Recruitment Dynamics in a Southern California Mountain Sheep Population

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Abstract
Lamb: ewe ratio data spanning the years 1953-82 for mountain sheep (Ovis canadensis) in the Santa Rosa Mountains of southern California were analyzed by multiple regression to investigate hypothesized controlling factors. Precipitation during November, January, and Feburary as independent variables each showed a significant positive influence on recruitment rate, in accord with expectations for this desert environment. Between 1962 and 1976, an unidentified 4th factor (probably rising population density) caused a slow decline in recruitment ratio. This ratio plummeted in 1977 and averaged about 25% of the ratio predicted from precipitation alone for 1977-82, a period coincident with a suspected disease epizootic. A similar period of depressed lamb recruitment was found for the 1953-61 period, suggesting the possibility of a recurrent phenomenon. Data needs and various hypotheses concerning factors potentially underlying the observed and future population dynamics are discussed.