Range-wide variation in the effect of spring snow phenology on Dall sheep population dynamics

Understanding the underlying mechanisms of species distributions and range limits is critical to predicting species responses to climate change. The relationship between climate factors and population performance is often assumed to be constant across species' ranges, but factors that limit the persistence and abundance of species may vary spatially. We examined the consistency of climate effects on population performance across the global range of Dall sheep (Ovis dalli dalli) in northwestern North America. Using a linear mixed modeling approach, we evaluated the effects of temperature, precipitation, and snow cover on rates of lamb recruitment, which is a key measure of population performance. We used snow cover products derived from NASA's moderate resolution imaging spectroradiometer (MODIS) imagery and gridded climate data as predictors. Lamb recruitment rates were estimated using range-wide aerial survey data from 2000-2015. These consisted of 127 833 records of sheep sightings from 1570 surveys conducted in 24 mountain units over an area of 634 271 km2. MODIS-derived spring snow cover had stronger effects on lamb recruitment than did temperature or precipitation. Lamb recruitment increased with higher spring snowline elevations, earlier snow disappearance dates, and fewer snow-covered days per year, while the strength of these effects increased strongly with latitude. Simple population models indicate that, based on elasticity estimates, population growth would be reduced by 2% in years with late snow disappearance dates at intermediate northern latitudes, and by 5% at high latitudes. These results suggest that northern Dall sheep populations are more sensitive to changing snow conditions than their southern counterparts. In addition, variable relationships between climatic factors and population performance should be accounted for when modeling species distributions and projecting climate-induced range shifts.