The Positive Effects of Wolf Populations Flow into Rivers and Beyond

When learning about keystone species in high school biology classes, many of us discuss a case study looking at the wolf population in Yellowstone National Park--and for good reason. The decimation and reintroduction of wolves in Yellowstone has been a concrete example of how ecosystems often rely on a homeostatic balance of diverse species to thrive, and this was one of the first times in American environmental history that humans attempted to remedy the ecological disruption they caused by allowing nature to find its balance once again. Now, we are still seeing how the reintroduction of wolves has impacted the park today. A new study conducted by researchers at Oregon State University revealed an exciting find: the reintroduction of wolves in Yellowstone has changed stream morphology.

While wolves in Yellowstone have been the topic of many studies, recently published research has found that willow growth along the East and West Forks of Blacktail Deer Creek in the northern range of Yellowstone has increased from 2004 to 2017, indicating an improvement of the health in the riparian communities, or the space where terrestrial and aquatic ecosystems meet. In addition to observing increased willow growth, their research also found improved stream morphology and more beaver activity, which further helps streams recover. While these may seem like trivial findings, they are quite remarkable. Taking place over the course of thirteen years, this study was the first of its kind to suggest that the reintroduction of land-living carnivores can bolster stream function and development.

  Figure 1. Yellowstone National Park wolf numbers in early winter, 1995-2015. NPS

Figure 1. Yellowstone National Park wolf numbers in early winter, 1995-2015. NPS

When the wolf and cougar populations in Yellowstone bottomed out due to human hunting by the 1920s, elk populations soared, and they overgrazed much of the park’s northern range. On top of getting wolves named as endangered species under the Endangered Species Act in 1978, the widespread loss of wolf populations also appeared to affect the ecosystem as a whole. Known as trophic cascades, these trickle down or trickle up effects can happen when species are lost or introduced and cause dramatic shifts up and down the food chain. Although thinking of these trophic cascades may be an oversimplification of ecology in some ways, there is truth to the theory that apex predators have a huge influence on the ecosystems they live in.

Through the analysis of wolf, elk, and beaver population trends and the measurements of height and canopy cover in riparian willows, horizontal and vertical channel dimensions of the creek, and beaver dam heights, the research implies that the trophic cascade caused by the reintroduction of wolves to Yellowstone is the cause for a recovering Blacktail Deer Creek. With wolves helping to moderate the elk populations at lower numbers, elk graze the riparian plant areas to a lesser degree, thus improving willow growth and stream morphology. This shows that the channel is recovering from the period when elk overgrazed its banks.

 by Ian McAllister

by Ian McAllister

Robert Beschta, the lead scientist on the research, is excited by these results and other studies. “We’re finding that the effects of having a large carnivore, and particularly wolves, back on the landscape is incredibly important ecologically not only for streams but also for birds, for small mammals, for bears, for a whole host of species that do better when these habitats improve,” Beschta said.

Beschta mentioned that these observations demonstrating the importance of wolves “suggest that others should be looking at large predators elsewhere in the western U.S.”

Part of what makes this research on wolves and trophic cascades so essential is that healthy ecosystems, ones that promote many different species to thrive, are imperative for continued function of that ecosystem and for rich biodiversity. In light of the House of Representative passing a bill on Friday, November 16th, to remove federal protection of the gray wolf under the Endangered Species Act, research on wolves in America is more important than ever to consider. According to Beschta, who has been in the field for several decades, the bill is truly upsetting. “[The Endangered Species Act] has had some very positive outcomes in regards to species, at least in the American west where I work, so for the House of Representatives to pass a bill that makes that null and void particularly for wolves--and probably for other species also--is really really unfortunate,” Beschta said.