Does restoring apex predators to food webs restore ecosystems? Large carnivores in Yellowstone as a model system

Abstract

Modification of food webs is a frequent cause of shifts in ecosystem states that resist reversal when the food web is restored to its original condition. We used the restoration of the large carnivore guild including gray wolves (Canis lupis), cougars (Felis concolor), and grizzly bears (Ursus arctos horribilis) to the northern range of Yellowstone National Park as a model system to understand how ecosystems might resist reconfiguration after the restoration of apex predators to the food web. The absence of wolves, cougars, and grizzly bears for nearly a century from the northern range was the primary cause of dramatic changes in riparian plant communities. Willows (Salix spp.) were suppressed in height by intense browsing by the dominant herbivore, elk (Cervus canadensis). The loss of activity by beavers (Castor canadensis) coincided with the loss of tall willows. We hypothesized that intense elk browsing interrupted the mutualism between willow and beavers: ecosystem engineering by beavers was a critical component of willow habitat and tall willows were a critical component of habitat for beavers. This interruption made riparian communities resilient to the disturbance caused by the restoration of apex predators. We hypothesized further that reductions in elk browsing attributable to reductions in elk population size were not sufficient to prevent the suppression of willow growth. To test these hypotheses, we conducted a 20‐year, factorial experiment that crossed simulated beaver dams with the exclusion of browsing. We found that willows grew to heights expected for restored communities only in the presence of dams and reduced browsing. Willows experiencing ambient conditions remained well below this expectation. We found no difference in heights or growth rates of willows in experimental controls and willows in 21 randomly chosen sites, confirming that the results of the experiment were representative of range‐wide conditions. A reorganized community of large herbivores was implicated in the suppression of willow growth. We conclude that the restoration of large carnivores to the food web failed to restore riparian plant communities on Yellowstone's northern range, supporting the hypothesis that this ecosystem is in an alternative stable state caused primarily by the extirpation of apex predators during the early 20th century.

Reintroduced, but not accepted: Stakeholder perceptions of beavers in Germany

“…Examples of such keystone species are wolves (Canis lupus) (Laundré et al, 2001;Manning et al, 2009;Ripple & Beschta, 2003) and Eurasian lynx (Lynx lynx) (Kramer-Schadt et al, 2005;Linnell et al, 2009) that shape ecosystems via the landscape-of-fear effect, European bison (Bison bonasus) that change grasslands by grazing (Cromsigt et al, 2018) or beavers (Castor spp.) that alter entire landscapes through their dam-building activities (Hobbs et al, 2024;Law et al, 2017;Wright et al, 2002). The general aim of species reintroductions is to increase ecosystem resilience (Carroll & Noss, 2021) and create self-sustaining ecosystems (Brown et al, 2011) that oppose invasive human interference (Prior & Brady, 2017).…”

“…The general aim of species reintroductions is to increase ecosystem resilience (Carroll & Noss, 2021) and create self-sustaining ecosystems (Brown et al, 2011) that oppose invasive human interference (Prior & Brady, 2017). The effects of rewilding are still not comprehensively understood, however, and ongoing investigations seek to ascertain if they yield the expected benefits (Hobbs et al, 2024). Attempts to 'rewild' landscapes are often justified by pointing to habitat destruction due to colonization, industrialization and urbanization (Brown et al, 2011;Foreman, 2010).…”