Bottom-up and top-down factors influence large mammal populations across a gradient of fire severity

Background/Question/Methods

Ecological disturbance is a driving force that structures ecosystems and shapes landscape pattern. Wildfire is a widespread form of ecological disturbance, and in some vegetation communities it is reported that fires are increasing in extent, frequency, and severity due to human activities and changing climatic conditions. Across the gradient of fire severity, fire can have both positive and negative effects on animals, where some species require mature climax vegetation conditions and other species select for early successional stages. However, further work is necessary to understand the role of bottom-up and top-down forces in shaping large mammal populations and their use of areas across the gradient of fire severity. The objective of this project was to evaluate how bottom-up and top-down factors influenced the occupancy and habitat use of black bear, elk, mountain lion, mule deer, and wolves across a gradient of fire severity. It was predicted that animals would select for areas of moderate to high fire severity, where food resources were expected to be greatest. In addition, it was predicted that top-down factors (i.e., human disturbance) would potentially influence habitat use of animals. Remote wildlife cameras sampled the gradient of fire severity seven years post a major wildfire (Wallow Fire, Arizona, 2011, 538,000 acres). Camera data were evaluating using single-species occupancy and Royle-Nichols models.

Results/Conclusions

As predicted, all species exhibited positive occupancy and/or habitat use in relation to fire severity or fire heterogeneity (bottom up). In addition, top carnivores selected for areas with prey species (bottom up). These results indicate that large mammals were selecting for areas with increased food resources, which were associated with areas of moderate to high fire severity. Top-down factors also appeared to influence habitat use of some species, although results were more varied in this system. Some species avoided areas of higher road density, likely avoiding human disturbance, whereas other species selected for areas with high road density, likely due to increased forage in these areas. This research can help inform management and conservation plans of large mammals that increasingly occupy landscapes experiencing mixed-severity fires.