Entangling Ecology: Integrating Mechanistic Theories of Food Webs and Mutualistic Networks to Understand Function and Dynamics in Terrestrial Ecosystems

Abstract

Ecological networks describe the diverse and complex interactions between species that occur in nature. Studying these networks provides powerful conceptual and technical tools to understand how disturbances propagate through species-rich ecosystems. In particular, trophic and mutualistic interactions like predation and pollination co-occur in terrestrial communities, with feedbacks between them affecting both population- and ecosystem-level responses to global change. However, these different types of interactions have historically been studied separately, impairing our ability to predict and mitigate such responses. Yet, theory suggests that they share similar underlying mechanisms: ecological traits determine both the presence and the rates of trophic and mutualistic interactions between species. Furthermore, theory in both the trophic and mutualistic network traditions has tended to ignore variation in plant life histories, a mechanism which may be critical to understand the diversity observed in terrestrial ecosystems. In this dissertation, I develop ecological network theory that synthesizes mechanisms known to determine terrestrial species interactions and evaluate how these mechanisms affect structure and dynamics from the population- to ecosystem-scales. Specifically, I ask: How do mechanisms in terrestrial ecosystems affect (Q1) the dynamics of trophic networks, (Q2) the dynamics of mutualistic interactions, and (Q3) the interconnected structure of trophic and mutualistic networks? (Q1) I integrate previous models of the structure and dynamics of trophic and mutualistic networks into a common bioenergetic framework, where production and consumption, including on mutualistic rewards, scale allometrically with body size. I find that simulated ecosystems with pollination mutualisms are more diverse, stable, and productive than otherwise identical ecosystems with only trophic interactions. This is because mutualistic rewards increase and stabilize oscillations in productivity at the base of the food web, supporting a richer and more abundant community of animal consumers. (Q2) I review historical models of pairwise mutualisms and develop new consumer-resource models, focusing on mechanisms of plant reproduction. I find that mutualisms robustly exhibit stable coexistence at high density and destabilizing thresholds at low density. However, differences in interaction mechanisms can cause unique threshold dynamics, like an Allee threshold in plants under which they become too rare to attract sufficient pollinator visits to achieve outcrossing, causing population collapse. (Q3) I develop a model that uses species traits to simulate ecological networks composed of interconnected subnetworks of different interaction types. I find that trait correlations due to, e.g., allometry, constrain species’ functional roles within and between different subnetworks but that subnetwork structure is independent of potential trait constraints imposed by other interaction types. I also present a hyperrich network of ~250,000 trophic and mutualistic interactions among ~4000 species observed in terrestrial ecosystems at the University of Michigan BioStation according to public data, student projects, and expert opinion. I characterize how mechanisms of scale, including species size and structural complexity, trophic level, and taxonomic resolution, affect the overall network structure and species’ functional roles across interaction types. Critical ecosystem functions such as plant productivity and reproduction depend on both trophic and mutualistic interactions. This dissertation provides mechanistic insight into the structure, dynamics, and function of terrestrial ecosystems by accounting for the previously ignored integration of these interactions in ecological networks.