It’s been 25 years. It was mid-January of 1995 when the first wolves arrived in Yellowstone National Park. The gray wolves arrived from Jasper National Park in Alberta, Canada and were the first to roam Yellowstone since the 1920s when the last pack was killed.
According to Doug Smith, the park’s senior wildlife biologist, as of January 2020 there are at last 94 wolves in the park, living in eight packs. In general, wolf numbers have fluctuated between 83 and 108 wolves since 2009.
Mentioning wolves can elicit fevered responses on both ends of the spectrum. There are those who ardently and vocally decry the reintroduction of wolves to the Yellowstone ecosystem. There are those on the other side who are just as vocal and just as ardent in support of the wolves’ return.
Regardless, in the 25 years since reintroduction, scientists have been very active studying many aspects of both direct and ripple effects due to the addition of this top-tier predator.
Early on, vegetation changes were noted as willows, cottonwoods and aspen returned. Elk avoided feeding in the wetland areas for fear of becoming wolf dinner. As a consequence, the vegetation in stream bottoms thrived.
The impacts on elk numbers in Yellowstone were initially quite dramatic. Basically, as wolf numbers went up, elk numbers went down. That was true for the first decade following reintroduction but, since then the predator-prey relationship has become more complicated. Grizzly bear predation on elk calves went up, possibly due to reduction in cutthroat trout because of the increase in lake trout in Yellowstone Lake. Bison numbers increased, likely due to reduced competition with elk for forage; and vegetation changes continued.
There’s new research, published in Feb. 2020 in the “Journal of Animal Ecology,” analyzing 20 years of data concerning predation by wolves on elk. In that 20 year span, Christopher Wilmers, a professor of environmental studies at the University of California, Santa Cruz, worked with Smith and other biologists to figure out which elk were getting killed by wolves.
The general assumption is “survival of the fittest” with weaker individuals becoming prey. Wilmers said they noticed something that seemed counterintuitive to this assumption.
“At the end of the winter, we’d see wolves killing the biggest, baddest male elk,” Wilmers said in an April 17, 2020 article by Joshual Rapp Learn in The Official eWildlifer of The Wildlife Society. “It seemed like something was going on – that there was a relationship between the time of year and the climate, and which type of animals they were killing.”
Predation on a bull elk has less impact on the elk population than if a mature cow elk, in the prime of its reproductive years, is lost. In other words, killing a bull affects the overall elk population less than killing a cow elk. Wolves taking bulls would have less impact on elk numbers compared to taking reproductive cows.
The 20 years of data collection included close inspection of the amount of fat in the bone marrow of the elk carcasses. Higher fat content meant better nutrition. The biologists found that nutritional condition of wolf-killed elk varied markedly with summer plant productivity, snow depths and the winter period.
In the year or two following poor vegetation growth when elk numbers were down, bulls were more likely to be killed by wolves instead of cows or calves. Wilmers said that, by late in the winter, bull elk may not be the fittest. They bulk up heading into the fall to woo the females and fend off the other bulls. All that fighting and chasing takes a toll. While they are running around, the cows are eating, preparing for the upcoming winter.
The result is that by late winter, the bulls run out of gas. They were more often taken by wolves in low snow years, though. I tough winters, all the elk deteriorated, making them all easier prey for wolves. By comparison, in low snow years, the bulls are comparatively less fit and become easier prey.
Elk abundance also comes into play; when elk numbers are up, more cows are killed simply because there are more of them.
The general conclusion is that climate may have an effect on predator-prey dynamics as weather becomes more variable. Wilmers concludes that snow depth in late winter, growing conditions in previous summers and the overall severity of winter can impact the elk population in Yellowstone over time.
For now, the findings could be used to better inform hunting quotas outside the park. The number of bull tags allowed could go up or down based on weather conditions.