Precipitation and desert bighorn sheep in the Mazatzal Mountains, Arizona.

We examined demography of a population of desert bighorn sheep (Ovis canadensis) in the Mazatzal Mountains, Arizona, in association with annual and seasonal rainfall and relative herd density over an 11-year period that encompassed increasing, but normal median, annual precipitation (1989-1993) and drought conditions (post-1993). During wet years of 1989-1993 and through 1994, the population was stable or increasing slightly, and desert bighorn were distributed throughout the study area. Relative abundance of yearlings was positively correlated during wet years with total annual rainfall from the same year, and winter rainfall was positively correlated with number of groups observed 1.5 years later. In contrast, total males, Class I and II males, lambs observed per hour of survey time, and lambs:100 females declined during the post-1993 drought. Declining winter rainfall during drought was correlated with total desert bighorn, female and yearling observations per hour, and with yearling recruitment 1.5 years later. We found no clear evidence suggesting density dependent effects on recruitment during wet or drought years. However, lower population densities during drought corresponded with a contraction of bighorn distribution. The proportion of Class IV males in the total population of males was lower than expected throughout the study and was independent of rainfall. Our results indicate that annual and seasonal rainfalls are important variables driving the dynamics of bighorn sheep populations in desert environments.