Drought and growing season phenology over 35 years modulates species interactions among domestic and wild herbivores.

Domestic animals represent the largest mammalian biomass on Earth, creating an urgent need to understand their effects on native species. Environmental change may further alter these interactions, particularly for large herbivores that are highly sensitive to climate-driven resource availability. Interactions among assemblages of large herbivores include interference and exploitation competition, but the grazing optimization hypothesis paradigm suggests that interactions are often facilitative because defoliation from grazing stimulates compensatory regrowth of high-quality forage that benefits other species. We used 35 years of contemporaneous telemetry data on elk and domestic cattle to quantify spatial interactions within the context of climate-change-induced drought conditions. We fit step-selection functions to assess the immediate response of elk to spatiotemporal cattle distribution and tested whether drought and phenological advancement of the growing season-mediated interactions. We also modelled elk selection for pastures that were ungrazed, currently grazed or previously grazed by cattle to assess competition and facilitation. Our results refute the grazing optimization hypothesis and suggest elk are not attracted to areas previously grazed by cattle. Instead, elk avoided cattle throughout the entire growing season and at multiple scales. Elk were 2.9 times more likely to make movements to locations without cattle versus locations with 1 cow-calf pair/hectare, 23% less likely to use a pasture with cattle present, and 27% less likely to use one grazed by cattle earlier in the season, compared to an ungrazed pasture. After controlling for current season grazing status, elk were 30% less likely to use a pasture if it was grazed the previous year. Resource availability, influenced by growing season phenology and drought, mediated the magnitude of avoidance of cattle by elk. In the periods of most severe drought, elk reduced their avoidance of cattle ostensibly to seek out the remaining palatable forage regardless of cattle presence. Increased overlap between domestic and wild ungulates should be anticipated as climate change continues to increase the intensity and frequency of drought, which may result in decreased weight gain or fitness consequences to both species, increased potential for disease transmission, and more encounters between predators and domestic livestock.