –– . 9 0 397 0 191 0 049 0 089 0 313 0 572 0 932 0 291 0 0 242 25 0 146 0 066 0 08 0 215 oe a oe (b) 1 Model (a) 1 Model SE ...... 1 206 166 183 138 0 139 113 136 134 095 0 174 154 0 136 101 ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ 0 0 0 0 0 0 0 0 0 0 0 ...... 2 0 12 063 022 0 039 066 15 0 23 10 08 10 0 0 046 021 0 035 051 — * *** *** * * ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ 0 0 0 0 0 0 0 0 0 0 B ...... 3 0 036 0 026 0 008 0 075 9 0 298 0 336 0 192 0 068 0 016 0 036 0 201 0 091 0 312 0 119 0 191 0 237 SE ...... 2 0 028 035 123 0 108 08 0 08 163 134 149 11 0 11 089 0 0 108 106 075 0 138 ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ ≠ 0 0 0 0 0 0 0 0 0 ...... 39 35 056 022 007 016 042 024 08 039 063 09 011 008 004 018 — *** *** * **

Note: †Reference category. *p<0.05, **p<0.01, ***p<0.001, N = 541. Dummy variable gender was coded as 0=male, 1=female. Pos. in Business was coded as 0=other (e.g., head of dept. / administration), 1=general manager or branch manager. 149

C Supplementary Material of Chapter 4

C.1 Families of the Global species list

Table C.1: The 23 Families of the Global species list, with information on the number of species and genera, and the data source per family.

Family #species Reference #genera Amaryllidaceae 2199 (77) (Govaerts et al., 2013g) Apocynaceae 1783 (163) (Govaerts and Leeuwenberg, 2013) Araceae 3278 (115) (Govaerts et al., 2013c) Araliaceae 1439 (40) (Govaerts et al., 2013e) Arecaceae 2508 (183) (Govaerts et al., 2013d) Asparagaceae 2876 (124) (Govaerts et al., 2013m) Asteraceae 29457 (1799) Global Compositae checklist, (Flann C. (ed.), 2009) Begoniaceae 1579 (2) (Govaerts and Sands, 2013) Campanulaceae 2361 (83) (Govaerts and Lammers, 2013) Cyperaceae 5700 (103) (Govaerts et al., 2013h) Euphorbiaceae 6490 (222) (Govaerts et al., 2013a) Fabaceae 19981 (732) ILDIS World Database of Legumes, (Roskov et al., 2005) Iridaceae 2002 (71) (Barker, 2013) Lamiaceae 6252 (228) (Govaerts et al., 2013j) Myrtaceae 5847 (140) (Govaerts et al., 2013l) Orchidaceae 27139 (814) (Govaerts et al., 2013b) Pandanaceae 993 (4) (Govaerts, 2013a) Phyllanthaceae 2096 (58) (Govaerts, 2013b) Poaceae 11243 (733) (Clayton et al., 2013) Rubiaceae 13515 (607) (Govaerts et al., 2013k) Sapotaceae 1289 (57) (Govaerts et al., 2013f) Verbenaceae 1014 (33) (Govaerts and Atkins, 2013) Zingiberaceae 1565 (51) (Govaerts et al., 2013i) Total 152606 (6438) 150 APPENDIX C. SUPPLEMENTARY MATERIAL OF CHAPTER 4

C.2 100 most frequently oered species

Table C.2: Overview over the 100 species with the highest average number of oers per day on eBay.com. Included are species from all lists used in this study. G: Global, I: Invasives, IUCN: 100 of the World’s Worst Invasive Alien Species. Number of countries / number of regions denote item locations.

