Predicting community interactions under grizzly bear rewilding and anthropogenic change

Abstract

Rewilding is increasingly recognized as an impactful conservation strategy, but a key question remains: how do ecological systems respond to the return of species long absent from the landscape? Predicting these responses is challenging due to complex direct and indirect interactions, especially amid anthropogenic changes. The ongoing range expansion of grizzly bears (Ursus arctos) in western North America offers a unique opportunity to develop and test predictions about the effects of a large, generalist omnivore returning to its historic range. We developed a priori predictions that grizzly recovery would lead to (1) declines in sympatric large carnivores due to competition, (2) mesopredator release, (3) increased top-down control on large herbivores, and (4) stronger effects under anthropogenic stressors. Our fuzzy interaction webs (FIWs) supported these hypotheses, predicting that in habitats where grizzlies reach high density, black bears (Ursus americanus), mountain lions (Puma concolor), coyotes (Canis latrans), grey wolves (Canis lupus), scavenging birds, and ungulates may experience small population reductions through interference competition, exploitation competition, and predation. Small carnivores may increase, while reduced precipitation and human hunting of ungulates may intensify declines in mountain lions and ungulates. While FIWs offer a tractable framework for anticipating community change in complex, data-poor, multitrophic systems, they are still limited by data quality, assumptions of equilibrium dynamics, and the absence of spatial output. Nevertheless, FIWs serve as useful tools for generating testable hypotheses, identifying knowledge gaps, and guiding research and conservation efforts as species recover and ecosystems reorganize under global change.