By Max Allen of the Santa Cruz Puma Project
Pumas and black bears are the two large carnivores found throughout California. Both species kill deer and other ungulates and as a result they often compete with each other. In Mendocino National Forest, where I completed my PhD project, black-tailed deer, including adults and fawns, make up the vast majority of puma diets. In contrast black bears only prey on fawns, although fawns can make up a large part of their diet during spring and summer. Since pumas prey on both adults and fawns year-round while bear predation is restricted to young fawns, it would be easy to assume that pumas are more responsible for the dynamics of ungulate populations than bears. However, this is leaving out an important part of the story.
While bears do hunt and kill prey themselves, they also frequently scavenge dead animals that have been killed by other predators, such as pumas, or that have died from other causes. Bears’ strong sense of smell helps them locate carrion, while their large size make them a dominant scavenger as they can both usurp and defend the carcass from other animals. One focus of my Ph.D. research was trying to understand how bear scavenging affects puma feeding behavior. In order to do this I visited puma kills to determine how often bears were present, and I placed cameras on fresh puma kills and compared behaviors at those with and without bears.
A black bear feeding on a deer it usurped from a puma (Video by Max Allen)
What I found surprised me: black bears were having large detrimental effects on pumas. Based on camera data, pumas abandoned over 70% of their kills as soon as bears arrived (Allen et al. 2015). Furthermore, because bear activity is highly seasonal, puma killing and feeding behavior exhibited an interesting seasonal pattern. In the graph below, the time pumas spent at their kills (handling time) is noted in red, while the frequency of how often they killed (kill rate) is in blue. During summer and autumn, the seasons of high bear activity, pumas spent half as much time at their kills of adult deer compared to winter. This is likely due to bears usurping their kills, and as a result, pumas nearly doubled their kill rates during those seasons (Allen et al. 2014). This suggests that bear scavenging significantly effects puma kill rates and feeding behavior.
Because bears regularly usurped puma kills and therefore reduced the energy available to them, I expected pumas to try to limit or avoid scavenging by bears. One option would be for pumas to be more aggressive in defending their kills. Although I did document a puma killing a bear that was feeding on its kill (Allen et al. 2015), this did not appear to be a commonly used tactic. Another frequently used tactic is for subordinate species to shift their habitat use to areas not used by the dominant species, in order to find a refuge from competition. We analyzed this, but found that no matter in which habitat pumas killed ungulates, bears located and usurped the puma kills (Elbroch et al. 2015). This suggests that spatial refuges from bears do not exist, and thus, it is likely that the seasonal refuge, when bears are hibernating, is the only refuge from bear scavenging and competition for pumas.
In addition to usurping carcasses and forcing pumas to make more kills, I suspect that bears are also having a negative impact on puma populations by directly killing kittens. I was only able to find limited evidence of this, however, field biologists in the Yellowstone region have found that bears, both black bears and grizzlies, are important causes of mortality for puma kittens. This may be simply be bears killing in order to eat, as bears eat the cubs they kill, or it could be a mechanism to reduce the population of their competitors.
A young puma feeding at a deer killed by its mother. The kill was later usurped by a black bear (Video by Max Allen)
What does all of this mean for pumas in areas without bears? My current study area in Santa Cruz, California is only a few hours drive from my previous study area in Mendocino County, California. Santa Cruz does not have bears, and as you might expect puma densities are over five times higher. What is surprising is that people seem to have similar effects as bears. Pumas spend less time feeding at kills near houses, retreat further from their kills in these areas to bed down during the day, and as a result exhibit higher kill rates in more human-dominated areas (Smith et al. 2015). It seems that maybe pumas just can’t catch a break.
Puma ecology and feeding behavior is an ongoing area of my research. Keep up to date with my research and findings at facebook.com/thewildlives.
Allen, M.L., L.M. Elbroch, C.C. Wilmers, and H.U. Wittmer. 2015. The comparative effects of large carnivores on the acquisition of carrion by scavengers. American Naturalist 185: 822-833.
Allen, M.L., L.M. Elbroch, D.S. Casady, and H.U. Wittmer. 2014. Seasonal Variation in the Feeding Ecology of Pumas (Puma concolor) in northern California. Canadian Journal of Zoology 92: 397–403.
Elbroch, L.M., P. Lendrum, M.L. Allen, and H.U. Wittmer. 2015. Nowhere to hide: pumas, black bears, and competition refuges. Behavioral Ecology 26: 247–254.
Smith, J.A., Y. Wang, C.C. Wilmers. 2015. Top carnivores increase their kill rates on prey as a response to human-induced fear. Proceedings of the Royal Society B 282: 20142711.