Humanities › Geography Wolves and Beavers in Yellowstone National Park The Reintroduction of 2 Animal Species Into Yellowstone National Park Share Flipboard Email Print A beaver eating bark from an aspen tree branch. Robert McGouey / Getty Images Geography Physical Geography Basics Political Geography Population Country Information Key Figures & Milestones Maps Urban Geography By Jackie Guz Updated July 27, 2018 The elimination of two animal groups from Yellowstone National Park changed the course of rivers and decreased plant and animal diversity. What two animals had such a big impact? Creatures that humans have long considered competitors and pests: wolves and beavers. Why Eliminate Wolves? It all started with good intentions. In the 1800s, wolves were seen as a threat to settlers’ livestock. Fear of the wolves also made it seem logical to eliminate them. Other predator populations such as bears, cougars, and coyotes were also hunted during this time in order to enhance other, preferred species. By the early 1970s, a survey of Yellowstone National Park showed no evidence of a wolf population. How Did a Lack of Wolves Change the Physical Geography of the Park? Without wolves to thin herds, elk and deer populations surpassed the park carrying capacity. Despite efforts to manage deer and elk populations, their preferred food sources of aspen and willow trees were decimated. This resulted in a lack of food for beavers and their populations declined. Without beaver dams to slow the flow of rivers and create appropriate habitat, water-loving willows nearly disappeared. The lack of shallow marshes created by beaver dams also decreased the quality of habitats for birds, amphibians, and other animals. Rivers became faster and deeper. Reintroduction of the Wolves The process to restore habitat conditions was made possible with the passage of the 1973 Endangered Species Act. The law compelled US Fish and Wildlife Service to reestablish endangered populations when possible. Yellowstone National Park became one of three designated recovery sites for the Gray Wolf. Amid much controversy, wolf reintroduction finally began in 1994 with the capture of wild wolves from Canada that were released in Yellowstone. A few years later, wolf populations stabilized and a wonderful story emerged about the restoration of park ecology. It was hoped that with reduced elk populations, beavers would have access to their favored food and return to create lush wetlands. The return of the previously maligned wolf would transform the ecosystem for the better. It was a wonderful vision and some of it has come true, but nothing is ever easy in the restoration of complex ecosystems. Why Yellowstone Needs to Have Beavers Come Back Beavers have not returned to Yellowstone for a simple reason — they need food. Willows are preferred by beavers for dam construction and nutrition; however, despite the decline in the elk population, willows have not been recovering at the pace predicted. The potential reason for this is a lack of the marshy habitat that favors their growth and expansion. Willows thrive in areas where soil is kept moist from regular flow of nearby water. Rivers in Yellowstone run faster and have steeper banks than they did during the era with beavers. Without beaver ponds and meandering, slow-flow areas, willow trees are not thriving. Without willows, beavers are less likely to return. Scientists have tried to resolve this dilemma by building dams that recreate beaver habitats. So far, willows have not spread into these man-made ponding areas. Time, rainy conditions, and still lower elk and deer populations may all need to converge before there will be mature willows to lure back a large beaver population. Yellowstone Wolf Restoration Still a Great Story The great debate over how fully wolves have restored Yellowstone ecology may go on for years, but scientists seem to agree that the wolves have improved conditions. Wildlife biologists have noted that endangered grizzly bears often manage to steal wolf kills. This could be critical if other food sources such as fish populations continue to decline. Coyote and foxes still thrive, but in smaller numbers; perhaps due to competition with wolves. Fewer small predators have allowed populations of rodents and other small mammals to recover. It has even been suggested that deer and elk health has improved because they must move more quickly and remain alert with wolves in the area. Wolves in Yellowstone Today The expansion of the wolf population has been amazing. In 2011, the US Fish and Wildlife Service estimated that there were about 1,650 wolves in Yellowstone National Park. In addition, the wolves were taken off the endangered-species list in Idaho and Montana. Today, the packs in Yellowstone range from two to eleven wolves. The size of the packs varies with the size of the prey. Wolves are currently hunted in areas surrounding Yellowstone National Park. The National Park Service is still monitoring the wolf population in the park and surrounding areas. Hope for the Beaver? Beavers are among the most persistent wildlife on the planet. Their reputation of nuisance comes from the challenge of discouraging them once they become attached to a stream or river. While they prefer willows, they can survive from other tree species, such as aspens. The National Park Service continues to monitor the beaver population. It is possible that over time the combination of reduced elk populations, improving aspens and willows, and a wet weather period could combine to create ideal conditions for their return.