Effects of biological invasions and habitat degradation on amphibian populations in Cerro Largo, Uruguay.

Abstract

Amphibians are the most threatened vertebrates globally due to a range of stressors. In this study spanning 11 years and covering 61 Pampas ponds in Cerro Largo Department, Uruguay, we aimed to evaluate the combined effect of the invasion of the American bullfrog (Aquarana catesbeiana) and the loss of environmental heterogeneity on native anuran assemblages. We conducted annual breeding season sampling for which we recorded native anuran species presences and abundances across different ponds. Additionally, we documented bullfrog invasion history and abundance and key environmental and climatic variables. We measured pond heterogeneity, which is influenced by variations in livestock grazing intensity and historical human use of native forests. We used generalized linear mixed models to predict richness and abundance of native anurans. We recorded 18 native amphibian species across the ponds, with declines in both richness and abundance as bullfrog invasion intensified. Models showed that ponds could lose up to 40% of amphibian species and 50% of amphibian abundance within 10 years of bullfrog invasion. Although environmental heterogeneity, particularly microhabitat edge diversity, attenuates these impacts, it is insufficient to halt this process. Among the most affected native anuran species were 3 highly aquatic-associated species. Our results highlight stressor effects across a broad spatiotemporal scale through an integrative approach, which we recommend for future research. Our findings underscore the threat of invasions to amphibian conservation. By examining the combined impacts of biological invasions and environmental heterogeneity, we have advanced understanding of how global change drives amphibian declines. Urgent control measures are needed to eradicate and prevent invasions in amphibian breeding sites and to preserve and restore these habitats, the focus of which should be microhabitat diversity.