Taxon #countries Species list (avg. number of oers per day) (#regions) Adenium obesum (3154.5) 12 (5) G Zinnia elegans (177.5) 13 (5) G Plumeria rubra (114.3) 5 (4) G Cosmos bipinnatus (103.9) 9 (4) G Helianthus annuus (93.2) 11 (4) G Passiﬂora edulis (90.0) 17 (5) I Echinacea purpurea (84.1) 12 (4) G Centaurea cyanus (81.7) 10 (5) I Arbutus unedo (80.6) 9 (3) I Ficus carica (80.3) 8 (3) I Ocimum basilicum (79.5) 15 (5) G Laurus nobilis (78.1) 11 (3) I Zea mays (73.8) 11 (4) G Punica granatum (71.2) 19 (5) I Mimosa pudica (69.3) 14 (3) G Rudbeckia hirta (68.4) 8 (4) G Delonix regia (68.0) 13 (6) G, I Jacaranda mimosifolia (62.7) 12 (5) I Callistephus chinensis (62.2) 9 (4) G Eschscholzia californica (61.3) 11 (5) I Calendula ocinalis (56.7) 13 (4) G Stevia rebaudiana (54.8) 13 (3) G Achillea millefolium (54.3) 10 (4) G, I Paulownia tomentosa (52.8) 15 (4) I Opuntia ﬁcus-indica (52.5) 9 (4) I Digitalis purpurea (50.8) 10 (4) I Psidium guajava (50.2) 11 (4) G, I Bellis perennis (48.8) 13 (4) G Phaseolus vulgaris (47.7) 5 (4) G Lycopersicon esculentum (47.6) 8 (4) I Coea arabica (45.8) 9 (3) G, I Gardenia jasminoides (44.3) 7 (4) G Lathyrus odoratus (44.0) 10 (4) G C.2. 100 MOST FREQUENTLY OFFERED SPECIES 151

Acer ginnala (43.7) 7 (3) I Liatris spicata (42.9) 8 (4) G Wisteria sinensis (41.6) 12 (4) G, I Nerium oleander (41.0) 13 (4) G, I Albizia julibrissin (40.4) 13 (5) G, I Robinia pseudoacacia (40.2) 11 (3) G, I Datura stramonium (40.1) 5 (2) I Agave americana (40.0) 10 (5) G, I Euphorbia milii (39.7) 6 (5) G Colocasia esculenta (38.9) 4 (3) G, I Cercis siliquastrum (38.4) 13 (4) G Lactuca sativa (37.0) 10 (4) G Rosa rugosa (36.5) 8 (4) I Agastache foeniculum (36.2) 6 (3) G Trachycarpus fortunei (36.1) 13 (4) G, I Panax ginseng (35.3) 10 (4) G Cassia fistula (35.2) 11 (4) G, I Humulus lupulus (34.5) 10 (4) I Olea europaea (34.5) 9 (4) I Senna alata (34.3) 5 (3) G, I Lycium barbarum (34.0) 10 (4) I Agave parryi (33.9) 8 (3) G Caesalpinia pulcherrima (33.5) 8 (5) G, I Etlingera elatior (33.4) 5 (4) G Pinus nigra (33.3) 6 (2) I Ceratonia siliqua (32.4) 12 (4) G, I Neofinetia falcata (32.3) 5 (3) G Pachypodium saundersii (31.7) 6 (4) G Lobelia erinus (31.5) 10 (5) G Salvia apiana (31.5) 5 (4) G Cercis canadensis (31.4) 6 (4) G Cortaderia selloana (31.4) 9 (4) G, I Sorbus aucuparia (31.3) 7 (2) I Pachypodium lamerei (31.1) 9 (4) G Acer buergerianum (31.0) 8 (3) I Thunbergia alata (30.5) 7 (4) I Sambucus nigra (30.5) 6 (3) I Myosotis sylvatica (30.5) 5 (4) I Physalis peruviana (30.3) 16 (4) I Salvia coccinea (29.6) 6 (4) G Allium tuberosum (29.6) 9 (4) G Sabal minor (29.4) 4 (3) G Washingtonia filifera (29.06) 7 (3) G, I 152 APPENDIX C. SUPPLEMENTARY MATERIAL OF CHAPTER 4

