An apex predator is at the top of a food chain. It has no predators of its own. In 1872 when Yellowstone National Park was established, the gray wolf was a critical apex predator in the park’s ecosystem. But wildlife in the new park had no legal protections. Gray wolves were especially at risk.
From the time colonists began arriving in North America, wolves were objects of fear and hatred. With this mind-set, park officials and local residents began an aggressive program to kill them off. Wolves were trapped, poisoned, and shot. By the mid-1920s gray wolves had become virtually extinct within the park’s boundaries.
It didn’t take long for the effects of this policy to become apparent. Elk had been one of the wolves’ primary prey. Without wolves to control their numbers, elk populations rose significantly. Elk behavior changed, too. When wolves are nearby elk tend to be in motion a lot. They don’t spend much time grazing in any one area. Without pressure from wolves to keep moving the elk began settling down. They consumed large stands of young aspen and willow trees, preventing them from reaching full growth.
This consumption caused a cascade of negative effects on the park’s biodiversity, or the variety of life found in the area. Fewer numbers of mature trees meant fewer songbirds because they had fewer places to build their nests. The area’s beaver population was affected. Beaver need willow trees for food and to build dams. Without beaver dams streams became narrower and deeper as their flow rates increased. Because willow trees need relatively shallow, slow-moving water in which to put down their roots, the increased flow meant even fewer willow trees. As a result, much of the landscape changed from forested to grass- and shrubland.
Park rangers and naturalists became aware of the problem. They began a decades-long struggle to return some wolves to Yellowstone. The Endangered Species Act of 1973 was a major step in that direction. The list of endangered species included the gray wolf. Finally in 1994 the secretary of the interior accepted the recommendation to reintroduce wolves to Yellowstone.
Park officials moved quickly. They captured and tagged a number of wolves from western Canada. Hundreds of people turned out when the first eight wolves arrived at the park early the following January. Six more wolves followed soon afterward. Within a year a total of 31 had arrived at their new home. Officials placed them in temporary holding pens to allow the wolves to get used to their new surroundings. After several weeks the wolves were released into the wild.
The results soon became obvious. Elk had to keep moving because of the renewed pressure from wolves. That meant they had less time to forage among the aspens and willows. Many more trees were able to grow to their full size, which benefited beavers. When the wolves were reintroduced, a single beaver colony was located in the park. Within 15 years that number had increased to nine. The increase in colonies meant many more dams. In turn Yellowstone’s streams flowed more slowly, becoming shallower and broader. And the increased number of large trees provided shade for fish and homes for songbirds.
“It is like kicking a pebble down a mountain slope where conditions were just right that a falling pebble could trigger an avalanche of change,” said biologist Doug Smith, a key figure in the wolf restoration project. “In the entire scientific literature, there are only five or six comparable circumstances. What we’re seeing now is a feeding frenzy of scientific research.” Scientists had the rare opportunity to study what happens when an apex predator returns to an ecosystem.
While nearly everyone agrees that the reintroduction of the wolves has helped parts of Yellowstone restore its previous biodiversity, the benefits haven’t spread to the entire park. “You put the predator back, that’s great, but conditions have changed so much in the intervening decades that putting the predator back is not enough to restore the ecosystem,” said Tom Hobbs, an ecology professor at Colorado State University. “There’s not a quick fix for mistakes like exterminating apex predators.”
Yellowstone’s wolf reintroduction may have implications far beyond the park itself. Professor Hobbs added, “Maintaining intact ecosystems may be easier than fixing them after you’ve lost some of the parts.” So if other apex predators, such as lions, tigers, and even sharks, become endangered or extinct, it’s impossible to predict what the consequences might be. It serves to point out the interconnection of all species on earth.