Global change effects on the stability of food-web motifs
von Florian Schwarzmüller
Datum der mündl. Prüfung:2015-03-26
Betreuer:Prof. Dr. Ulrich Brose
Gutachter:Prof. Dr. Ulrich Brose
Gutachter:Prof. Dr. Stefan Scheu
EnglischGlobal change affects ecosystems worldwide and has already caused a massive decline in the world's biodiversity. As the processes behind environmental change continue at ever-accelerating rates, this leads to a severe threat of ecosystem functioning, ecosystem services, and, in the end, human well-being. The most prominent drivers of global biodiversity loss are climate change, increasing nitrogen deposition, land-use change and biotic exchange. Their correlation with species extinctions has been documented in numerous studies some of which have identified the underlying mechanisms they operate on. However, it still remains difficult to predict the exact effects of specific drivers of environmental change on populations. This makes it hard to identify particularly endangered species and to develop adequate conservation strategies. In my thesis, I focus on small-scale effects of global-change drivers on single individuals or populations. I use bioenergetic modelling to show how these low-level effects scale up to higher levels of ecological organisation and influence the stability of food-web motifs. Finally, I provide experimentally testable hypotheses on environmental-change effects and their compensation. Throughout the research chapters of my thesis, I study the effect of different environmental-change drivers on the stability of different trophic motifs. In Chapter 2, I focus on single consumer-resource interactions and how environmental warming influences their stability. The relationship between temperature and species' biological rates (metabolism, growth and feeding) is well-known from empirical warming experiments. However, their interactive effects on the stability of consumer- resource systems are still under debate. I show that warming leads to dynamic stabilization of biomass oscillations. These results are based on an extensive literature research about temperature scaling of metabolism, feeding rates and maximum population size. Implementing these relationships into a generalized bioenergetic model yields information on the dynamical consequences of the different scaling relationships. The vast majority of possible parameter combinations predicts a dynamic stabilization of consumer-resource interactions at the risk of predator starvation. Consequently, this is tested in a microcosm experiment using bacterial prey (Pseudomonas fluorescens) and a cilliate predator (Tetrahymena pyriformis). Time-series analyses of these experiments confirmed the hypothesis of warming leading to an increased population stability while, at the same time, undermining species diversity. In Chapter 3, I investigate the effect of nutrient enrichment which has been reported to induce unstable dynamics in consumer-resource systems. The resulting oscillations have been shown to endanger species persistence in trophic systems of low complexity. However, in more complex natural systems this effect seems to be dampened which indicates that some intrinsic properties of complex systems prevent unstable dynamics. Identifying these “ecosystem buffers” is crucial for our understanding of the stability of ecosystems and an important tool for environmental and conservation biologists. Earlier theoretical studies suggested that this stabilization might be caused by so-called “weak interactions”. However, their relevance has rarely been tested experimentally. I use network and allometric theory for an a-priori identification of species that buffer against externally induced instability of increased population oscillations via weak interactions. Afterwards, the hypotheses are tested in a microcosm experiment using a soil food-web motif. I show that large-bodied species feeding at the food web's base, so called “trophic whales”, can buffer ecosystems against unstable dynamics induced by nutrient enrichment. In Chapter 4, I investigate the combined effects of habitat fragmentation and nutrient enrichment as they occur under increasing land-use intensity. Moreover, this chapter tackles the challenges of an integrative ecological theory on how different drivers of global change interact. I thus study the combined effects of habitat isolation and nutrient enrichment on the stability of a tri-trophic food-chain. Therefore, I expand bioenergetic models towards spatially explicit systems of two habitat patches using empirically- derived allometric scaling relationships of animal migration. I find that extinctions that occur at high levels of habitat fragmentation are caused by reduced bottom-up energy supply. Thus, conservation activities that focus only on single species might not prevent biodiversity loss if they ignore the respective lower trophic levels. The starvation effects of isolation are counteracted by nutrient enrichment which increases energy fluxes along the food chains. Thus, habitat isolation stabilizes eutrophic systems but undermines species diversity in oligotrophic systems. The three research chapters provide good examples of how a generalized bioenergetic modelling approach provides an in-depth understanding and can generate testable hypotheses on the behaviour of simple trophic systems under global change. The general findings are combined and discussed in the Synopsis which also provides a categorization of environmental stressors according to their respective influence on ecosystem stability. The Synopsis elucidates the interplay of multiple environmental stressors and how their combined effects endanger biodiversity. In an ever changing world, our understanding of ecosystem processes and their underlying mechanisms is of striking importance. This conceptual work will foster future research by (1) applying general modelling tools to investigate the effects of different environmental stressors, (2) testing the generated hypotheses in experimental systems, and (3) synthesizing the findings according to their respective influence on systems stability. Furthermore, it will contribute to new and well-founded conservation approaches.
Keywords: stability; global change; food-web motifs; theoretical ecology; energy fluxes; climate change; enrichment; habitat fragmentation