White-tailed deer in northeastern Washington live in a world shaped by two competing constraints: predators and food availability.
But while wolves and mountain lions get more attention, they are not the most influential when it comes to the success or failure of deer populations.
“The most important factor limiting this deer population is the amount of food available to them,” said University of Washington researcher Taylor Ganz.
Ganz is lead author of a new study published in the journal Ecological Applications, titled “Population dynamics of white-tailed deer in human-created, predator-diverse habitats.”
The paper is the latest to emerge from the Washington State Predator-Prey Research Project, a multiyear effort by the Washington Department of Fish and Wildlife and the University of Washington to analyze how large animal species are responding to the increasing population of wolves and the presence of other predators in Washington state. Researchers are conducting multiple studies based in Okanogan, Stevens and Pend Oreille counties.
Ganz's dissertation focuses on white-tailed deer in Stevens and Pend Oreille counties. Between 2016 and 2021, he and his team radio-collared 280 white-tailed deer, including newborns, juveniles and adults, to track the animals' survival and reproduction. They also radio-collared 33 bobcats, 50 mountain lions, 28 coyotes and 14 wolves.
They used collar data to investigate how exposure to predators and use of human-modified landscapes affect population trends.
Although the predators killed many deer, studies have found that wolves have a weaker impact on deer populations than coyotes or cougars.
Coyotes allegedly killed 20 juvenile deer but no adult deer, while cougars allegedly killed 17 juvenile deer and 17 adults.
Wolves were potentially involved in three deaths, but only one involved wolves alone; investigations into the other two found that other predators may have been the cause.
Ganz was surprised that the wolf impact was so weak, but said there are a few answers as to why: First, there are fewer wolves than cougars in the region, and wolves avoid humans, while deer spend more time in populated areas, providing a kind of shield against canine predators.
Brian Carston, a WDFW biologist and one of the study's co-authors, added that mountain lions are more effective hunters than wolves.
“Compared to wolves, I think mountain lions are just better adapted to the environment that white-tailed deer currently inhabit,” Carston said.
Overall, the study found that while predation did have a negative impact on populations, human modification of the environment led to even greater increases in populations.
The study looked at how the landscape has changed over the past 20 years or so and found that recently logged areas have seen a 55% increase in forage, as less timber has allowed shrubs and other plants that deer love to eat to regrow.
Deer that spent the summer in these areas were also less likely to be preyed on, another surprise to Ganz: You'd think that more open land would give predators an advantage in finding and pursuing deer, but that wasn't the case.
“The benefits of having more food outweighed the challenges of escaping predators,” Ganz said.
At the same time, food isn't infinite: If wildlife managers take steps to reduce the predator population in the area, the white-tailed deer population may increase slightly, but it will still be limited because there will be more deer competing for the same amount of food, and not all of them will survive.
Humans' impact on deer is not one-sided: collisions with vehicles were the leading cause of deer deaths during the study period, resulting in the deaths of 22 deer.
But Ganz said the amount of time deer spend near roads doesn't make them more likely to be killed by predators specifically. Deer closer to roads may be at lower risk of being killed by predators, she said.
This shows how complicated the life of a deer is, as they must live in a world populated by predators and humans.
“Humans and wildlife are increasingly overlapping, mainly because humans continue to encroach on their habitats and, in some cases, species such as wolves are returning to their native ranges,” Ganz said. “This research is critical for understanding how we manage wildlife in these situations.”
