Projected impacts of climate change on the distribution of bighorn sheep inhabiting California’s Peninsular Ranges

Background/Question/Methods

Predicting ecological responses to climate change is critical for conservation planning, especially for protecting endangered species. However, despite broad agreement that climate change is happening, understanding how individual species will respond to that change, at scales relevant to management responses, has proved challenging. A useful tool in assessing species sensitivity to changing conditions is niche modeling (also called habitat suitability modeling or species distribution modeling) which includes variables that describe habitat characteristics as well as climate in an attempt to assess the complex interaction of factors that constrain a species’ distribution. We employed the Mahalanobis distance (D²) statistic to model the current niche of an endangered population of bighorn sheep (Ovis canadensis nelsoni) that inhabits the Peninsular mountain ranges of southern California. We then manipulated that model with a gradient of climate change scenarios to determine where suitable habitat for this species will shift as temperatures rise. 

Results/Conclusions

When current temperature was increased by just 1°C suitable habitat increased 5.4%. At a modeled warming of 2°C and 3°C, the model indicated an increase in suitable habitat of 8.5% and 15.2%, respectively, from current conditions. New habitat that is not currently occupied by bighorn sheep in the northwestern portion of the San Jacinto Mountains was identified in the climate-change niche models. When this habitat was excluded to focus only on habitat known to be currently occupied or adjacent to occupied areas, suitable habitat area increased 2% when temperature increased 1°C, and decreased by 1.4% and 5.9% when temperature was shifted +2°C and +3°C, respectively. Although precipitation shifts were not explicitly included in the modeled climate change scenarios, temperature was strongly correlated with annual precipitation in the study area which provided a realistic implicit proxy for rainfall. Our modeled habitat responses, coupled with the sheep’s potentially high mobility over a landscape characterized by broad elevation ranges with few internal habitat barriers, indicate the sheep should adjust to changes in climate with gains of suitable habitat.