Evaluation of Factors Potentially Influencing a Desert Bighorn Sheep Population

Abstract We studied a desert bighorn sheep (Ovis canadensis) population in the Mazatzal Mountains (primary study area) in central Arizona and population indices on reference areas between 1989 and 2003. We evaluated disease exposure and nutritional status of desert bighorn sheep, vegetation parameters, predator diets, and mountain lion (Puma concolor) harvest and abundance (1999–2003) and mountain lion predation (1995–2003) as factors potentially affecting desert bighorn sheep and population parameters. We measured rainfall monthly, monitored demography and relative abundance of desert bighorn sheep using aerial surveys, captured and placed radio collars on desert bighorn sheep, and collected samples of blood, parasites, and other pathogenic agents from captured animals. We measured mineral content, relative use, and structural composition of vegetation and determined diets of desert bighorn sheep adults and lambs, dietary intakes of nitrogen (FN), 2,6-diaminopimelic acid (FDAPA), neutral detergent fiber, and minerals using fecal analyses. We incorporated mountain lion reductions as an experimental element, monitored harvest, and used track surveys as an index of relative abundance of the predator and monitored radio-collared desert bighorn sheep to determine mortalities and causes of death. We determined diets of bobcats (Lynx rufus), coyotes (Canis latrans), and mountain lions using fecal analyses. Drought conditions occurred during summer (July–September) and winter (November–April) during 4 and 3 years, respectively, between 1999 and 2003. Annual surveys indicated that the Mazatzal Mountains population declined during drought between 1994 and 1997, experienced low growth and lamb production coincident with above-normal rainfall in 1998 and drought in 1999, and exhibited higher growth, production, and productivity during 2000–2003 despite persistent drought conditions during this period. We observed no clinical symptoms of disease in radio-collared desert bighorn sheep, and hematological and other evidence of exposure to disease agents was unremarkable. Population indices on the primary study and reference areas were positively correlated with winter (November–April) rainfall. We found no evidence of forage overutilization on the primary study area. Rainfall on Mazatzal Mountains was associated with differences in primary production, particularly of forbs, forage mineral concentrations, and diets, nutritional status, and demographic attributes of desert bighorn sheep between 1999 and 2003. Higher winter rainfall was associated with higher forb growth, and higher rainfall was associated with higher concentrations of P and Se but lower levels of Fe in browse; higher concentrations of Ca, P, and Zn in forbs; and higher levels of P, Se, and Zn in grasses. Narrower mean Ca:P ratios of browse and forbs were associated with higher rainfall. Diets of desert bighorn sheep adults and lambs generally were similar, particularly near summer, and forbs tended to predominate in diets during wetter and drier years. Higher winter rainfall was associated in adult feces with more prolonged winter-to-spring increases in FN and FDAPA concentrations, higher fecal phosphorus, lower fecal Ca levels, and narrower fecal Ca:P and Na:K ratios, but levels of fecal Na increased during the driest year. Higher winter rainfall corresponded in lamb feces with higher levels of FN, FDAPA, and fecal P; lower concentrations of fecal Ca; and narrower fecal Ca:P ratios. Thus, we hypothesized that diets and nutritional status of desert bighorn sheep adults and lambs tended to correspond with rainfall patterns and associated differences in relative abundance and mineral content of forages. We found no evidence that bobcats or coyotes preyed on or scavenged desert bighorn sheep. Decline of desert bighorn sheep abundance during 1994–1997 was greater than declines on reference areas lacking mountain lions despite continually higher, and a lesser decline in, winter rainfall on the primary study area. In comparison, population indices on a reference area and on Mazatzal Mountains increased between 1999 and 2003 in association with predator reductions and lower abundance of mountain lions and predation of radio-collared animals despite continued occurrences of drought during this period. We thus identified 2 proximate factors that likely acted to influence demographic trends of the Mazatzal Mountains desert bighorn sheep population: nutritional status (higher rainfall [ultimate factor] was associated with higher availability and differences in mineral content of forages and improved indices of desert bighorn sheep nutritional status) and predation by mountain lions. We hypothesize that nutritional status and mountain lion predation during a period of drought influenced desert bighorn sheep population parameters in Mazatzal Mountains and that short-term removal of mountain lions by lethal harvest contributed to higher growth and productivity of the small, isolated population, even during periods of drought.