Effects of mountain lion predation on reducing feral horse population growth rates: panacea or pipedream?

David Stoner^1*Brian Folt²Kathryn Schoenecker³

• ¹Utah State University, Logan, United States
• ²University of Nevada, Reno, Reno, Nevada, United States
• ³United States Geological Survey (USGS), United States Department of the Interior, Reston, Virginia, United States

A major goal of applied ecology is to evaluate how demographic rates, such as fertility and survival, contribute to population growth, and consequently, how demography might be manipulated to achieve management objectives. In western North America, some feral horse (Equus caballus) populations occupying public lands are protected by federal law and managed for site-specific population targets, but rapid population growth rates (>20% per yr) can result in overpopulation and conflict with other protected land uses. In response, management agencies and stakeholder groups have sought natural solutions to the problem of horse overabundance. Mountain lion (Puma concolor) predation has been hypothesized to have suppressive effects on horse population growth rates under some conditions. Here, we evaluated the degree to which this phenomenon might reduce horse growth rates using elasticity analysis, scenario analysis with simulations, and an empirical state-space model for a population in southeastern Nevada subject to chronic predation. Age-specific elasticities revealed that annual population growth rates (λ) were more sensitive to perturbations in foal and yearling survival rates than for older age-classes. Combined with empirical measures indicating that foals comprise approximately 60% of horses killed by lions, suggests that predation may have greater potential to reduce horse population growth rates than previously recognized. Scenario analysis predicted that horse populations could decrease to target levels within 10 years if predation reduced annual survival rates of: (1) foals by 80% each year, or (2) foals, yearlings, and 2-year olds by >60%. Furthermore, the state-space model suggested that mountain lion predation may have reduced horse λ by 23.6% in 2021 and 24.2% in 2022, but populations continued to grow despite this pervasive source of mortality. Together, our results suggest that predation alone is unlikely to induce stable or negative population growth of feral horses. That said, our findings suggest two aspects of predation that may benefit managers. First, chronic predation may reduce λ, thereby increasing the time required for populations to recover from declines or exceed management objectives; and second, predation may be most effective achieving management objectives when combined with other interventions, such as removals or fertility control treatments.