Allium cepa (28.92) 7 (4) G Pinus sylvestris (28.88) 5 (2) I Cosmos sulphureus (28.4) 7 (6) G Nepeta cataria (28.34) 9 (4) G Jatropha curcas (28.24) 8 (4) G, I Morus alba (27.02) 8 (3) I Clitoria ternatea (26.74) 8 (4) G Beaucarnea recurvata (26.48) 9 (5) G Tagetes erecta (26.32) 7 (4) G Artemisia absinthium (26.06) 8 (4) G Dracaena draco (26.02) 7 (3) G Pisum sativum (25.32) 6 (4) G Morinda citrifolia (25.06) 10 (5) G, I Syringa vulgaris (25.02) 7 (3) I Euphorbia obesa (24.92) 9 (4) G Annona squamosa (24.78) 11 (4) I Vigna unguiculata (24.66) 5 (3) G Cryptomeria japonica (24.64) 8 (4) I Allium schoenoprasum (24.52) 8 (4) G Bauhinia purpurea (24.48) 11 (4) G, I Ulmus parvifolia (24.1) 6 (4) I Picea abies (24.1) 5 (2) I Lobelia cardinalis (24.02) 5 (3) G Eugenia uniﬂora (23.68) 4 (3) G, I C.3. IUCN’S WORST INVASIVE ALIEN SPECIES OFFERED 153

C.3 IUCN’s worst invasive alien species oered

Table C.3: The 13 invasive plant species oered on eBay.com that are listed on IUCN’s list of 100 of the world’s worst invasive alien species (out of a total of 35 plant species). For each species the average number of oers per day across the 50 search days, the countries where the species oers were located and the number of regions these countries belong to are shown.

Taxon average Item locations #oers (#regions) per day Lantana camara L. 16.74 Germany, Italy, Malta, Romania, Spain, Turkey, UK, USA (2) Leucaena leucocephala 10.20 Australia, Indonesia, Italy, Malta, (Lam.) de Wit Mexico, Portugal, Spain, Thailand, UK, USA (5) Spathodea campanulata 8.88 Germany, Hungary, Italy, UK, USA (2) Beauv. Psidium cattleianum 7.98 Portugal, USA (2) Sabine Lythrum salicaria L. 6.80 Australia, China, Ireland (3) Acacia mearnsii 4.66 Australia, Germany, Italy, UK (2) De Wild. Hedychium gardnerianum 4.28 Germany, Malta, USA (2) Sheppard ex Ker-Gawl Pinus pinaster 2.94 Lithuania, Turkey, USA (2) Soland., non Ait. Arundo donax L. 2.42 Portugal, USA (2) Ulex europaeus L. 2.08 Lithuania, USA (2) Imperata cylindrica 1.56 Turkey, USA (2) (L.) Raeusch. Schinus terebinthifolius 0.56 Hungary, USA (2) Raddi Eichhornia crassipes 0.24 USA (1) (Mart.) Solms 154 APPENDIX C. SUPPLEMENTARY MATERIAL OF CHAPTER 4

C.4 Cumulative frequency distributions of species offered

Figure C.1: Cumulative frequency distributions of plant species oered over a period of 50 days on eBay.com. Data is given for the world species list of 23 families (Global, dark line, Ntotal = 2115) and invasive species (Invasives, light line, Ntotal = 510). On the ﬁrst day of our search we found oers for 75% of the species of the Global list, and for 92% of the invasive species. C.5. REGIONS WHERE NON-WOODY INVASIVES WERE OFFERED 155

C.5 Number of regions where non-woody invasives were oered

Figure C.2: Number of regions where non-woody invasive plant species (N = 123) were oered for sale on eBay.com (x-axis) and where they were assessed to be invasive (y-axis). Non-woody species account only for 24% of all invasive plant species oered on eBay.com (N = 510). The red line indicates the average number of regions where a species is classiﬁed as invasive. 156 APPENDIX C. SUPPLEMENTARY MATERIAL OF CHAPTER 4

C.6 References

Barker, C. (2013). World checklist of Iridaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013.

