The modern biodiversity crisis is characterized by an unprecedented loss of species, owing to threats such as habitat loss, global change, and emerging pathogens. If these threats cause local extinctions nonrandomly across communities, then among-community biotic distinctiveness, or beta diversity, will be lost. This underappreciated loss of diversity, known as biotic homogenization, can rob ecosystems of important functions and disproportionately affect regional diversity, resulting in more regional and global extinctions than would be expected under random extinction. In particular, global amphibian declines and extinctions that are associated with a pathogenic agent of global amphibian decline, Batrachochytrium dendrobatidis (Bd), may be causing selective extinctions and homogenizing amphibian biotas. In this study, we use data from thorough surveys of eight rainforest sites in Costa Rica and Panamá to determine if recent declines and extirpations associated with Bd have resulted in selective extinction and homogenization of these diverse tropical American amphibian biotas. To address this we applied two analyses: 1) a null model to determine if Bd-associated extirpations have caused changes in biotic similarity and beta diversity, irrespective of changes in species richness; and 2) a null model to determine how selective local extinctions are affecting regional and global amphibian diversity relative to random extinction processes.

Results/Conclusions

We report that that Bd-associated extinctions in Costa Rica and Panamá were nonrandom and primarily targeted low-occupancy and endemic species. This resulted in a loss of biotic distinctiveness and homogenization of the remnant amphibian fauna, such that amphibian biotas are now more similar than would be expected by chance. This regional homogenization eliminated known biogeographic patterns that previously structured the biotas. Additionally, many more species were extirpated from the region than would be expected if Bd-associated extinctions occurred randomly. This indicates that Bd did not act indiscriminately on the amphibian community, but instead primarily affected low-occupancy and endemic species, resulting in disproportionately high rates of extinction. Our results indicate that Bd is acting as an ecological filter, reducing amphibian biodiversity to relict assemblages of threat-tolerant species and ultimately causing amplified biodiversity loss at regional and global scales.