Disentangling Factors Contributing to Population Performance of Zion Bighorn Sheep

Bighorn sheep populations across the western United States face increasing challenges from habitat loss, disease, and predation. In Zion National Park and surrounding areas, survey-based estimates indicate that the desert bighorn sheep population declined from over 800 individuals in 2018 to fewer than 500 in 2021. Understanding what drives these population changes is critical for effective wildlife management and conservation. This research examined key factors influencing population growth and individual survival to inform strategies for recovery in this bighorn population. First, we analyzed which vital rates contributed most to population trends by estimating survival and reproduction rates from ground and aerial surveys, as well as GPS collar data from individual sheep. We found that adult survival was the most important factor affecting population growth, explaining nearly 90% of the variation in population growth rate. Further investigation revealed that mountain lion predation was a leading cause of adult mortality, so we assessed the impact of varying levels of predation on the population trajectory. While predation was a significant source of mortality, our results suggested that addressing it alone would likely not be enough to stop the population decline. Since predation was a leading cause of mortality, we also examined whether individual movement patterns influenced survival. Using locations from GPS collared individuals, we quantified seasonal home ranges and linked individual space use to survival. Surprisingly, we found that individuals with larger seasonal home ranges had lower mortality rates from predation. This suggests that sheep that move across larger areas may be better able to find necessary resources and avoid predators than those that stay in one place. These findings provide insight into bighorn sheep population dynamics for wildlife managers working to conserve desert bighorn sheep in Zion. Trying to mitigate other sources of mortality, such as disease or human disturbance, may increase effectiveness of recovery actions than focusing solely on predator control. By improving our understanding of the factors influencing bighorn sheep survival, this research helps guide management actions that balance predator-prey dynamics and ensure the long-term persistence of this herd.