Study: Food availability more important than predators for white-tailed deer in northeast Washington

White-tailed deer in northeast Washington live in a world shaped by two competing limitations – predators and the availability of food.

But while wolves and cougars garner more attention, they’re not the most influential force when it comes to the deer population’s success or failure.

“The most important thing limiting this deer population is the amount of food that’s available to them,” said Taylor Ganz, a research scientist at the University of Washington.

Ganz is the lead author of a new study published in the journal Ecological Applications, titled “White-tailed deer population dynamics in a multi-predator landscape shaped by humans.”

The paper is the latest to come out of Washington’s predator-prey project, a long running effort by the Washington Department of Fish and Wildlife and the University of Washington to analyze how big game species are responding to the expansion of wolves and the existence of other predators in the state. Researchers have produced several studies based in Okanogan, Stevens and Pend Oreille counties.

Ganz’s paper focused on white-tailed deer in Stevens and Pend Oreille counties. From 2016 to 2021, she and her team placed radio collars on 280 white-tailed deer, including newborns, juveniles and adults, and they tracked the animals’ survival and reproduction. They also collared 33 bobcats, 50 cougars, 28 coyotes and 14 wolves.

Using the collar data, they examined how the population trend was affected by predator exposure and their use of human-modified landscapes.

Predators killed plenty of the deer, but the study found that wolves had a weaker impact on the population than coyotes and cougars.

Coyotes were deemed responsible for 20 juvenile deer deaths and no adults, while cougars were responsible for killing 17 juveniles and 17 adults.

Wolves may have played a role in three deaths. Of those, just one was linked solely to a wolf. Investigations into two others showed that other predators may have been responsible.

Ganz said she was surprised that the impact of wolves was so weak, but she had a couple of answers as to why. For one, there are fewer wolves than cougars in the area. For another, wolves avoid people while deer spend more time in populated areas, giving them a sort of shield against the canine predators.

Brian Kerston, a WDFW biologist and one of the study’s co-authors, added that cougars are more effective hunters than wolves.

“I think cougars are just better adapted to those landscapes that whitetails currently occupy compared to wolves,” Kerston said.

Overall, the study found that while predation did hurt the population, humans tinkering with the landscape gave it a more significant boost.

The study looked at how the landscape had been changed over the past 20 years or so, and found that areas that had been logged recently offered 55% more forage. Less timber allowed the regrowth of plants like shrubs that deer like to eat.

Deer that spent time in those areas in the summer also had a better chance of not becoming prey. That was another surprise for Ganz – it would stand to reason that more open country would give predators an advantage in spotting and stalking deer, but that wasn’t the case.

Ganz said the “benefits of increased food outweighed any challenges of escaping predators.”

At the same time, food isn’t unlimited. If wildlife managers were to take steps to reduce predator populations in this area, the white-tailed deer population might increase a little, but it would still be constrained – there would be more deer competing for the same amount of food, and not all of them would live.

Humans’ impact on deer isn’t one-sided. Vehicle collisions were a significant cause of deer deaths during the study, resulting in the killing of 22 deer.

However, Ganz said the time a deer spent near roads didn’t increase its chances of dying, particularly when it came to predation. She said it’s possible that deer closer to roads are at a lower risk of being killed by predators.

That shows just how complex a deer’s life is, having to navigate a world inhabited by predators and people.

“Humans and wildlife are increasingly overlapping, largely because we continue to encroach on their habitat and in some cases species like wolves are returning to their native range,” Ganz said. “This study is really important to understanding how we manage wildlife in those contexts.”