Research Spotlight: Fear of Predation May Prevent Mule Deer from Taking Advantage of Forage Resources in Mojave Sky-Islands

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A recent publication from the USGS Western Ecological Research Center reports findings from a study of mule deer and cougars in the Mojave Desert. The results indicate that fear of predation by cougars may restrict mule deer access to food even in areas of relatively greater resources.

A female mule deer with a GPS-satellite collar at a water source with two fawns.

A female mule deer with a GPS-satellite collar at a water source with two fawns.

(Public domain.)

The study took place in a “sky-island” system in Nevada’s Desert National Wildlife Refuge, where forested mountain habitats are isolated from one another by expanses of desert. Mule deer take refuge on these sky-islands, but so do their predators: cougars. To study mule deer activity in the sky-island system and their interactions with cougars, USGS scientists and collaborators fit 19 female adult mule deer and five cougars with GPS-satellite collars that tracked their locations every 4 hours for 2.5 and 2 years, respectively. The researchers used the animal locations along with satellite-derived estimates of vegetation biomass and biomass change (“green-up”) to assess how food availability and predation influenced deer movements throughout the year. Fawn birth dates were estimated with cameras placed at water sources.

Satellite imagery revealed that vegetation growth rates peaked in the Mojave sky-island system in March. Mule deer location data showed that deer took advantage of these resources during spring, spending their time in areas with the highest “green up,” but did not do so at other times of year, even when there was growing vegetation available. During summer, mule deer favored higher elevation habitats with less food availability but greater refuge from harsh summer conditions and cougar predation. However, deer could not fully avoid predation by moving to higher elevations. Cougars also moved to cooler forested habitats in summer, following deer upslope. 

The study found that sky-island mule deer may also be vulnerable due to timing of reproduction. Unlike in other regions, where fawning and peak green up coincide in late spring, the early spring green up in the sky islands means that female deer give birth and nurse their young when food resources are less available and predation risk is high.

Overall, the findings suggest that mule deer in the Mojave sky islands have not developed a local strategy for maximizing nutritional resources and minimizing risk. Vegetation growth and cougar presence both influence deer behavior, but the isolation of the sky-islands limits food availability and traps deer with their predators in the summer months.

This spotlight refers to:
Lowrey, C., Longshore, K. M., Choate, D. M., Nagol, J. R., Sexton, J., & Thompson, D. (2019). Ecological effects of fear: How spatiotemporal heterogeneity in predation risk influences mule deer access to forage in a sky‐island system. Ecology and Evolution. 9:7213–7226. https://doi.org/10.1002/ece3.5291
 

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