Predator-Mediated Competition
Charles E. Kay. 2010. Predator-Mediated Competition: What happens when there is a second, alternative prey in a system? Muley Crazy, July/August 2010.
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In systems with a single predator and single prey, the predator cannot generally take the prey to extinction due to declining return rates — that is the predators usually starve to death before they can find the last few prey. So while mountain lions, for example, can have a negative impact on mule deer, the cats can only take the deer population so low before the lions begin to run out of food and increasingly turn to killing each other. But what happens when there is a second, alternative prey in a system? Counter intuitively, the additional prey species does not buffer, or reduce, the predation pressure on the first prey animal. Instead, fueled by alternative prey, the predator takes the more vulnerable species to even lower levels. This is called predator-mediated or apparent competition and where this occurs habitat and habitat improvements are largely irrelevant, contrary to what most biologists would have you believe.
A classic example of predator-mediated competition is now playing itself out in Yellowstone National Park. For over 60 years, 600 to 700 food-limited elk wintered in the thermal areas along the Firehole, Gibbon, and Madison Rivers in the west-central portion of the park. With the arrival of introduced wolves, however, the elk population began a precipitous decline with researchers predicting extinction — see The Ecology of Large Mammals in Central Yellowstone. The wolves have been able to do this because they have bison as an alternative prey. In fact, if the elk did not have a partial refugia by fleeing into the depths of the Madison River when confronted by wolves, the elk would already be extinct. The habitat is still there, after all this is a national park, but the elk are all but gone.
Similarly, moose-fueled wolves are in the process of eliminating mountain and woodland caribou across the length and breadth of Canada. While in Alaska, wolves fueled by salmon, yes salmon, have taken black-tailed deer, moose, and caribou to very low levels — much lower then if the wolves did not have salmon as an alternative prey. In Nevada, mountain lions that prey on wild horses have a much greater impact on mule deer than cougar populations without feral equines as alternative prey. It has also been reported that mountain lions have taken bighorn sheep to near extinction on several western ranges where the cats subsist on alternative prey.
In many parts of the West, white-tailed deer and mule deer are sympatric; that is the two species occupy the same areas. Researchers in Alberta have identified predator-mediated competition as a key reason mule deer are declining. Due to behavioral differences, mule deer are more vulnerable to coyote predation than are whitetails. But by preying on both mule deer and whitetails, the coyotes are able to exert much greater predation pressure on mule deer, then if mule deer were the canids only prey. Again the addition of a second prey species, whitetail deer, allowed the predator, coyotes in this case, to have a much greater impact on the more vulnerable prey, mule deer.
While in British Columbia, predator-mediated competition between whitetails, mule deer, and mountain lions has been documented. Again, mule deer are the more vulnerable prey, but by subsisting mainly on whitetails, the cats are able to take mule deer populations to very low levels — much lower than if whitetails were not present. Whitetail-fueled cougars have also been identified as the factor driving British Columbia’s southern, mountain caribou to extinction. Similarly, in Canada’s Banff National Park, elk-fueled wolves have been instrumental in the elimination of both mountain caribou and moose.
Which brings us to the question of predator-mediated competition between ever-increasing numbers to elk in the West and declining mule deer populations. By subsisting on elk, could mountain lions be taking mule deer numbers even lower? Given the fact that mule deer are easier for cougars to kill than elk, predator-mediated competition is certainly possible. Although no one has specifically studied this problem, work that I have been doing for San Juan County in southeastern Utah does shed some light on this issue.
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