The potential effect of forest type on the structuring of animal communities in western Amazonia remains poorly understood. In this study, we tested the hypothesis that amphibian species richness, composition and abundance differ across forest types in the lowland rain forest of south-eastern Peru. By using 320 individual transects, we compared the amphibian assemblages across four major forest types (floodplain, terra firme, bamboo and palm swamp) at each of four sites separated by 3.5–105 km. We identified 1967 individuals of 65 species in 11 families and found that a large proportion of the amphibian diversity in this region is attributed to habitat-related beta diversity. Overall, we found that forest type is more important than site in predicting both species composition and abundance. We also found that, when analyses are conducted separately for each forest type and include species abundance data, similarity between assemblages decreases with increasing geographic distance. In contrast to studies that considered species presence/absence but ignored species abundances, our results highlight the importance of including abundance data in the assessment of animal diversity patterns in western Amazonia. We conclude that evaluating community structure across forest types can improve our understanding of diversity patterns in this region.

Diversity is positively correlated with water availability at global, continental and regional scales. With the objective of better understanding the mechanisms that drive these relationships, we investigated the degree to which variation in water availability affects the performance (recruitment, growth and survival) of juvenile trees. Precipitation was supplemented throughout two dry seasons in a seasonal moist forest in south-eastern Peru. Supplementing precipitation by 160 mm mo−1, we increased soil moisture by 17%. To generate seedling communities of known species composition, we sowed 3840 seeds of 12 species. We monitored the fates of the 554 seedlings recruited from the sown seeds, as well as 1856 older non-sown seedlings (10 cm ≤ height < 50 cm), and 2353 saplings (> 1 m tall). Watering significantly enhanced young seedling growth and survival, increasing stem density and diversity. Watering diminished the recruitment of species associated with upland forests, but increased the survival of both upland- and lowland-associated species. Though supplemental watering increased the growth of older seedlings, their density and diversity were unaffected. Sapling performance was insensitive to watering. We infer that variation in dry-season water availability may affect seedling community structure by differentially affecting recruitment and increasing overall survival. These results suggest that differential seedling recruitment and survival may contribute to the observed relationships between water availability, habitat associations and patterns of tree species richness.