Aimee Tallian, Douglas W. Smith, Daniel R. Stahler, Matthew C. Metz , Rick L. Wallen, Chris Geremia, Joel Ruprecht, C. Travis Wyman, Daniel R. MacNulty
‘Prey switching’ refers to a change in the diet of a predator as the abundance of two prey species changes. Prey switching is ecologically important because it can stabilize prey populations. When predators switch prey, they kill disproportionately more of an abundant prey species, and less of a rare prey species, which can release the rare prey species from predation. Prey switching has been observed in natural systems where prey are small and generally helpless when attacked by a predator (imagine lynx preying on hares and squirrels). Yet, we know little about the capacity of predators to prey switch in wildlife systems that include dangerous prey species that resist predation. We used long-term (1995-2015) data from northern Yellowstone National Park, USA, to understand how prey preference of a wild predator (wolves) responds to a shift in prey abundance, involving rising numbers of dangerous prey (bison) and falling numbers of relatively safer prey (elk). Our results suggest that wolves did not switch to bison as their relative abundance increased, but attacked and killed disproportionately more of the rarer, but safer elk. Wolves maintained a strong preference against bison, even when bison were more than twice as abundant as elk. There was also evidence that wolves were increasingly averse to hunting bison as relative bison abundance increased. Wolves seldom hunted bison because their hunting success was limited to a narrow set of conditions: larger packs (>11 wolves) chasing smaller herds (10-20 bison) with calves. Wolves scavenged bison carcasses instead, and did so more frequently as bison abundance increased. Our study demonstrates the importance of prey vulnerability to understanding the prey preferences of predators in systems with dangerous prey. The formidable defenses of such prey diminish the potential to prey switch. In these systems, shifting from hunting to scavenging is perhaps more likely than shifting prey preference. The assumption that predators prey switch, regardless of prey vulnerability, may overestimate the stability of prey populations in systems that include dangerous prey species
Wolves and bison. Photo provided by authors.