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MS Raptor Biology Thesis Defense - Caitlin Davis
June 14 @ 11:00 am - 12:00 pm MDT
Recreation, Fire, and Disease Creates a Mosaic of Threats in Southwestern Idaho
Advisor: Dr. Julie Heath, Biological Sciences
Committee Members: Dr. Jen Forbey, Biological Sciences, Ethan Ellsworth, BLM, Karen Steenhof, Owyhee Desert Studies
Anthropogenic stressors have resulted in ecosystem impoverishment and biodiversity loss worldwide. As the strength and reach of the human footprint increases, investigation of the additive or interactive effects of synergistic stressors on the landscape is imperative for conserving ecosystems and species within them. Golden eagles (Aquila chrysaetos) are apex predators of North American sagebrush-steppe ecosystems that are impacted by a suite of stressors, including wildfire, outdoor recreation disturbance, and habitat loss. We investigated whether multiple threats had additive or interactive effects on golden eagle occupancy, reproduction, and diet. We used a before-after-control-impact (BACI) design to study the effects of fire and recreation on eagle reproduction at 22 historical territories in southwestern Idaho. In 2015, the Soda wildfire burned 14 historical eagle territories and 8 territories were unburned. We collected data on recreation and eagle territory occupancy, apparent non-egg-laying rates, productivity per egg-laying pair, and diet in 2017 and 2018 and compared these data to pre-fire levels of recreation and eagle metrics in 2013 and 2014. Recreation use, including off-road vehicle (ORV) users, increased in unburned areas after the fire and remained the same in burned areas. Regardless of whether the time period was before or after the burn, or whether an area had burned, ORV recreation was negatively associated with eagle territory occupancy. However, the effect of pedestrian recreation changed after the fire. Before the Soda fire, pedestrian use was positively associated with the probability of apparent non-egg-laying. After the Soda fire, pedestrian use decreased, and eagle pairs were less likely to be categorized as apparent non-egg-laying pairs, suggesting that the decrease in pedestrian use had an interactive, positive effect on eagle reproduction. Diet composition differed between burned and unburned territories, but overall diet diversity and prey delivery rates were similar across fire and recreation gradients. In burned areas, eagles brought less leporid prey (rabbits and hares) and more sciurid prey (ground squirrels and marmots) to nests than in unburned areas and more leporid prey in the presence of higher recreation use. Additionally, in areas with low recreation, eagle diets utilized a higher proportion of rock pigeon prey (Columba livia), which are vectors of disease. Our results suggest that recreation is a major threat to eagle occupancy and reproduction, even compared to large-scale wildfires, that can have massive effects on shrub-steppe ecosystems and may lead to decreases of preferred prey, like black-tailed jackrabbits (Lepus californicus), in eagle diets. Together, these results suggest that a mosaic of stressors threaten eagles across the southwestern Idaho landscape. It is therefore imperative that we understand the additive or interactive effects of synergistic stressors acting on ecosystems so that we can best manage lands and conserve biodiversity in a time of rapid global change.