Sierra Nevada Bighorn Sheep: History and Population Ecology.

Abstract

The Mount Baxter and Mount Williamson bighorn herds in the Sierra Nevada were studied from 1974 to 1979 with the objective of investigating population status and controlling factors. Minimum herd sizes of 217 and 30 were established for the Baxter and Williamson herds, respectively, in 1978. Recruitment ratios, ram age structure, and an index of population density all indicated a substantial increase in the Baxter herd since the 1960's. Recruitment ratios for the Williamson herd suggested that it is approximately static. Lungworm infection in the Williamson herd was 10 times that of the Baxter herd for adults and about 100 times for lambs and yearlings. Infection levels of both herds were low compared with Rocky Mountain herds, and no clinical signs of infection were apparent; lungworms were not considered an important influence on demography in the Sierra. Human disturbance of ewe-lamb groups was investigated in summer. There was no evidence that long term spatial displacement was occurring in the Baxter herd. Also, with its increasing population trend, it could not be argued that disturbance from humans was adversely affecting reproduction. A small sample of interactions with the Williamson herd suggested greater wariness than the Baxter herd. Human use of Mount Williamson has increased exponentially since World War II. Coincident with this increase has been a loss of bighorn summer range. The current summer range boundaries coincide with regular routes of human use; a causal relationship may be involved. The nutrient content of the bighorn diet was found to begin rising in early February, reaching a peak in early May, then declining until October. The timing of plant growth in spring lagged with elevation at a rate of one day per 17.8m, and elevational differences in diet quality of bighorn reflected this; forage quality was traded off for higher, safer habitat during lambing in late April and May. An elevational difference of 1100m and large differences in diet quality were found between a lambing area in the Baxter herd range and one in the Williamson herd range. However, milk consumption did not differ between the two herds. This suggests that nutrition of ewes in both herds exceeded minimum nutrient requirements of early lactation, and that lambs received maximum milk rations. The annual pattern of diet quality closely matched that of nutrient requirements of pregnant and lactating ewes. It was predicted that lamb mortality would be low and recruitment rates would correlate well with ovulation rates, which are determined by prior nutrition. Summer nutrition in the Sierran alpine depends on snow pack, because this is the major source of water in summer. Winter nutrition appears to be influenced by the timing of winter storms. It was thus predicted that winter precipitation would influence recruitment rate two years later. It was found that 73% of the variation in recruitment of the Baxter herd was explained in a multiple regression by Nov-Dec and Jan-March precipitation two years prior. An index of population density was entered as a third independent variable, but was not significant. It was thus concluded that population density effects are not yet important, and the Baxter herd can be expected to increase further.