Behavior altered to find food, protection
Wildfires have burned vast tracts of forests surrounding the Methow Valley in recent decades, forcing wildlife to adapt to changing landscapes.
Mule deer, one of the Methow Valley’s signature species, change how they use forests after forests burn. Wildlife researchers, in a recent paper, described how mule deer alter their behavior to find food and protect themselves from predators in burned areas.
“Our study and others show that deer and other wildlife use burned areas after wildfire, even when these zones have been intensely burned. But these fires bring both costs and benefits to wildlife,” wrote Taylor Ganz, a wildlife ecologist and the primary author of the research paper.
“Understanding how mule deer respond to burns and interact with predators in burned areas may be essential for conserving and restoring wildlife communities,” Ganz wrote in an article about her research that recently appeared in the online publication “The Conversation” (theconversation.com).
Ganz has been studying mule deer in the Methow Valley and north central Washington for several years as part of the University of Washington’s Predator-Prey Project. That project is examining how the presence of wolves and other predators is affecting deer and elk in Washington state.
“I’m particularly interested in how these species interact in changing landscapes … and how wildfires affect deer interactions with cougars and wolves,” Ganz said.
“We found that mule deer use these burns in summer but avoid them in winter. Deer also adjusted their movement to reduce predation risk in these burned landscapes, which varies depending on whether cougars or wolves are the threat,” Ganz said.
Forests in western North America are adapted to withstand fire, and some trees depend on fires to disperse seeds for reproduction. Wildfire plays a role in clearing the forest understory, burning away shrubs and small trees.
In more severe fires, flames reach into the treetops, opening up the canopy and allowing more light to reach the forest floor. As a result, burned forests can be lush with shrubs and other fire-adapted vegetation in summer, providing deer an abundant source of forage, Ganz said. The ample food sources in burned areas can even have a “magnet effect” on deer, attracting them from surrounding areas.
“In our study, deer generally preferred burned areas for about 20 years post-fire, which is the time it takes for the forest to move beyond the initial regrowth stage,” Ganz said.
Restructuring landscapes
Just as fires trigger forest regeneration, they also restructure landscapes. Ganz found this process influences interactions between mule deer and the animals that prey on them.
While the open canopy created by large wildfire produces a bountiful food source for mule deer in spring and summer, it also means that more snow reaches the forest floor in winter. The snow is often deeper in burned areas than in unburned forest.
The deep snow makes it difficult for deer to feed, and also makes them more vulnerable to carnivores, Ganz said. Their hooves sink into the snow, while predators like wolves and cougars have wide paws that help them travel over the snow. “For these reasons, the mule deer we tracked avoided burns in winter,” Ganz said.
Ganz and her fellow researchers found that mule deer adapted their behaviors depending on the type of predator present. Cougars hunt by stalking and ambushing their prey, and often rely on foliage and complex terrain to approach undetected. Wolves hunt by chasing their prey over long distances, a strategy that works best in open terrain.
“After fires, vegetation growth and the accumulation of fallen trees and branches can create stalking cover for cougars and also provide refuge for deer to hide from wolves,” “Ganz said. Researchers in the Washington Predator-Prey Project found that deer were generally less likely to use burned forests in areas of high cougar activity, although that depended on severity of the fire and time elapsed since the fire occurred.
“Deer had to balance the availability of improved summer forage in burns with increased predation risk from cougars. In areas heavily used by wolves, however, burns created a win-win for deer: more food and less risk of being detected by a predator,” Ganz said.
“I was surprised to see that mule deer used even severely burned forests in the summer months,” she said.
Collared wildlife
Wildfires in the West have become larger, more severe and more frequent due to climate change, the loss of indigenous burning practices and a century of fire suppression, Ganz said.
To evaluate the impacts of wildfire on deer behavior, Ganz and her research colleagues used satellite data to map 35 years of impacts from fires that occurred between 1985 and 2019. During those years, fires have burned nearly 40 percent of the landscape in the study area — the Methow Valley and northern Okanogan County. More than half of those burns have occurred in the past decade, researchers found.
Researchers captured 150 mule deer and fit them with GPS collars that provided locations every four hours. They also captured and collared five wolves and 24 cougars to track their movements.
By analyzing the animals’ behavior, the researchers were able to determine how much deer chose or avoided burned areas that had varying levels of cougar and wolf activity.
“Mule deer may benefit from the opportunity to feed on better summer forage. But avoiding burns in the winter, when the ground is covered with snow, could reduce the deer’s range at a time when the animals already gather at lower elevations to avoid the deepest snow,” Ganz said.
Understanding how wildfires impact wildlife and the way animals interact is important for conserving and restoring wildlife communities, Ganz said.
“As policymakers debate suppressing wildfire, treating forests to reduce fuels and logging after fires, I believe they should consider how these strategies will affect wildlife — a key part of biodiverse, resilient landscapes,” Ganz said.
“We showed that in summer, mule deer used even severely burned forests, highlighting the value of burned forests for wildlife, Ganz said. “This should be considered when considering policies to let fires burn versus extinguishing them, and when considering salvage logging.”
