Free-roaming horses exceeding appropriate management levels affect multiple vital rates in greater sage-grouse

Abstract

Since the passage of the Wild Free-Roaming Horses and Burros Act of 1971, federal agencies have been responsible for managing free-roaming equids in the United States. Over the last 20 years, management has been hampered by direct opposition from advocacy groups, budget limitations, and a decline in the public's willingness to adopt free-roaming horses (Equus caballus). As a result, free-roaming equid numbers have increased to >3 times the targeted goal of 26,785 (horses and burros [E. asinus] combined), the cumulative sum of the appropriate management levels (AML) for all 177 designated herd management areas (HMA) managed by the Bureau of Land Management. This increase is one of the causes of greater sage-grouse (Centrocercus urophasianus) population declines, owing to habitat alteration from free-roaming equids exacerbated by ongoing drought. To evaluate potential demographic mechanisms influencing these declines, we compiled survival data from 4 studies in central Wyoming, USA, including 995 adult female (first-year breeders or older) sage-grouse during the breeding season, 1,075 nests, 372 broods, and 136 juveniles (i.e., overwinter survival for fledged young), from 2008–2022. During this period, we also obtained population information for free-roaming horses from 9 HMAs used by individual grouse in our sample. Population estimates of free-roaming horses for these HMAs ranged from 59% to 7 times of the maximum appropriate management level (AML_max). Sage-grouse monitored outside of HMAs represented control populations and, because we assumed they were not exposed to populations of free-roaming horses, we set values of AML_max to zero for all grouse located outside of HMAs. To evaluate whether free-roaming horses were negatively affecting sage-grouse, we modeled daily survival of breeding age females, nest, broods, and juveniles. There was strong or moderate evidence that overabundant free-roaming horses negatively affected nest, brood, and juvenile survival. When horse abundance increased from AML_max to 3 times AML_max, survival was reduced 8.1%, 18.3%, 18.2%, and 18.2% for nests, early broods (≤20 days after hatch), late broods (>20 days to 35 days after hatch), and juveniles, respectively. These results indicate increasing free-roaming horse numbers affected vital rates for important life stages of sage-grouse, and that maintaining free-roaming horse numbers below AML_max would reduce negative effects to sage-grouse populations.