As mountain lions continue to expand eastward, there is growing concern about how the apex predator will affect their prey. In much of their North American range, mountain lions compete with grizzlies, wolves, and black bears for their prey. But what happens to the prey population when lions are the only apex predator?
To attempt to answer this question, a research team from the Black Hills of South Dakota identified an area where mountain lions were the only apex predator in the ecosystem. The team wanted to know if mountain lions were preying more or less on elk calves, and compare their results with elk populations where bears and wolves were also present. They set out to collar as many elk calves as possible, and document their cause of death to estimate how the elk population was faring.
The team collared a total of 184 elk, 125 of which were calves, between 2011 and 2015. The goal was simple: If one of the collared elk died, the team would go to the site of death and document any predator sign.
The results reveal a bleak but interesting picture. The elk calves were found to have a 26% chance of surviving, with mountain lions being the primary cause of death. However, as the elk calves grew older, their chances of survival improved greatly, indicating mountain lions left them alone once they grew too large and able to defend themselves.
This 26% chance at survival was well below the average survival of 35% reported in elk populations with multiple predators, i.e. systems with mountain lions, grizzlies, black bears, and wolves. These results indicate that when mountain lions are the only elk predator in an ecosystem, they prey on elk calves more aggressively.
In an interesting twist, the researchers reported the mountain lion’s favorite prey, the white-tailed deer, were doing exceedingly well. In another Black Hills study, deer made up 86% of their diet with white-tailed deer reported to have a population of 51,000 in South Dakota and increasing.
This led the researchers to conclude that the blossoming deer population was possibly supporting the mountain lion population at a higher than normal density. This would mean the struggling elk calves may be getting hit harder because of an abnormally large mountain lion population.
Based on these results, the two management options the research team suggests are to reduce the mountain lion population and increase the number of tags for white-tailed deer in an effort to decrease their population. The team offers that the second option is a more long term solution to improving the elk population.