Gray Wolf Reintroduction
Yellowstone National Park
Competitive interactions are not the only type of interrelationship that can occur between wolves and other organisms in Yellowstone. For example, elk carcasses left by wolves (after they have fed) provide food for magpies, ravens, golden eagles, bald eagles, coyotes and bear.
While the exact causative agent(s) responsible for the regrowth of aspen in Yellowstone is still being studied, another ecological benefit from forest regrowth is the resources it provides to other species. For example:
1. Beaver have returned to Yellowstone National Park since the abundance of aspens has provided beaver with food, as well as a wood for their dams and lodges.
2. Beaver ponds have created habitat for aquatic and semiaquatic life (e.g., muskrats, ducks, geese, frogs, turtles, etc.).
3. The shade conditions on nearby streams have helped to cool water so that it will be more conducive to trout.
4. Soil stabilization by aspen trees has reduced erosion and enhanced water quality.
1. Beaver have returned to Yellowstone National Park since the abundance of aspens has provided beaver with food, as well as a wood for their dams and lodges.
2. Beaver ponds have created habitat for aquatic and semiaquatic life (e.g., muskrats, ducks, geese, frogs, turtles, etc.).
3. The shade conditions on nearby streams have helped to cool water so that it will be more conducive to trout.
4. Soil stabilization by aspen trees has reduced erosion and enhanced water quality.
Elk foraged on certain species of trees such as aspen (depicted in the above diagram). This reduced the ability of the trees to survive and repopulate the forest. Erosion along streams reduced water quality.
When wolves hunted in these forests, elk were vulnerable to capture. Therefore, they moved to open fields where their ability to see approaching wolves improved. Aspens were able to survive and regenerate, thereby encouraging the population growth of these trees.
Another example of how the presence of wolves may be affecting the distribution and abundance of species is the regrowth of certain tree species in Yellowstone. When wolves were absent from Yellowstone National Park, the other predators that killed elk (shown on the previous page) were not able to control the population growth of elk. As a result, willow, cottonwood and aspen stands were heavily browsed by the elk. When wolves arrived, increased predatory pressure on the elk occurred. Some scientists believe that this forced elk to leave woodland areas and move into open habitat in order to be able to better see predatory wolf packs. This antipredatory behavior by elk reduced browsing pressure on the trees, thereby enhancing the ability of these trees to produce new trees. However, recent research indicates that while wolves have increased predation on elk, other factors helped to promote the growth of these trees.
This type of impact on a trophic level, lower on the food chain, that was created by the introduction of wolves is called a trophic cascade.
This type of impact on a trophic level, lower on the food chain, that was created by the introduction of wolves is called a trophic cascade.
Magpies feeding on an elk carcass
Ravens (55 of them) feeding at a wolf kill
An elk carcass being fed on by a coyote. Coyote abundance in the northern range of Yellowstone National Park decreased significantly (30 - 50%) following the reintroduction of wolves in the mid 1990s. Scientists now believe that coyotes have adapted to the presence of wolves, and their population size is increasing.
Chief Joseph wolf pack has surrounded a bull elk.
Ecosystem Changes
Credits: Wolves Surrounding Elk (Christine Smith, National Park Service) / Coyote Feeding on Elk, Magpies Feeding on Elk, Ravens Feeding at a Wolf Kill (Jim Peaco, National Park Service) / Aspen (Sasham | Dreamstime)
