Humans drove wolves to extinction in Washington state around the 1930s. Thanks to conservation efforts, the wolves had returned about 80 years later: They had first crossed from the Canadian border into Washington around 2008 and later entered the state from Idaho. Since then, the number of wolves in Washington has steadily grown, raising questions about what the return of this large predator species means for both ecosystems and people.
In northeastern Washington, where wolves have recovered most successfully, researchers from the University of Washington and the Washington Department of Fish and Wildlife tracked one of their top prey — white-tailed deer — in part to see what impact wolf packs have on deer populations. The answer? So far, wolves don’t affect deer as much as other factors.
In an article published on June 18 in Ecological applications, the team reports that the biggest factors determining white-tailed deer populations in northeastern Washington are the quality of available habitat and another long-standing large predator in the state: the cougar, also known as the mountain lion or cougar. Wolves were a distant third in their impact.
“A key takeaway from this study is that wolves do not return to empty landscapes. These are places with people and other carnivores, such as cougars, which will influence the impact that wolves can have,” said lead author Taylor Ganz, who conducted this research for her UW doctorate as part of the Washington Predator-Prey Project. “This area has a relatively high human footprint compared to other areas where wolves have been surveyed. These are not national parks or dense, old-growth forests. These are areas with active logging, agriculture, livestock farming and cities. Our findings show that these factors likely limit the impact of wolves on one of their primary food sources.”
It’s not that wolves don’t hunt white-tailed deer. According to the research, they alone are not enough to take a big bite out of the population as a whole.
White-tailed deer are widespread east of the Cascades. The highest density population of this species in the state is within the study area, which includes farmland and timber forests in parts of Stevens and Pend Oreille counties in northeastern Washington. For the study, researchers recorded 280 white-tailed deer, 14 wolves, 50 mountain lions, 28 coyotes and 33 radio-collared bobcats between 2016 and 2021. At the time of the collar, the researchers also recorded vital statistics, including body condition, age and whether females were pregnant. When collared animals died, the team, if possible, conducted a mortality study and attempted to determine the cause of death.
The team, which also includes researchers from Washington State University and the Spokane Tribe of Indians, used the resulting data set to estimate the growth rate of the white-tailed deer population over the four-year study and identify the key factors that shaped it. The analysis found that the whitetail population in the study area was likely stable, or declining slightly, but that wolves were not largely responsible.
The biggest factor affecting the deer population was habitat quality, including the amount of forage available to deer. For white-tailed deer, which are highly adaptable to human activities, foraging areas can range from forests and shrublands to agricultural fields. The study area includes both agricultural land and forests recently cleared for timber, both of which could provide deer with high caloric density foraging sites, according to Ganz.
After habitat quality, the study found that cougar predation had a smaller effect on the whitetail population. Wolf predation had an even smaller impact. Bobcats and coyotes – both medium-sized predators – had a negligible impact on deer numbers.
“Studies like these provide valuable insights into the complexity of these systems and how managing predator and prey populations is challenging and dynamic,” said co-author Melia DeVivo, a research scientist at WDFW. “It is important to continue to evaluate these systems to understand the impact of management decisions. Prior to this study, one might have expected that relying solely on wolf management strategies would result in a thriving deer population, when it is clearly more complex than that.”
Since their return, the number of wolves in Washington has risen steadily, reaching a minimum of 260 by 2023, state researchers said. Four packs of wolves live in the northeastern study area of the Predator-Prey Project of Washington. The total number of wolves in the study area – approximately 23 – remained stable overall over the study period.
The team’s findings contrast with studies of long-standing wolf populations in protected areas, such as Yellowstone National Park, which show a greater impact of wolves on the population dynamics of their prey species. For the authors of this new study, these differences highlight the importance of studying wolves in different habitats.
“This study reminds us that the population dynamics of predator and prey species can vary quite a bit,” said senior author Laura Prugh, associate professor of environmental and forest sciences at the UW. “The quality of habitat, the species present and the level of human activity all influence the impact that large predators such as wolves will have. It is crucial to compare different types of sites.”
The article is part of the Washington Predator-Prey Project, a partnership between the UW and WDFW to investigate the impact of wolf returns on state ecosystems. Additional co-authors are Sarah Bassing, a UW doctoral alum in environmental and forest sciences; Lauren Satterfield, a UW doctoral student in environmental and forest sciences; biologists Brian Kertson and Benjamin Turnock at WDFW; Lisa Shipley, a professor at WSU; Savannah Walker and Derek Abrahamson, both biologists with the Spokane Tribe of Indians; Beth Gardner, associate professor of environmental and forest sciences at UW; and Aaron Wirsing, professor of environmental and forest sciences at the UW. The research was funded by WDFW, the National Science Foundation, the Rocky Mountain Elk Foundation and the UW College of the Environment.
