UvA-DARE (Digital Academic Repository) Ecological effects of plant invasions van Hengstum, T. Publication date 2013 Document Version Final published version Link to publication Citation for published version (APA): van Hengstum, T. (2013). Ecological effects of plant invasions. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:01 Oct 2021 THOMAS VAN HENGSTUM van Hengstum, T. 2013. Ecological effects of plant invasions. PhD thesis, University of Amsterdam, The Netherlands. The research presented in this thesis was funded by the Dutch Organization for Scientific Research (NWO) as part of the ERGO program (838.06.111). ISBN: 978-90-821099-0-0 ECOLOGICAL EFFECTS OF PLANT INVASIONS Academisch proefschrift ter verkrijging van de graad van doctor aan de Universiteit van Amsterdam op gezag van de Rector Magnificus prof. dr. D.C. van den Boom ten overstaan van een door het college voor promoties ingestelde commissie, in het openbaar te verdedigen in de Agnietenkapel op woensdag 11 september 2013, te 10.00 uur door Thomas van Hengstum geboren te ‘s-Gravenhage Promotor Prof. dr. P.H. van Tienderen Co-promotor Dr. J.G.B. Oostermeijer Overige leden Prof. dr. S.B.J. Menken Prof. dr. J.H.D. Wolf Dr. A.T. Groot Prof. dr. G.R. de Snoo Prof. dr. J.C. Biesmeijer Dr. T.J. de Jong Faculteit der Natuurwetenschappen, Wiskunde en Informatica Chapter 1 7 General introduction Chapter 2 15 Impact of experimental plant invasions on plant-pollinator and plant-herbivore interactions Chapter 3 35 Contrasting effects of experimental plant invasions on invertebrate communities Chapter 4 57 Impact of plant invasions on local arthropod communities: a meta-analysis Chapter 5 77 Causes and consequences of plant introductions: insights gained from long-term monitoring Chapter 6 95 General discussion References 107 English summary 115 Nederlandse samenvatting 119 Acknowledgements 123 6 1 General introduction Due to growing human mobility the number of species introductions outside their native range has increased dramatically over the past decades, leading to a steep rise in the number of plant invasions (Davis 2009, Hulme 2009). There is great concern over the ecological and economic effects of invasive species on their introduced en- vironment. More recently, this concern has been further fuelled by the increased commercial cultivation of genetically modified organisms (GMOs), which are feared to exert even more harmful effects on the environment if they would become invasive (Ellstrand et al 2013; Andow and Zwahlen 2006). In this introduction I will explain how plant invasions may affect native biodiversity and ecosystem func- tioning. In particular, I will address the potential effects of invaders on plant-herbi- vore and plant-pollinator interactions, as well as on arthropod communities. What are invasive species? There is a lack of consensus on the definition of ‘invasive species’, and many different interpretations can be found in the literature (Colautti and MacIsaac 2004, Rich- ardson and Pyšek 2006). All include rapid expansion as characteristic of invasive species, but many also take detrimental effects on economy, ecology and health into account. This can be problematic, because some effects of invasive species are temporal, spatially restricted or may not have been noticed yet (Colautti and MacIsaac 2004; Strayer et al 2006). Moreover, some effects may be dramatic while others may be negligible. Therefore, in this thesis I use a definition modified from (Richardson and Pyšek 2006), which only describes the expanding behaviour of invasive species: “exotic or native species that produce reproductive offspring, often in large numbers, at substantial distances from the parent plants and therefore have the potential to spread over a considerable area”. While most plant invasions are the result of a human-mediated introduction outside their native range, native species may also develop invasive behaviour following large-scale habitat modification, predator- or herbivore removal or climate change (Carey et al. 2012). In addition, native species may become invasive as a result of hybridisation and introgression of genes from a related (wild or crop) gene pool (Ellstrand 2003). An example of a native invader is Lactuca serriola in the Netherlands, which was considered rare before the 1950s, but is currently found in most of the country, as evidenced by its presence in 60% of all 5x5 km grid cells in recent surveys. Climate change, ruderalisation and hybridization with crop lettuce, L. sativa, were discussed as possible drivers for its rapid expansion (Hooftman et al. 2006). 8 Chapter 1 / General introduction Fig. 1 Diagram illustrating the potential effects of plant invasions on invertebrate communities and different ecosystem functions. The outlined box indicates the focus of this thesis. Ecological effects of invasive plants By definition, invasive species change the composition of the invaded plant com- munities, which may reduce the density of resident natives, and potentially drive some natives to local extinction. Besides a direct competitive effect on resident natives, invasive species may cause indirect effects that may in turn affect ecosystem functioning (Mack et al. 2000, White et al. 2006). A well-known example includes the invasion by a Spartina hybrid (S. alterniflora × S. foliosa) on the west coast of the United States, which dramatically increased sedimentation rates and caused the system to shift from an algae-based to a detritus-based food web (Levin et al. 2006). Another example of an invader that caused significant indirect effects is Bromus tectorum in the Western United States, which increased fire frequency by producing highly flammable biomass and by connecting previously separated patches of shrubs (Keeley 2006). Many of the indirect effects of invasions involve plant-animal interactions, including plant-pollinator and plant-herbivore re- lationships, which often play an important role in ecosystem functioning. In the next sections I will elaborate on some commonly occurring effects of invaders on plant-animal interactions (Fig. 1). 9 Arthropod communities and herbivory Plant invasions can have significant impact on the composition, diversity and size of arthropod communities (French and Major 2001, Harris et al. 2004, Pearson 2009). It has often been hypothesized and sometimes been demonstrated that species richness and abundance of arthropods decreases following invasions (e.g. Greenwood et al 2004; Magoba and Samways 2011). At the same time, there are studies that found that arthropod communities increased in diversity or abundance (e.g. Ostoja et al. 2009, Pearson 2009). Thus far, it has remained unclear what are the actual factors influencing arthropod communities in response to invasions. Nevertheless, several possible drivers and mechanisms have been discussed in the literature: (i) exotic species are released from their natural enemies and therefore have fewer associated (specialist) herbivores than the natives they replace (Keane and Crawley 2002), (ii) the diversity of the plant community may be reduced (or enhanced) following invasion, providing local arthropod communities with less (or more) diverse resources and reduced (or enhanced) habitat complexity (Crooks 2002). Finally, (iii) arthropod community composition may be affected by environ- mental factors that are altered by the invader, such as water availability, irradiation and resource quality (Kimmins 2004; Levine et al. 2003). In response to changes in arthropod community composition, plant-herbivore interactions may be affected considerably (White et al. 2006; Weiser and Siemann 2004). Invaders may affect the abundance or distribution of herbivores due to processes including resource concentration and spillover (Blitzer et al 2012; Rand and Louda 2004). As a result, invaders may indirectly alter the consumption rate and population dynamics of co-occurring natives (Holt 1977). This indirect effect, known as apparent competition, has been commonly observed in nature (Orrock et al. 2010; Meiners 2007). Thus, co-occurring native species may not only be affected by direct competition with invaders, but also by herbivore-mediated effects. Plant-pollinator interactions Besides through plant-herbivore interactions, invaders may also indirectly affect the fitness of native species through disruption of native plant-pollinator networks (Bjerknes et al. 2007, Memmott and Waser 2002, Morales and Traveset 2009). In the presence of invaders, co-flowering natives may experience increased competi- tion for pollinators that are shared with the invader (Brown et al. 2002, Chittka and Schurkens 2001). Alternatively,