Food Web Structures Under Global Change

Abstract

Global change is threatening the stability and function of ecosystems by shifting underlying food web structures (i.e., rewiring food webs). The stability of ecosystems, in other words the ability of ecosystems to persist and thus maintain function in the face of environmental perturbations, is driven by food web structure. Food web structures are being rewired through alterations in spatial and temporal resource availability and consumer responses to changing conditions, such as climate change and landscape modification. Examining how changes in spatial and temporal resource availability and consumer responses shift in response to changing conditions can elucidate how they are altering food web structure, and ultimately influencing ecosystem stability and function. Throughout this thesis I explore how fundamental structures and fundamental characteristics of food webs that can contribute to ecosystem stability (i.e., omnivory (structure), portfolio effects and subsidies (characteristics)) are altered through various types of global change. My work contributes important knowledge towards understanding food web structure and stability outcomes under global change.

First, I combine theoretical and empirical approaches to develop a novel conceptual framework of dynamic omnivory. Omnivory, a common structure within food webs that describes feeding at more than one trophic level, has predominantly been considered a static (non-changing) trait in food webs. It is increasingly recognized that omnivorous consumers respond to changing relative resource densities through space and time, employing consumptive portfolio effects. I synthesize existing omnivory theory to develop novel theory incorporating dynamic omnivory and demonstrate the powerful stabilizing force of the dynamic omnivory structure in food webs in the face of changing conditions. I then synthesize empirical examples under the conceptual framework to demonstrate the ubiquity dynamic omnivory across ecosystems. Second, I identify a previously unconsidered aspect of subsidies, subsidy accessibility (ability of recipient species to access and thus forage on subsidies) that influence recipient food web dynamics. Using a combined bio-tracer approach I demonstrate how net-pen aquacultures can redistribute available resources by introducing a high-quality point-source subsidy, and how accessibility to this subsidy is a strong driver of recipient food web responses (trophic structure and biomass distribution). Lastly, I show how agricultural land-use is altering the seasonal availability of resources and consumer responses in streams, thereby altering abundance distributions and food web structure in ways that indicate increasing instability. Combined, this thesis demonstrates how changing conditions alter resource distributions and consumer responses that alter how key food web structures and characteristics contribute to stability of food webs under global change.