INTRODUCTION:

Escape terrain and vegetation cover are two vital components in bighorn sheep habitat use. Understanding these components is essential for conserving and managing this species. Despite the importance of assessing habitat use, there are few studies in Mexico and most efforts have focused on population surveys. The lack of studies of bighorn sheep habitat in Baja California does not allow predicting possible effects of climate change, for example, reduction in availability of forage and less rainfall and increasing local extinctions of sheep populations in desert areas. Our objectives were to assess availability of escape terrain in topographic areas of varying ruggedness (flat, low hills, canyons, and steep mountains) and availability of plant cover, using an index of normalized difference in vegetation within the Sierra Santa Isabel. Additionally, selection of topographic ruggedness and vegetation cover by bighorn sheep was analyzed. The results of this study will serve to identify sites for conservation and management of Ovis canadensis and their habitat in the Baja California Peninsula.

METHODOLOGY:

From July 2011 through February 2012, we observed free-ranging herds of desert bighorn sheep in three time frames in two representative locations in the Santa Isabel mountains. We used terrain roughness and vegetation index, derived from Landsat images to determine the available escape terrain and vegetation cover. From the frequency of sightings and availability of habitat variables, habitat selection was analyzed, using a Chi-squared test and constructing Bonferroni confidence intervals.

RESULTS:

The sites with greater availability of escape terrain were more frequently selected by herds of ewes and yearlings (χ² = 9.83, P < 0.05), while herds of mixed adults selected sites having less escape terrain. During the dry season, sheep selected sites had low vegetation cover (χ² = 11.58, P < 0.05). During the rainy season, sheep selected greater vegetation cover at only one of the study sites (χ² = 8.72, P < 0.05). During the post-rainfall season, we found no relationship between vegetation cover and presence of sheep.

DISCUSSION AND CONCLUSIONS:

Selection of escape terrain and vegetation cover seems to be related to the sheep's anti-predatory strategy. More females (n = 140) and yearlings (n = 62) occurred where continuous habitat and permanent water was available, which confirms the importance of the Sierra Santa Isabel for breeding and recruitment of bighorn sheep in the Baja California Peninsula. The vegetation index derived from satellite imagery measured only the phenological stage of the plant community, but did not indicate which plant species are present and used by the sheep. However, the index facilitates locating areas with high availability of food and defining sampling areas to monitor populations and carry out censuses.