How Body Sizes, Preferences and Habitat Structure Constrain Predator-Prey Interactions
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Towards an understanding of complexity: How body sizes, preferences and habitat structure constrain predator-prey interactions Vom Fachbereich Biologie der Technischen Universität Darmstadt zur Erlangung des akademischen Grades eines Doctor rerum naturalium genehmigte Dissertation von Dipl.-Biol. Gregor Kalinkat aus Leipzig Berichterstatter (1. Referent): Prof. Dr. Ulrich Brose Mitberichterstatter (2. Referent): Prof. Dr. Nico Blüthgen Tag der Einreichung: 25. Mai 2012 Tag der mündlichen Prüfung: 20. Juli 2012 Darmstadt 2012 D 17 2 “Stuff your eyes with wonder . live as if you'd drop dead in ten seconds. See the world. It's more fantastic than any dream made or paid for in factories.“ Ray Bradbury, Fahrenheit 451 3 4 für meine Familie 5 6 Table of Contents 1.General Introduction................................................................................................9 1.1. Aims and scope................................................................................................9 1.2. Ecology: interactions and stability ..............................................................10 1.3. Predators and prey........................................................................................15 1.4. Optimal foraging theory ...............................................................................16 1.5. The ecological meaning of body size.............................................................17 1.6. Theoretical population ecology.....................................................................20 1.7. A short outline of this thesis.........................................................................25 1.8. Contributions to the included articles..........................................................26 2. Articles....................................................................................................................29 2.1. Allometric functional response model: body masses constrain interaction strengths.................................................................................................................29 2.2. Taxonomic versus allometric constraints on non-linear interaction strengths ................................................................................................................44 2.3. Generalised allometric functional responses facilitate predator-prey stability...................................................................................................................60 2.4.The allometry of prey preferences.................................................................88 2.5. Habitat structure alters top-down control in litter communities............108 3. General Discussion..............................................................................................122 4. Bibliography.........................................................................................................127 5.Summary...............................................................................................................148 6. Zusammenfassung...............................................................................................150 7. Appendix...............................................................................................................153 7.1. Curriculum vitae.........................................................................................153 7.2. List of publications and talks.....................................................................155 7.3. Ehrenwörtliche Erklärung.........................................................................157 7.4. Acknowledgements / Danksagungen .........................................................158 7 8 1. General Introduction 1.1. Aims and scope With the concepts, experiments, models and results presented in this thesis I have interwoven knowledge, ideas and approaches from all relevant hierarchical levels in ecology: While optimal foraging models from behavioural ecology focus on mechanisms at the very individual level there is the consumer-resource functional response serving as a bridge to population ecology in its classical sense. Above, the macro-ecological theme of the metabolic theory of ecology with its core element “allometric scaling” provided a general framework beyond detailed species-specific classifications. This gave the possibility to unify all these levels under the umbrella of community ecology by addressing questions about general rules that govern energy flows within natural food webs as well as the structure and stability of these fundamental entities of ecosystems. 9 1.2. Ecology: interactions and stability a) The science of interactions or how this thesis unfolds Before I go into the details of food-web theory and functional responses it seems worthwhile to recapitulate what the subject I have been dealing with is all about: “Ecology is the scientific study of the interactions that determine the distribution and abundance of organisms” (Krebs 2001). Although Häckel (1866) primarily introduced the term “ecology” almost 150 years ago, it was not until Krebs' definition (firstly formulated in the early 1970ies) that the central importance of interactions has been carved out word by word. Nevertheless, interactions have always been the focus of the scientific endeavours under ecology's roof including abiotic as well as biotic factors but also interactions at intra- and interspecific, or even at population or community levels. Consequently, the intriguing complexity that we find in nature has to be addressed by research aiming to understand the rules and patterns that shape the interaction networks connecting individuals and populations. Regarding our huge knowledge gaps about earth’s biodiversity (Mora et al. 2011) - even in well- sampled temperate zones (Creer et al. 2010) - the exploration of the vast number of possible interactor pairs in nature appears absolutely intractable. This implies that there is an urgent need for an alternative framework to understand individual and population interactions as well as ecosystem processes in a generalised and rather quantitative way as we cannot wait until all species on earth are determined.1 Nevertheless, ecologists of the past decades have managed to get new insights by the application of the scientific method comprising description, theory and experiment. Hence, observations and descriptions of patterns should serve as starting points for the formulation of hypotheses with the help of sound theoretical thinking and subsequent testing in experiments. New insights from 1 Obviously, I do not want to bad-mouth traditional taxonomic classification, which is definitely worthwhile and important to understand evolutionary and ecological processes and needs much more support than it gets nowadays (Boero 2010; Bacher 2012). It is rather that most, if not all, problems we are facing today need a diversity of approaches to tackle them and I think that quantitative, continuous data have many considerable advantages over categorical data like species lists. 10 experiments might then serve to sharpen the profile of the hypotheses and go back to the experimental (and the observational) arena again. Following this almost-trivial (sic!) scientific cooking recipe I built my thesis on the observations of regular patterns in body-size distributions in natural communities [(Elton 1927; Sheldon, Sutcliff, Jr., & Prakash 1972; Peters 1983; Cohen et al. 1993; Brose et al. 2006a); see chapter 1.5a]. The conceptual framework that then helps to interpret and understand this patterning is fostered by allometric scaling rules [(Kleiber 1932; Peters 1983; Brown et al. Box 1.1.: Glossary Allometric scaling: Describes the relationships of various biological traits and rates in dependence of an organism's body size. The fundamental principle follows simple geometric arguments, as surface to volume ratios do not increase (or decrease) linearly with changing sizes. Today, most allometric scaling relations are formally defined as power laws (i.e., linear in on log-scale). The most important allometric scaling in terms of population and community ecology is that of the metabolic rate as it is critical for the flow and dynamics of the fundamental biological currency: energy (see chapter1.5 and Box 1.3.). Consumer: Following a broad definition, any heterotroph organism. Consumers can be classified into functional groups (see Box 1.2.). This thesis focusses on true predators. Food web: The description of who eats whom in an ecological community. Functional response: The feeding rate of a consumer (i.e., predator) described as a function of the density of the resource (i.e., prey). The two fundamental parameters are the capture rate (alternatively: attack rate) and the handling time. Interaction strength: Generally, the net effect of one species on another species within a community. In the majority of cases, this implies direct trophic effects. Theory predicts that linear interaction-strengths cause highly unstable food webs and empirical evidence suggests that they usually are non-linear. Functional responses are widely-accepted as a way to describe non-linear interaction strength and to implement them in food web models. See Berlow et al (2004) for a detailed review on the topic. Metabolic rate: The amount of energy expended by an organism in a given time. Resource: Any organism that gets (partly or entirely) consumed by the consumer. The empirical data in this thesis includes resources that are herbivores, detritivores and omnivores that were