|Free-Roaming Horses Adversely Impact Greater Sage-Grouse Population Dynamics in Sagebrush Ecosystems|
|Shawn O’Neil, Peter Coates, Diana Munoz, Ian Dwight, John Tull|
Free-roaming horse (Equus caballus) populations have increased in sagebrush ecosystems and have exceeded maximum appropriate management levels (AMLmax) designated by the Bureau of Land Management for more than a decade. Concomitantly, greater sage-grouse (Centrocercus urophasianus) populations have declined from loss and degradation of critical habitats. Overgrazing can degrade sagebrush communities, but the effects that feral horses have on sage-grouse population dynamics are largely unknown. We employed Bayesian state-space models to estimate sage-grouse population rate of change (λ) using 15 years of lek surveys in relation to horse abundance (relative to AMLmax) and other environmental covariates such as sagebrush cover, wildfire, and precipitation indices. Additionally, we employed a post-hoc impact-control design to validate existing AMLmax values in relation to sage-grouse population responses that help control for environmental stochasticity and broad-scale oscillations in sage-grouse abundance. For every 50% increase in horse abundance over AMLmax, a 2.6% annual decline in sage-grouse abundance was predicted. When horse abundance was at or below AMLmax, sage-grouse λ estimates mirrored trends at areas with no horses. Conversely, results indicated a 75%, 97%, and 99% probability of λ decline relative to controls when horse abundance relative to AMLmax was 200%, 250%, and 300%, respectively. For context, horse herds were estimated at 405% AMLmax in Nevada, USA during 2019. Model projections indicate ~70% declines in sage-grouse populations within horse occupied areas by 2034 if horse population trends continue unabated, compared to 18% declines in areas not currently occupied by horses. Monitoring frameworks that consider sage-grouse and other ecosystem indicator species can guide management decisions that promote co-occurrence of horses with sensitive wildlife and other managed livestock within multiple-use landscapes. Findings are preliminary and provided for best timely science.