Relationship Between Endemic and Invasive Frogs on Grenada

Invasive species, loss of habitat, and climate change are just some of the many threats accelerating biodiversity loss, and understanding their impacts on endangered species is key to implementing effective conservation. The endemic Grenada frog (<i>Pristimantis euphronides</i>) is found only in high elevation cloud forests, habitat that is being invaded by the introduced Lesser Antillean frog (<i>Eleutherodactylus johnstonei</i>) and threatened by climate change. Between 2004 and 2020, our field team surveyed three key sites in the central highlands of Grenada to monitor populations of both frogs. We used generalized linear models and Spearman's rank analysis to evaluate the effects of site and invasive frog relative abundance on the endemic frog. Although the relative abundances of the two species were negatively correlated overall (<i>ρ</i> = -0.501, 95% CI: (-0.660, -0.300)), the relationship between them was weakly positive in the model that included site as a covariate. The two species appear to respond similarly to environmental fluctuations at local scales, but the negative overall correlation implies that competition with the Lesser Antillean frog may have affected the distribution of the Grenada frog across the island. Grenada frogs were much more abundant than Lesser Antillean frogs at the highest elevation site, while the reverse was true at the lower sites. If higher elevation sites are indeed acting as refugia for the Grenada frog from its invasive competitor, the effects of climate change at those high elevation sites will likely be critical to the future of the species.