Climate change models for the continental United States predict the arid southwest, especially southern California, will likely experience the greatest departure from current conditions. This region is rich in biodiversity, however predicting the impact of climate change on that biodiversity is hampered by the inability to down-scale continental scale models to local environments with sufficient precision. Rather than depend on climate models for local scale predictions, I assessed the sensitivity of selected species within southern California’s Joshua Tree National Park to a range of climate change conditions. I used a partitioned Mahalanobis D2 niche modeling analysis, a method requiring only “presence” data to construct the models. Species locations were coupled with independent, GIS based variables, including climate variables (precipitation, maximum July temperatures and minimum January temperatures), along with abiotic metrics such as slope, ruggedness, and soils, to project current habitat suitability for six reptile species. Combining climate variables with structural components of a species’ niche more accurately constrains the response to climate change to locations where structural niche requirements occur. Once a current niche was modeled, the underlying climate variables were incrementally shifted to determine the sensitivity of each species to increases in temperature and decreases in annual rainfall.
My results revealed temperature and rainfall related shifts in the distribution and availability of suitable habitat for species ranging from the threatened desert tortoise, Gopherus agassizii, chuckwalla, Sauromalus obsesus, to the desert horned lizard, Phrynosoma platyrhinos. The Mahalanobis-generated model output includes distributions of suitable habitat based on how close a given location, or map cell, comes to the multivariate mean for known locations. The resultant habitat Suitability Indices (HSI) range between 0 (no similarity to occupied locations) to 1 (complete similarity). Using an arbitrary HSI range of ≥ 0.333 and ≥ 0.500, the model for the current distribution for desert tortoises indicates 90,726-58,828 ha of suitable habitat occurs within the park. Raising the mean maximum July temperature by 1-5°C resulted in a 58-100% reduction in suitable habitat. Keeping the temperature constant, but decreasing mean precipitation resulted in a similar pattern; reducing rainfall by 35-75 mm resulted in a 62-99% reduction in suitable habitat. An interesting pattern resulted from varying temperature and rainfall in concert; rather than a multiplier effect, there appeared to be a buffering interaction. Suitable tortoise habitat shifted to higher elevations and further west, to locations where tortoises are either rare or absent today.