A 2013 film explains the importance of apex predators in simple, beautifully illustrated terms. Called “How Wolves Change Rivers,” the film describes the environmental changes that occurred when wolves, hunted out of Yellowstone National Park in the 1930s, were re-introduced to the park in the mid-90s. Although criticized by some as a bit over-simplistic, the film’s thesis is worth re-telling here as an example of the influence apex predators have on an entire ecosystem.
Since wolves had been gone from the park for nearly 70 years, the native elk had few natural predators and the elk population exploded. The unnaturally large population of elk had grazed much of the park down to bare roots and many of the park’s native plants were destroyed. When wolves were reintroduced, they naturally killed some of the elk. As a result, the elk began to avoid certain parts of the park where they were more vulnerable to wolves.
The native plants in the areas the elk avoided regenerated. When aspen, willow and cottonwood trees started to grow again, native birds returned. Beavers, who eat the bark of the trees and use the branches for lodges and dams, moved back in. They dammed the river, because that is what beavers do, causing ponds to develop and providing habitat for creatures like river otters, muskrat, fish and ducks.
The wolves also preyed on coyotes, which reduced their population and caused the population of rabbits and mice to increase. The return of small rodents—prey of animals like weasels, hawks and foxes—caused more of those species to move back into the park. Ravens and bald eagles returned to feed on the carrion left by the wolves. Bears, who also eat carrion, returned too.
But the ultimate changes in the ecosystem of the park were even more significant. Because of the reintroduction of wolves into Yellowstone Park, the nature of the rivers actually changed. When more native trees were allowed to grow along the banks of the rivers, the banks were reinforced and the channels of the rivers deepened. Rivers that had become wide and shallow with the absence of wolves became narrow and deeper, creating habitat for creatures that had not been there in decades.
A recent review of “How Wolves Change Rivers” summed up this “trophic effect” eloquently: “Human/wildlife conflict is a reality of growing populations around the world, and the fact is that we need to learn to live beside wildlife if we are to maintain our wonderful thriving ecosystems in the future.”