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Prey size plays surprising role in competition among wolves, bears and cougars

New research from the University of Minnesota upends long-held understanding about how wolves, bears and cougars—three of Yellowstone National Park's most iconic carnivores—compete for prey.

For years, scientists theorized that when prey becomes scarce, predators become more aggressive toward each other. It's a straightforward theory: fewer resources and more competition suggest that dominant predators—like wolves and bears—will steal food from subordinate predators—like cougars.

New findings, in Communications Biology, reveal that the size of prey animals plays a surprisingly pivotal role in competition patterns among predators.

The study is based on over two decades of data collected from a team of ecologists studying cougar predation in Yellowstone National Park. They found:

• As the availability of larger prey, like elk, declined over the past few decades, cougars in Yellowstone began targeting smaller animals, like deer. The smaller animals are easier to hunt, and cougars can kill and consume them more quickly while attracting less attention from more dominant predators.
• When cougars hunt smaller prey, wolves and bears are less likely to steal their kills.
• As cougars shifted to hunting smaller animals, the frequency with which wolves and bears displaced them from their kills decreased.

"This work really showcases the complex ways large carnivores make a living," said lead author Jack Rabe, a graduate student in the College of Food, Agricultural and Natural Resource Sciences.

"We found that shifting cougar behavior is likely a key factor in the ecosystem's stability. By opting for smaller prey, cougars reduce the amount of time they spend hunting to make up for kills lost to other carnivores. This allows them to maintain a relatively stable kill-rate, which in turn contributes to the overall balance of the predator-prey dynamics in the park."

These findings underscore the importance of maintaining a diverse array of prey species in ecosystems with large carnivores. In Yellowstone National Park, the abundance of different types of prey—from bison to elk to mule deer—helps keep competition in check and allows different predator species to coexist. In places where prey populations are dwindling or becoming more homogeneous, the balance between predators may become more fragile, leading to increased competition and potential disruptions in the ecosystem.

Partner organizations on this project include the Yellowstone Wolf, Cougar, and Elk Project.