Towards an understanding of complexity: How body sizes, preferences and habitat structure constrain predator-prey interactions

Many, if not all, ecosystems on earth are threatened by increasing human populations and resource-intensive economical growth entailing pollution, eutrophication and habitat fragmentation, to name some of the threats. As many of these ecosystems provide vital services for mankind there is an urgent need to understand how the populations and communities within these systems function, how their stability comes about and might be protected. Therefore we need more than knowledge about diversity (e.g., how many species are there) but, most critically, how the complexity of all interacting ecological entities (e.g., populations) is structured and constrained. To evaluate critical processes, the categorical, Linnean classification of biodiversity might hamper an ecosystem- wide approach resulting in generalised suggestions. Rather, we should address the interdependent dimensions of organisms' body-sizes and biomass flows as continuous variables being key to a better understanding of nature. Hence, this thesis was motivated by the findings of several recent and prominent studies that highlighted two aspects of community ecology: (1) There are general patterns in the body-size distributions within food webs that seem to prevail in ecosystems as different as a coral reef and a forest-floor. (2) These body-size relations in food webs have profound effects on the quality and quantity of the interactions that govern the flow of energy and nutrients within these webs and therefore are fundamental for our understanding of their dynamics and their stability. To investigate generalities in body-size effects on interaction strengths I performed various laboratory experiments where prey-density dependent feeding rates of terrestrial arthropod predators were examined under different experimental settings. Accordingly, a model framework was established on the base of taxonomic predator-prey pairs with distinct size-ratios that unravelled particular size dependencies on the fundamental parameters (i.e., biological mechanisms) of the interactions (Chapter 2.1.). Furthermore, the statistical modelling approach was tailored to incorporate these findings in a framework where exclusion of taxonomic information is the next possible step providing the opportunity to build a global null model for the interactions of many species- pairs: In Chapter 2.2. I have shown how allometric information alone explains a large fraction of the variation in feeding rates although more complex models comprising taxonomic and allometric information perform better. Nevertheless, for the analyses of a much more comprehensive dataset in Chapter 2.3. I decided to skip taxonomic information for the sake of clarity and showed far-reaching consequences on the community-level through the general existence of sigmoid response curves. One of the outstanding findings of this thesis is that these rules also comply with body-size relations of predator-prey pairs in natural food webs. The experiments with systematic variance in predator and prey body sizes providing the database for the chapters 2.1. - 2.3. were build upon the simplifying assumption of an idealised environment with one predator and only one prey species per replicate and a constant level of habitat structure. In contrast, I increased the complexity of the experimental setting in the concluding two chapters: In Chapter 2.4. I tested how predictions from single-prey experiments presented in Chapter 2.1. are suitable to interpret the outcomes of more complex experiments with two different(-sized) prey species. I found that in contrast to the allometric null models larger predators favour larger prey to an unanticipated extent therefore potentially contributing to the mix of weak and strong interactions stabilising empirical food webs. Finally I tested how the effects of changing habitat structure (i.e., leaf litter) affects predator-prey interactions particularly in dynamically changing habitats (chapter 2.5.). It could be shown that alternating amounts of litter were translated into a dilution effect impairing predator-prey encounter rates and thereby reducing the potential for top-down control in litter-dominated systems to a minimum. Altogether, the results in this thesis emphasise that the regular patterns of body-size distributions in nature are interdependent with the allometric constraints on the interactions that connect individuals and populations in food webs. Furthermore, active preferences towards larger, but usually rarer, prey together with habitat-structure effects might create the general framework where patterns of strong and weak feeding links promote the stability of natural communities. The diversity and intricacy of nature with millions of species connected by a multiple of interactions often leaves us ecologists with more questions than answers. Nevertheless, the approaches and results in this thesis suggest that complexity-reducing, quantitative model frameworks represent a suitable tool to understand how interactions are shaped and, accordingly, the functioning and stability of real ecosystems.