The comeback of a top predator and its effects on a population of feral horses

The absence of top predators can lead to significant ecological consequences, especially in association with changes in bottom-up ecosystem regulation. In the context of biological invasions, the enemy release hypothesis explains how invasive alien species can reach high densities in the absence of natural antagonists, often leading to negative effects on biodiversity and ecosystem services. Feral equids frequently reach problematic densities in natural environments, where their management faces operational and communication challenges. In this study, we evaluated the impact of puma (Puma concolor) presence on the demography of a feral horse (Equus caballus) population in an Argentinian grassland nature reserve. We compared periods with (2023–2024) and without predation (1995–2002). Foal survival was significantly lower in 2023–2024, while adult survival increased compared to 1995–2002. These changes, coupled with direct evidence of puma predation on young horse carcasses, suggest that pumas are affecting population growth rates by reducing recruitment, and may limit population size. Concomitantly, mean female body condition improved in 2023–2024, suggesting that the loss of foals to predation may reduce lactation-related energy costs. These findings suggest that pumas could play an important role in the dynamics of feral horse populations, marking the first documented case of such a regulation in South America. Incorporating predator activity into management strategies could reduce the need for direct interventions, offering a more sustainable approach to controlling the ecological impacts of feral horse populations.