Desert bighorn sheep home range and disease transmission risk responses to temporally dynamic environmental variation

Abstract

Pathogens introduced into wildlife populations can cause population declines and pose a threat to conservation. Understanding how pathogens spread through wildlife populations requires information about animal space use across the landscape. The abundance of bighorn sheep (Ovis canadensis) in western North America has declined in response to pathogens introduced concurrently with the expansion of domestic sheep. Wildlife and land management agencies use bighorn sheep seasonal home range models to assess and mitigate pathogen transmission risk among bighorn and domestic sheep populations. Current risk assessment tools assume that seasonal home ranges are annually consistent, but the extent to which deviations from this assumption could render erroneous predictions about pathogen introduction risk has not received extensive attention. To evaluate the influence of temporally variable environmental conditions on pathogen introduction risk, we used locations of desert bighorn sheep (O. c. nelsoni) gathered across 6 populations residing in the Mojave Desert, an environment characterized by high interannual variation in precipitation and forage production. Our objectives were to assess whether seasonal home range sizes varied consistently with temporally varying attributes of the environment and assess the influence of varying environmental attributes on pathogen introduction risk using our model results and the United States Forest Service's Risk of Contact (ROC) tool. Home range sizes varied by sex and season, with higher environmental moisture levels (Palmer drought severity index) associated with larger male home ranges in summer and fall-winter. Higher spatial variation in primary productivity was associated with smaller male home range sizes in summer but with larger male home range sizes in fall-winter. Female home ranges were also smaller when variation in primary productivity was high during spring but had no detectable relation with environmental moisture or spatial variation in productivity during the rest of the year. We combined these results with the ROC tool to simulate the influence of varying environmental attributes on pathogen introduction risk. Those simulations suggested that home range expansions associated with above-average environmental moisture only increased the contact risk between 2 adjacent populations by 0.02%. Consequently, risk forecasts in the Mojave Desert system might be relatively robust to temporally dynamic home range sizes, so long as the environmental attributes driving home range size do not deviate dramatically from historical levels. However, major departures from long-term trends in environmental variation could lead to more dramatic effects on home range size and subsequent risk and should be reassessed if environmental variation changes substantially in the future.