A functional ecology framework for understanding and predicting animal responses to plant invasion

Abstract

Plant invasions can alter food resources and habitat conditions that structure animal communities. These effects are negative for many native animals, but neutral or even positive for others. Understanding why we see this variation in responses is critical for mitigating invasion outcomes, yet we lack a synthetic framework to explain and potentially predict effects of invasive plants on native animals. We propose a trait-based framework for understanding how invasive plants affect native fauna, which draws on community assembly, niche, and trait theories to define the mechanisms by which invasive plants alter ecological conditions relevant to native animals. This approach moves beyond prior frameworks by explicitly accounting for the context dependency that defines most ecological interactions and invasion outcomes. Namely, by characterizing the plant community in terms of functional effect traits (e.g., seed size) relevant to consumers and quantifying those traits along a consumer resource axis, we can map the functional relationship between plant resources and animals. We can then delineate how plant invaders alter the plant community and associated resource axes to restructure consumer communities. We apply this framework to case studies of rodents, spiders, and birds to demonstrate the process and explore its utility. For example, we show that by focusing on how a nonnative grass altered seed sizes (relative to the native plant community), we can better understand declines in abundance of granivorous rodents and increases in opportunists. This approach can elucidate which native animals will be most likely affected by plant invasion, as well as how and why they might respond. Moreover, these mechanistic explanations provide working hypotheses for how invasive plants impact native animals more generally, with potential for predicting impacts of future invaders.