How can adding wolves to an environment make more trees grow? One answer: something called the “ecology of fear.”

In the Lamar River Valley in Yellowstone National Park, there was a famous example of the ecology of fear playing out with three species: aspen trees, elk, and wolves. In this example, aspen trees were the main prey of elk, and elk were the main prey of wolves.

This story starts in the early 1900’s. Back then, due to hunting, wolves had entirely disappeared from the park. Without wolves, or predation, elk roamed freely, eating young aspen trees and other shrubs. The elk population grew rapidly and over time they started to deplete the aspen population until the trees were in serious decline.

Nearly 70 years after the wolves’ disappearance, there was a big scientific and political movement to reintroduce wolves into Yellowstone. It succeeded, and in 1995 wolves were brought back. Curiously, after the wolves returned, the aspen population began to recover.

This could have happened because the wolves were eating a lot of elk, meaning there were less elk eating the aspen, but scientists in the early 2000’s thought that this was not the case. Elk are large prey, weighing between 500 and 700 pounds. It takes wolves a lot of time and energy to track, hunt, chase, and eat elk. It was estimated that on average a wolf caught only one elk each month in the winter. This suggested the wolves weren’t eating enough elk to explain the substantial decrease in aspen grazing.

Instead of changing elk numbers, many scientists thought the wolves were changing elk behavior. Elk became fearful of grazing in open areas where they were vulnerable to wolves. They also spent more time watching out for wolves instead of eating aspen. This fear made elk graze less, and differently, choosing to stick only to areas that weren’t too exposed. Over time the reduction in predation allowed aspen trees to recover in the park.

While the wolves, elk, and aspen were one of the first examples of this ecology of fear, more recently scientists have suggested that there might be a lot of other factors contributing to the recovery of the aspen. However, the behavioral effects of predation risk remains an important idea that is used to study many biological systems.