Community assembly, stability and food web structure

Natural communities of species embody complex interrelationships between the structure of the interspecific interaction network, dynamics of species' populations, and the stability of the system as a whole. Studying these interrelationships is crucial for understanding the survival of species in nature. In this context, studying the food web (the network of who-eats-whom) embedded in each interaction network is particularly important because trophic interactions are the main channels of energy flow in all ecosystems. Using a combination of mathematical modeling and empirical data analyses, this study explores the interrelationship between food web structure and multi-species coexistence in local communities. Chapter 1 of this thesis places the overall dissertation study in context of the history of research on species interaction networks and food webs. In Chapter 2, I use a population dynamical model to show how the requirements of stable multi-species coexistence results in the emergence of specific, nonrandom configurations of food web structure during community assembly. These structural "signatures" can be used to empirically gauge the importance of interaction-driven dynamical stability constraints in natural communities. In Chapter 3, I extend the model analyzed in Chapter 2 by imposing biologically feasible constraints on its parameters. This is made possible by the allometric scaling between individual metabolism and body size, and the constraints on interspecific trophic interactions due to body size differences between pairs of interacting species. I show that, using this approach, it is possible to interlink three aspects of local communities that have typically been studied in isolation: the species' body mass distribution, the distribution of ratios of body sizes of consumer and resource species (e.g., predator and prey), and certain food web structural features. Some of these features have previously lacked explanatory models. Finally in Chapter 4, using empirical data from nine communities across a range of habitats, I test some theoretical predictions of the previous chapter. The results provide strong evidence that the food web structure of natural communities do indeed exhibit signatures of dynamical stability constraints, and that the model developed in Chapters 2 and 3 is successfully able to predict a number of empirically observed food web structural features.