Clayton, W. D., Govaerts, R., Harman, K. T., Williamson, H., and Vorontsova, M. (2013). World checklist of Poaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013.

Flann C. (ed.) (2009). Global compositae checklist. http://compositae.landcareresearch.co.nz. Accessed 05/22/2013.

Govaerts, Fernández Casas, F. J., Barker, C., Carter, S., Davies, S., Esser, H.- J., Gilbert, M., Homann, P., Radclie-Smith, A., Steinmann, V., van Welzen, P., and Whitmoore, T. (2013a). World checklist of Euphorbiaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013.

Govaerts, R. (2013a). World checklist of Pandanaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013.

Govaerts, R. (2013b). World checklist of Phyllanthaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013.

Govaerts, R. and Atkins, S. (2013). World checklist of Verbenaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013.

Govaerts, R., Bernet, P., Kratochvil, K., Gerlach, G., Carr, G., Alrich, P., Pridgeon, A. M., Pfahl, J., Campacci, M. A., Holland Baptista, D., Tigges, H. T., Shaw, J., Cribb, P., George, A., Kreuz, K., and Wood, J. (2013b). World checklist of Orchidaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013.

Govaerts, R., Bogner, J., Boos, J., Boyce, P., Cosgri, B., Croat, T., Gonçalves, E., Grayum, M., Hay, A., Hetterscheid, W., Ittenbach, S., Landolt, E., Mayo, S., Murata, J., Nguyen, V. D., Sakuragui, C. M., Singh, Y., Thompson, S., and Zhu, G. (2013c). World checklist of Araceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013.

Govaerts, R., Dransﬁeld, J., Zona, S. A., Hodel, D. R., and Henderson, A. (2013d). World checklist of Arecaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013.

Govaerts, R., Esser, H.-J., Frodin, D. G., Lowry, P. P., and Wen, J. (2013e). World checklist of Araliaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. C.6. REFERENCES 157

Govaerts, R., Harvey, Y., Jessup, L., Pennington, T. D., and Vink, W. (2013f). World checklist of Sapotaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R., Kington, S., Friesen, N., Fritsch, R., Snijman, D. A., Mar- cucci, R., Silverstone-Sopkin, P. A., and Brullo, S. (2013g). World checklist of Amaryllidaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R., Koopman, J., Simpson, D., Goetghebeur, P., Wilson, K., Egorova, T., and Bruhl, J. (2013h). World checklist of Cyperaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R. and Lammers, T. G. (2013). World checklist of Campanulaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R. and Leeuwenberg, A. (2013). World checklist of Apocynaceae. Facili- tated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R., Newman, M., and Lock, J. M. (2013i). World checklist of Zingiberaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R., Paton, A., Harvey, Y., Navarro, T., and del Rosario GarcÃa Peña, M. (2013j). World checklist of Lamiaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R., Ruhsam, M., Andersson, L., Robbrecht, E., Bridson, D., Davis, A., Schanzer, I., and Sonké, B. (2013k). World checklist of Rubiaceae. Facili- tated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R. and Sands, M. J. S. (2013). World checklist of Begoniaceae. Facili- tated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R., Sobral, M., Ashton, P., Barrie, F., Holst, B. K., Landrum, L. L., Matsumoto, K., Fernanda Mazine, F., Nic Lughadha, E., Proenca, C., Soares- Silva, L. H., Wilson, P. G., and Lucas, E. (2013l). World checklist of Myrtaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. Govaerts, R., Zonneveld, B. J. M., and Zona, S. A. (2013m). World checklist of Asparagaceae. Facilitated by the Royal Botanic Gardens, Kew. http://apps.kew.org/wcsp/. Accessed 05/22/2013. 158 APPENDIX C. SUPPLEMENTARY MATERIAL OF CHAPTER 4

Roskov, Y. R., Bisby, F. A., Zarucchi, J. L., and White, R. J. (2005). Ildis world database of legumes. 10th ed. LegumeWeb. www.ildis.org. Accessed 05/22/2013.