Correlates of Recruitment in Montana Bighorn Sheep Populations

Abstract

Relatively little is known about bighorn sheep (Ovis canadensis)population dynamics across Montana. In an effort to improve understanding of bighorn recruitment, we summarized demographic data collected by Montana Fish, Wildlife and Parks for 48 bighorn populations in five ecological regions (eco-regions) across Montana. For 22 bighorn populations, data were adequate for use in multiple linear regression estimation of baseline recruitment rates (indexed by lamb:ewe ratios) and to evaluate relationships between recruitment rates and annual variation in weather conditions and all-age disease die-off events. After accounting for all-age disease die-off events, recruitment rates of populations in three eco-regions were very similar, one had lower recruitment rates than all others in the state, and one had recruitment rates that were not comparable to others due to timing of data collection. There was substantial variation in baseline recruitment rates of populations within eco-regions. After all-age disease die-off events, recruitment rates were typically severely reduced for multiple years. Recruitment rates of individual populations were related to the average number of animals counted in a population, with small populations having lower baseline recruitment rates than those for larger populations. We failed to detect consistent correlations between recruitment and annual weather conditions across populations. We suspect that
the small size of many bighorn populations in Montana limits biological insight that can be gained, as accurate demographic data are difficult to collect from small populations, and small populations can be strongly influenced by unpredictable, chance events.