Do pathogens limit the distributions of tropical trees across a rainfall gradient?

1. Organisms are adapted to particular habitats; consequently, community composition changes across environmental gradients, enhancing regional diversity. In Panama, a rainfall gradient correlates with the spatial turnover of tree species. While strong evidence suggests that tree species common in the wetter forests are excluded from the drier forests by seasonal drought, the factor(s) excluding drought-tolerant species, common in the drier forests, from the wetter forests remain ambiguous. 2. Here, we show that seedlings were significantly more likely to suffer pathogen-caused damage and mortality in the wetter forest. While seedlings of dryand wet-forest species were equally likely to suffer pathogen attack, seedlings of dry-forest species were significantly more likely to die when attacked and tended to suffer more pathogen-caused mortality overall. Furthermore, seedlings of dry-forest species suffered pathogen-caused mortality in the forest in which they do not naturally occur and in which conspecific and/or congeneric adults are absent or rare, indicating that some pathogens are relatively widespread and/or are capable of damaging multiple host species. 3. Synthesis. Elevated risk of pathogen-caused damage and mortality in the wetter forests and a greater impact to host fitness from pathogen attack for seedlings of dry-forest species suggest that pathogens may enhance regional forest diversity by contributing to changes in tree species composition via the exclusion of dry-forest tree species from the wetter forests. This study highlights a potentially widespread and under explored mechanism by which pathogens shape plant communities at the landscape scale. An understanding of how species’ distributions are shaped by the interplay between abiotic and biotic factors is essential for conservation biology.