Stream hydrology limits recovery of riparian ecosystems after wolf reintroduction

Abstract

Efforts to restore ecosystems often focus on reintroducing apex predators to re-establish coevolved relationships among predators, herbivores and plants. The preponderance of evidence for indirect effects of predators on terrestrial plant communities comes from ecosystems where predators have been removed. Far less is known about the consequences of their restoration. The effects of removal and restoration are unlikely to be symmetrical because removing predators can create feedbacks that reinforce the effects of predator loss. Observational studies have suggested that the reintroduction of wolves to Yellowstone National Park initiated dramatic restoration of riparian ecosystems by releasing willows from excessive browsing by elk. Here, we present results from a decade-long experiment in Yellowstone showing that moderating browsing alone was not sufficient to restore riparian zones along small streams. Instead, restoration of willow communities depended on removing browsing and restoring hydrological conditions that prevailed before the removal of wolves. The 70-year absence of predators from the ecosystem changed the disturbance regime in a way that was not reversed by predator reintroduction. We conclude that predator restoration may not quickly repair effects of predator removal in ecosystems.

Can we save large carnivores without losing large carnivore science?

“…That wolves may not have been the cause of all the observed ecological changes in Yellowstone since the mid-1990s is argued by Kauffman et al (2010), Mech (2012) and others (e.g. Creel and Christianson, 2009;Winnie, 2012;Marshall et al, 2013;Marshall et al, 2014;Middleton, 2014;Peterson et al, 2014;see Table 2). The long term study of wolf-moose (Alces americanus)-habitat-climate relationships on Isle Royale illustrate the difficulties of attributing cause and effect even in very simple ecosystems (Vucetich and Peterson, 2004).…”

“…Environmental changes following the restoration of wolves to Yellowstone National Park are often given as a clear example of the beneficial effects of restoring large carnivores to ecosystems (Table 2), but there are alternative hypotheses to explain many of the observed changes (Vucetich et al, 2005;Marshall et al, 2013;Middleton et al, 2013b). There is strong evidence that wolves alone are not responsible for all the changes attributed to them (Mech, 2012;Winnie and Creel, 2017).…”

“…ansiotropic vs isotropic effects; sensu Ford and Goheen, 2015). Simply re-establishing or bolstering large carnivores may not fix the many environmental problems that occurred as a result of (and/or in addition to) carnivore extirpation (Marshall et al, 2013;Marshall et al, 2014;Wikenros et al, 2015). In some cases, food web structure and ecological context may have changed irreversibly (for whatever reason), some niches may no longer exist, and a carnivore's function in the new ecosystem might now be different from their previous function.…”

“…Indeed, several studies using strongly-inferential methods demonstrate that such top-down effects do not always occur, or if they do, they are far weaker than bottom-up processes (e.g. Gasaway et al, 1983;Boertje et al, 1996;Hayes et al, 2003;Vucetich and Peterson, 2004;Vucetich et al, 2005; Giordano, 2013;Marshall et al, 2013;Allen et al, 2014b;Ford et al, 2015a;Sivy, 2015; see also Schmitz et al, 2000;Bowyer et al, 2005;Sergio et al, 2008;McCoy et al, 2012;White, 2013;McPeek, 2014;Kuijper et al, 2017). Ford and Goheen (2015) showed that of five strongly-inferential experiments investigating large carnivores' roles, only two found evidence supporting the TCH.…”

Focusing Ecological Research for Conservation

“…Restoration of top predators can benefit biodiversity by alleviating the effects of mesopredators on prey (Ritchie and Johnson 2009), but recent evidence suggests that once predators have been removed, ecosystems can require a long time to recover even when predator reintroductions are successful (Marshall et al 2013). However, ecosystem recovery rates must be assessed in other systems in order to allow generalization.…”