On Biodiversity in Grasslands: Coexistence, Invasion and Multitrophic Interactions
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Zurich Open Repository and Archive University of Zurich Main Library Strickhofstrasse 39 CH-8057 Zurich www.zora.uzh.ch Year: 2009 On biodiversity in grasslands: Coexistence, invasion and multitrophic interactions Petermann, J S Abstract: In a rapidly changing world suffering from extensive diversity loss, the most pressing questions remain largely unanswered: how can diversity exist in the first place and what are the consequences of its decline for ecosystems? In grasslands, resource niches have to date been considered the major mechanism responsible for plant coexistence and diversity. The neutral theory has recently challenged this view by attributing species coexistence solely to stochastic processes. Whereas the negative effects of plant diversity loss on primary productivity have been demonstrated numerous times in biodiversity experiments, its effects on higher trophic levels have rarely been explored. Here, we used aglasshouse experiment, simulation modelling approaches and field studies in the Jena biodiversity experiment to examine diversity maintenance, invasion and community assembly in plant communities and effects of plant diversity loss on higher trophic levels. Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-24943 Dissertation Published Version Originally published at: Petermann, J S. On biodiversity in grasslands: Coexistence, invasion and multitrophic interactions. 2009, University of Zurich, Faculty of Science. On biodiversity in grasslands: Coexistence, invasion and multitrophic interactions Jana S. Petermann Die vorliegende Arbeit wurde von der Mathematisch-naturwissenschaftlichen Fakultät der Universität Zürich im Herbstsemester 2009 als Dissertation angenommen. Promotionskomitee: Prof. Dr. Bernhard Schmid (Vorsitz) Prof. Dr. Christine Müller Prof. Dr. Jasmin Joshi Prof. Dr. Wolfgang Weisser Dr. Gerlinde de Deyn I dedicate this thesis to my parents, the real biologists. And to Douglas Adams (1952-2001). CONTENTS GENERAL INTRODUCTION …………………………………………………. 5 CHAPTER 1 Janzen-Connell effects are widespread and strong enough to maintain diversity in grasslands Petermann JS, Fergus AJF, Turnbull LA, Schmid B (2008), Ecology 89, 2399-2406 …………………………………………... 21 CHAPTER 2 Species diversity reduces invasion success in pathogen- regulated communities Turnbull LA, Levine JM, Fergus AJF, Petermann JS, Oikos in press ……………………………………………………………… 37 CHAPTER 3 Biology, chance or history? The predictable re-assembly of temperate grassland communities Petermann JS, Fergus AJF, Roscher C, Turnbull LA, Weigelt A, Schmid B, Ecology in press ……………………………………..... 53 CHAPTER 4 Effects of plant species loss on aphid–parasitoid food webs Petermann JS, Müller CB, Weigelt A, Weisser WW, Schmid B, submitted to Journal of Animal Ecology …………………............. 91 CHAPTER 5 Plant species loss affects life-history traits of aphids and their parasitoids Petermann JS, Müller CB, Roscher C, Weigelt A, Weisser WW, Schmid B, Manuscript ………………………………………......... 123 GENERAL DISCUSSION ……………………………………………………… 149 SUMMARY ……………………………………………………………………….. 165 ZUSAMMENFASSUNG ……………………………………………………….. 169 ACKNOWLEDGEMENTS ……………………………………………………... 173 ADDITIONAL REFERENCES ………………………………………………... 177 CURRICULUM VITAE ………………………………………………………… 179 GENERAL INTRODUCTION "See first, think later, then test. But always see first. Otherwise you will only see what you were expecting. Most scientists forget that." (Douglas Adams) 5 GENERAL INTRODUCTION Biodiversity: how to coexist? "...., if there are really 30 or 40 million animal species, why didn't just a thousand evolve-or a billion?" (Wilson 1985) We know that biodiversity is currently declining at an unprecedented rate (UNEP 2008) and most people have acknowledged that we are largely responsible for this decline (Vitousek et al. 1997). Nonetheless, we have to date been unable to explain why this vast biodiversity exists in the first place. For example, "why are there so many species of herbivorous insects in tropical rainforests?" (Novotny et al. 2006), why 3,000 species of sea slugs in the world's oceans (Willan and Coleman 1984) or why several hundred to a thousand invertebrate species in a square meter of soil (Anderson 1975)? Many of these species are of similar appearance, have seemingly comparable life-styles and use a similar set of resources. So, how do they manage to coexist? This question lies at the very heart of Ecology and has intrigued naturalists from early on. The first studies of species coexistence attempted to assign unique jobs and living spaces even to very similar species. For example, Darwin (1859) described specific "lines of life" of each species and proposed that new species can