Yellowstone Wild Wolf Photography

Scientists have been researching and studying the impacts on the Yellowstone ecosystem since re-introduction in 1995.

As the wolf population in the park has grown, the elk population, their favored prey, has declined. Prior to reintroduction, the EIS predicted that wolves would kill an average 12 elk per wolf annually. This estimate proved too low as wolves are now killing an average of 22 elk per wolf annually.[33] This decline in elk has resulted in changes in flora, most specifically willows, cottonwoods and aspens along the fringes of heavily timbered areas. Although wolf kills are directly attributable to declines in elk numbers, some research has shown that elk behavior has been significantly altered by wolf predation. The constant presence of wolves have pushed elk into less favorable habitats, raised their stress level, lowered their nutrition and their overall birth rate.[34]

Yellowstone wolf
Limited Edition of 50 Exclusive high-resolution Museum Quality Fine Art Prints. Photos copyright © Jess Lee

The wolves became significant predators of coyotes after their reintroduction. Since then, in 1995 and 1996, the local coyote population went through a dramatic restructuring. Until the wolves returned, Yellowstone National Park had one of the densest and most stable coyote populations in America due to a lack of human impacts. Two years after the wolf reintroductions, the pre-wolf population of coyotes had been reduced to 50% through both competitive exclusion and intraguild predation. Coyote numbers were 39% lower in the areas of Yellowstone where wolves were reintroduced. In one study, about 16% of radio-collared coyotes were preyed upon by wolves. Yellowstone coyotes have had to shift their territories as a result, moving from open meadows to steep terrain. Carcasses in the open no longer attract coyotes; when a coyote is chased on flat terrain, it is often killed. They feel more secure on steep terrain where they will often lead a pursuing wolf downhill. As the wolf comes after it, the coyote will turn around and run uphill. Wolves, being heavier, cannot stop and the coyote gains a large lead. Though physical confrontations between the two species are usually dominated by the larger wolves, coyotes have been known to attack wolves if they outnumber them. Both species will kill each other's pups given the opportunity.[35][36]

Coyotes, in their turn, naturally suppress foxes, so the diminished coyote population has led to a rise in foxes, and "That in turn shifts the odds of survival for coyote prey such as hares and young deer, as well as for the small rodents and ground-nesting birds the foxes stalk. These changes affect how often certain roots, buds, seeds and insects get eaten, which can alter the balance of local plant communities, and so on down the food chain all the way to fungi and microbes."[37]

The presence of wolves has also coincided with a dramatic rise in the park's beaver population; where there was just one beaver colony in Yellowstone in 2001, there were nine beaver colonies in the park by 2011. The presence of wolves seems to have encouraged elk to browse more widely, diminishing their pressure on stands of willow, a plant that beavers need to survive the winter.[38] The renewed presence of beavers in the ecosystem has substantial effects on the local watershed because the existence of beaver dams "even[s] out the seasonal pulses of runoff; store[s] water for recharging the water table; and provide[s] cold, shaded water for fish."[39] Beaver dams also counter erosion and create "new pond and marsh habitats for moose, otters, mink, wading birds, waterfowl, fish, amphibians and more."[37]

Similarly, after the wolves' reintroduction, their increased predation of elk benefited Yellowstone's grizzly bear population, as it led to a significant increase in the growth of berries in the national park, an important food source for the grizzly bears.[40]

Yellowstone wolf
two packs of wolves facing off during breeding season.

Wolf kills are scavenged by and thus feed a wide array of animals, including, but not limited to, ravens, wolverines, bald eagles, golden eagles, grizzly bears, black bears, jays, magpies, martens and coyotes.[37]

Meanwhile, wolf packs often claim kills made by cougars, which has driven that species back out of valley hunting grounds to their more traditional mountainside territory.[37]

The top-down effect of the reintroduction of an apex predator like the wolf on other flora and fauna in an ecosystem is an example of a trophic cascade.