To understand how mule deer use fire-impacted areas, consider the season and account for their predators
This blog post was originally written for the Journal of Animal Ecology to summarize research findings from Predator-Prey Project's research paper, "Interactive effects of wildfires, season, and predator activity shape mule deer movements". It is reproduced with permission here, and the paper is available online or by emailing the authors to request a copy.
Across the American West, wildfires are becoming more frequent, larger, and more intense. In the Methow Valley and Okanogan region of north-central Washington, nearly 40% of the area has burned since 1985. Mule deer, white-tailed deer, moose, black bears, bobcats, cougars, coyotes, and recently returned wolves live across this area. While wildfires can shape wildlife habitat, the impacts of the recent fires on these species – and the interactions between them – is unclear. Given the decline of mule deer across the American West, primarily attributed to changes in habitat, we were especially interested in understanding how mule deer navigated burns while managing predation risk from cougars and wolves.
To examine how wildfire impacted mule deer, we captured and GPS collared 150 adult female deer along the Methow Valley and considered their movement as they encountered a range of fire impacts over the year. We also caught cougars and wolves to fit them with GPS collars. Using these predator locations, we mapped which areas they used most heavily, allowing us to consider how mule deer managed predation risk in fire-affected areas.
Wildfires can have a wide range of effects depending on the characteristics of the burn. We considered low, moderate, and high severity fires over time spans from 0 to 35 years post-fire. Fires impact forests by initiating nutrient cycling, triggering the growth of fire-adapted plants, burning understory vegetation, and in more severe fires, even removing the forest canopy. Generally, burns increase the shrubby and herbaceous vegetation deer favor for food in the summer. As expected, we found deer tended to select for burns in the summer to access this improved forage.
But the growth of this vegetation and increase in deadfall post-fire can also impact the hunting efficacy of predators. Cougars hunt by stalking and ambushing prey, and understory re-growth and accumulation of deadfall provide the hiding cover for cougars to approach deer undetected. Where cougar activity was higher, deer were generally more likely to reduce their selection of burned areas with better forage to avoid an increased risk of cougar predation. Mule deer’s response to cougars in burns also depended on the severity and time since the fire, suggesting that response to cougars reflected the ways that fire restructured the habitat. Deer balanced this food-safety tradeoff, and we found that they were no more likely to die from cougars or other predators in burned areas than in unburned areas.
Unlike cougars, wolves hunt by chasing down prey over long distances in open landscapes. Where wolf activity was higher, wildfire created a win-win situation for deer, improving summer forage quality and providing hiding cover from wolves. Because of this, deer were even more likely to use burns where wolf activity was high. Wolves have been naturally recolonizing the area over the last couple of decades. They occur at lower densities in the region compared to cougars, so effects on deer could increase if the local wolf population grows.
In the winter, snow can accumulate to deeper levels in burned versus unburned forests because snow accumulates in the understory rather than being intercepted by the canopy-cover. In this camera-trap photo, a mule deer buck moves through a burned forest in an early autumn snowfall. Photo: Sarah Bassing
In the winter, we found that deer avoided burned areas. When fire removes the canopy cover that would otherwise catch falling snow, deeper snow accumulates in burned forests than in unburned forests. Deeper snow in burns had two effects: First, deeper snow cover would inhibit deer access to the forage buried beneath it. Second, deer struggle to escape from predators in areas of deep fluffy snow where their hooves sink deeply, while predators’ paws act as snowshoes. Still, we found that deer avoided burns more strongly with increased cougar activity and less strongly with increased wolf activity, reflecting the hunting styles of these predators.
Our study showed that wildfires impact ungulate species such as mule deer through multiple pathways, shaping food availability and vulnerability to predators with seasonally dependent effects. In snowy areas where burns occur on winter range, these impacts could be concerning if wildfires functionally reduce available habitat for deer when they avoid burned areas.
Read the full paper here: Ganz, T. R., DeVivo, M. T., Kertson, B. N., Roussin, T., Satterfield, L., Wirsing, A. J., & Prugh, L. R. (2022). Interactive effects of wildfires, season and predator activity shape mule deer movements. Journal of Animal Ecology, 00, 1–16. https://doi.org/10.1111/1365-2656.13810