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The return of wolves had little impact on the deer population in Washington state, a study shows

The return of wolves had little impact on the deer population in northeast Washington, a study shows

Researchers with the Washington Predator-Prey Project have collared 280 white-tailed deer to monitor their movements, survival and reproduction. Photo credit: Laura Prugh

In the 1930s, humans drove wolves to extinction in Washington state. Thanks to conservation efforts, wolves returned about 80 years later, first entering Washington from the Canadian border in 2008 and later from Idaho. Since then, Washington’s wolf population has been steadily growing, raising questions about what the return of this large predator species means for ecosystems and people.

In northeastern Washington, where wolves have recovered best, researchers from the University of Washington and the Washington Department of Fish and Wildlife tracked one of their main prey species – white-tailed deer – in part to learn what effect wolf packs have on the deer population. The answer? So far, wolves have not had as much of an impact on the deer population as other factors.

In an article published on June 18 in Ecological applicationsThe team reports that the biggest factors influencing the white-tailed deer population in northeast Washington are the quality of available habitat and another long-standing large predator in the state: the cougar, also called the mountain lion or puma. Wolves were a distant third in their impact.

“One important finding from this study is that wolves are not returning to empty landscapes. These are places with people and other predator species like cougars, which will affect the wolves’ impact,” said lead author Taylor Ganz, who conducted this research for her doctorate at UW as part of the Washington Predator-Prey Project.

“This area has a relatively high human footprint compared to other areas where wolves have been studied. These are not national parks or dense virgin forests. They are areas of active deforestation, agriculture, livestock farming and cities. Our results show that these factors likely limit the wolves’ impact on one of their most important food sources.”

It’s not that wolves don’t prey on white-tailed deer. According to the study, there are just not enough wolves to cause significant damage to the overall population.

The return of wolves had little impact on the deer population in northeast Washington, a study shows

This image shows two adult female white-tailed deer and a fawn. A new study shows that humans have had both positive and negative impacts on this species in northeast Washington. Researchers found that some activities, particularly farming and logging, increased the food supply for deer. However, the study also found that car collisions are one of the leading causes of death for deer. Photo credit: Taylor Ganz

White-tailed deer are widespread east of the Cascade Range, with the highest population density of this species in the state occurring in the study area, which includes farmland and forests in parts of Stevens and Pend Oreille counties in northeastern Washington.

For the study, researchers radio-collared 280 white-tailed deer, 14 wolves, 50 cougars, 28 coyotes and 33 bobcats from 2016 to 2021. At the time of collaring, researchers also recorded key statistics such as body condition, age and pregnancy of females. When collared animals died, the team conducted a mortality examination when possible and attempted to determine the cause of death.

The team, which includes researchers from Washington State University and the Spokane Indian Tribe, used the resulting dataset to estimate the growth rate of the white-tailed deer population during the four-year study and to identify the key factors influencing it. The analysis found that the white-tailed deer population in the study area was likely stable or slightly declining, but that wolves were not a major contributor.

The biggest factor affecting the deer population was habitat quality, including the amount of forage available to the deer. For white-tailed deer, which are highly adaptable to human activities, foraging sites can range from forests and brush to agricultural fields. The study area includes both agricultural land and recently logged forests, which Ganz said could provide high-calorie-density foraging sites for the deer.

After habitat quality, the study found that predators such as cougars had a smaller impact on the white-tailed deer population. Predators such as wolves had an even smaller impact. Bobcats and coyotes, both medium-sized predators, had a negligible impact on the deer population.

“Studies like this provide valuable insights into the complexity of these systems and how challenging and dynamic managing predator and prey populations is,” said co-author Melia DeVivo, a scientist at WDFW.

“It is important to continue to evaluate these systems to understand the impacts of management decisions. Before this study, one might have expected that relying solely on wolf management strategies would lead to a boom in the deer population, although the issue is clearly more complex.”

The return of wolves had little impact on the deer population in northeast Washington, a study shows

The new study focused on the Washington Predator-Prey Project study area in northeast Washington, shown in green. Image credit: Taylor Ganz

Since their return, the number of wolves in Washington has steadily increased, reaching a minimum of 260 in 2023, according to state researchers. Four wolf packs live in the Predator-Prey Project study area in northeastern Washington. The total number of wolves in the study area – about 23 – remained stable overall during the research period.

The team’s findings contradict studies of long-standing wolf populations in protected areas such as Yellowstone National Park, which show a greater influence of wolves on the population dynamics of their prey species. For the authors of this new study, these differences underscore the importance of studying wolves in a variety of habitats.

“This study reminds us that the population dynamics of predator and prey species can be very different,” said lead author Laura Prugh, an associate professor of environmental and forest sciences at the University of Washington. “Habitat quality, species present, and levels of human activity all influence the impacts that large predators like wolves will have. It is critical to compare different types of sites.”

The paper is part of the Washington Predator-Prey Project, a partnership between the UW and WDFW to study the impacts of the return of wolves on the state’s ecosystems. Other co-authors include Sarah Bassing, a UW doctoral student in environmental and forest sciences; Lauren Satterfield, a UW doctoral student in environmental and forest sciences; WDFW biologists Brian Kertson and Benjamin Turnock; Lisa Shipley, a WSU professor; Savannah Walker and Derek Abrahamson, both biologists with the Spokane Indian Tribe; Beth Gardner, an associate professor of environmental and forest sciences at the UW; and Aaron Wirsing, a UW professor of environmental and forest sciences.

More information:
Taylor R. Ganz et al., Population dynamics of white-tailed deer in a human-shaped landscape with high predator populations, Ecological applications (2024). DOI: 10.1002/eap.3003

Provided by the University of Washington

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