Invasive Species in Europe: Ecology, Status, and Policy Reuben P Keller1*, Juergen Geist2†, Jonathan M Jeschke3† and Ingolf Kühn4†
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Keller et al. Environmental Sciences Europe 2011, 23:23 http://www.enveurope.com/content/23/1/23 COMMENTARY Open Access Invasive species in Europe: ecology, status, and policy Reuben P Keller1*, Juergen Geist2†, Jonathan M Jeschke3† and Ingolf Kühn4† Abstract Globalization of trade and travel has facilitated the spread of non-native species across the earth. A proportion of these species become established and cause serious environmental, economic, and human health impacts. These species are referred to as invasive, and are now recognized as one of the major drivers of biodiversity change across the globe. As a long-time centre for trade, Europe has seen the introduction and subsequent establishment of at least several thousand non-native species. These range in taxonomy from viruses and bacteria to fungi, plants, and animals. Although invasive species cause major negative impacts across all regions of Europe, they also offer scientists the opportunity to develop and test theory about how species enter and leave communities, how non- native and native species interact with each other, and how different types of species affect ecosystem functions. For these reasons, there has been recent growth in the field of invasion biology as scientists work to understand the process of invasion, the changes that invasive species cause to their recipient ecosystems, and the ways that the problems of invasive species can be reduced. This review covers the process and drivers of species invasions in Europe, the socio-economic factors that make some regions particularly strongly invaded, and the ecological factors that make some species particularly invasive. We describe the impacts of invasive species in Europe, the difficulties involved in reducing these impacts, and explain the policy options currently being considered. We outline the reasons that invasive species create unique policy challenges, and suggest some rules of thumb for designing and implementing management programs. If new management programs are not enacted in Europe, it is inevitable that more invasive species will arrive, and that the total economic, environmental, and human health impacts from these species will continue to grow. Keywords: Alien species, Biodiversity conservation, Biological invasions, Biotic resistance, Impacts of invasive spe- cies, Management, Pathways, Policy, Tens rule, Vectors Introduction pathways that move species mean that non-native spe- Globalization has integrated widely dispersed human cies are now recognized as one of the major drivers of communities into a worldwide economy. This process global biodiversity loss. They also cause significant provides many benefits through the movement of people damage to economies and human health [4-7]. and goods, but also leads to the intentional and uninten- Europe has been a centre for international trade for tional transfer of organisms among ecosystems that were many centuries and has consequently seen the establish- previously separate [1]. Some of these species become ment of a large number of species. Some of these spe- established beyond their native range, a subset of these cies have positive effects, including a subset of those spread, and some of these have negative impacts and introduced to enhance fisheries. Many other species, are termed invasive [2]. Although not all species moved however, cause large negative impacts. These species beyond their native range become established [3], the cover a broad taxonomic range - from viruses and bac- large number of species transported and the range of teria to fungi, plants, and animals - and affect all Eur- opeannationsandregions[6].Wheretimelinesare * Correspondence: [email protected] available, the number of non-native species established † Contributed equally in Europe is generally increasing exponentially in fresh- 1Program on the Global Environment, University of Chicago, Chicago IL 60637, USA water [8] and terrestrial ecosystems [5,8-11]. This Full list of author information is available at the end of the article © 2011 Keller et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Keller et al. Environmental Sciences Europe 2011, 23:23 Page 2 of 17 http://www.enveurope.com/content/23/1/23 pattern is consistent with exponential increases in trade The invasion process and travel [5]. Without increased efforts to manage To become invasive, a species must pass through a pathways of introduction, the number of invasive species number of transitions (e.g. [2,19,20]). First, it must be will continue to grow. Indeed, because there is often a entrained in a pathway and survive transit, where a significant lag period between species introduction and pathway is a human-mediated process that facilitates the spread, it is likely that many future invasions have movement of species from one region to another. When already been set in motion [12,13]. Consequently, the it arrives in a region beyond its native range due to task of designing policies toreducethetransportand direct or indirect human intervention, it is referred to as release of non-native species, and to manage those introduced. Species that are not able to maintain self- already established, has become a large priority for both sustaining populations, but that are occasionally found national governments within Europe, and for the Eur- beyond cultivation, are often termed casual species. opean Union [14,15]. Next, if a species survives, escapes, and begins reprodu- In contrast to their negative impacts, non-native spe- cing without direct human intervention it is referred to cies offer large opportunities for ecologists to test funda- as established. Finally, we refer to a species as invasive if mental theory [16,17]. In many cases, established non- it spreads widely and causes measurable environmental, native species spread rapidly into new ecosystems. economic, or human health impacts. These species interact with native species through com- Although the process described above is common to petition, predation, herbivory, introduction of disease, all invasions, different terminologies have been pro- and resource use. When carefully studied, the effects of posed, and these are often associated with different taxa these perturbations allow insight into basic ecological or regions. For example, botanists may speak of an inva- processes. For example, community ecology seeks to sive plant as one that exceeds some predetermined rates understand why certain species communities exist (or of spread, whether or not it causes negative impacts (e. do not), and how species within those communities g. [19]). While we acknowledge the range of terminolo- interact. Because invasions involve new species entering gies available, throughout this paper we use the defini- communities and interacting with resident species, they tions given in the previous paragraph. offer insight into these patterns and processes. The very The proportion of introduced species that become real need to better understand the process of invasion established can be quite low, as can the proportion of so that native habitats can be conserved, combined with established species that spread and become invasive. opportunities to test fundamental ecological processes, These proportions vary with the taxonomy of the spe- has led to huge recent growth in the field of invasion cies in question, and the regions to which they are biology (e.g. [18]), including in Europe. introduced. The ‘tens rule’ was proposed by Williamson The goal of this paper is to review the ecology and [3] as a rule of thumb to approximate the proportion of current status of non-native species in Europe, and to species that make it through each step in the invasion outline a framework for policy to reduce their number process. This rule holds that approximately 10% of and impacts. We begin with a description of the inva- introduced species will become established, and approxi- sion process through which non-native species are intro- mately 10% of those species will become invasive. duced, become established, and may then go on to have Hence, if 100 alien species were introduced, the tens negative impacts. This structures the sections that follow rule holds that one will become invasive. The tens rule which cover the pathways of species introduction, the was developed with a focus on terrestrial plants, but has characteristics of habitats and species that lead to estab- been applied by other authors to a wide range of spe- lishment, the number of non-native species in Europe, cies, often without thoroughly evaluating its validity. For and their impacts. Finally, we describe some characteris- example, recent studies show that for many animal taxa, tics of species invasions that create unique challenges the percentage of introduced species that become estab- compared to other environmental issues. We propose lished and the percentage of established species that some general principles to govern the policy response to become invasive can exceed 50% [5,21]. According to the risks from invasive species. Throughout the paper, the definition of invasive given above, Europe now con- we discriminate among three groups of organisms: ter- tains >100 terrestrial vertebrates, >600 terrestrial inver- restrial animals, terrestrial plants, and aquatic