evolve even in a common environment with related species. One of the first explicit references to the niche concept, which has since been widely used to explain species coexistence, is ascribed to Grinnell (1917). He wrote: "It is, of course, axiomatic that no two species regularly established in a single fauna have precisely the same niche relationships." An analogous concept was put forward by Charles Elton (1927). Both Grinnell and Elton viewed a species` niche mostly as its specific habitat (Schoener 1984), while Hutchinson (1957) defined the niche as an n‑dimensional hypervolume and included interactions between species into his niche concept, distinguishing between fundamental and realised niches. Today's niche models are predominantly based on resource competition with species occupying certain positions along several resource axes (e.g. MacArthur and Levins 1967) and their distance on these axes limiting the species` coexistence (the theory of limiting similarity, MacArthur and Levins 1967, Abrams 1983). One of the most cited models in this context is Tilman's R* model (1982), which predicts that species coexist if they are limited by different resources and each of them consumes most of the resource that most strongly limits its growth. After a first climax, the niche concept became unfashionable for some time, partly because of the lack of appropriate null hypotheses (Simberloff and Boecklen 1981, Lewin 1983, Chase and Leibold 2003), with some scientists even considering it "good practice to avoid the term niche whenever possible" (Williamson 1972). However, the concept 6 GENERAL INTRODUCTION experienced another recent rise (Chase and Leibold 2003, Leibold 2008) after the emergence of the neutral theory (Hubbell 2001, see section "Community assembly and invasion" below). Besides niches, other coexistence mechanisms have been proposed and tested. One example are trade-offs between species` traits that essentially equalise the inherent (i.e. density-independent) fitness of different species (Chesson 2000), preventing the emergence of a "super-hero species" (Rosenzweig 1995). These trade-offs can operate, for instance, between competition and colonisation abilities (Tilman 1994) or between competition and predator avoidance (Rochette and Grand 2004). Conversely, density-dependent coexistence mechanisms do not equalise the fitness of species but rather stabilise fitness differences between species via negative feedbacks. These negative effects only set in and limit a species with higher fitness when it becomes too abundant, preventing it from outcompeting others. Effectively, classical resource niches are such a stabilising mechanism (Chesson 2000). Another example for a stabilising mechanism is the Janzen-Connell effect. This hypothesis originally intended to explain the vast species diversity of coral reefs or tropical forests (Janzen 1970, Connell 1971, Augspurger and Kelly 1984). It proposed that adult organisms (corals or trees) accumulate species-specific predators or herbivores, thus reducing the survival or vigour of conspecific offspring in their vicinity. As a result, other species are enabled to establish, promoting the coexistence of many species. In fact, a similar effect was reported for temperate forests by a French forester as early as 1905 who referred to it as the "alternation of species" following the death of adult trees (Schaeffer and Moreau 1958, cited in Fox 1977). However, the Janzen-Connell effect has always been somewhat elusive and its actual importance as a coexistence mechanism has not been clearly demonstrated despite the immense effort in tropical forests (Hyatt et al. 2003, Gilbert 2005). Community assembly and invasion "Do natural communities assemble deterministically or does history play a strong role? …The answer…seems to be that it depends." (Chase and Leibold 2003) The mechanisms promoting species coexistence in established communities are certainly closely related to the mechanisms governing initial community assembly and the invasion of new species into established communities. Again starting with Elton (1958), species invasions have been explained on the basis of vacant niches in resident communities. Similarly, Jared Diamond's (1975) "assembly rules" associate patterns of community assembly primarily with resource competition. These rules have been hotly debated, again due to the lack of testable 7 GENERAL INTRODUCTION null hypotheses (Connor and Simberloff 1979). It was then suggested that the history of species` arrivals might be much more important than deterministic niche-based rules (Hubbell and Foster 1986, Drake 1991). This idea was supported by the fact that community assembly processes and species coexistence within established communities could in many cases be explained without the complications