Avian species composition across the A mazon R iver: the roles of dispersal limitation and environmental heterogeneity

Abstract Aim The roles of dispersal limitation and environmental heterogeneity in structuring tropical species composition can be better understood by accounting for dispersal barriers and possible niche differentiation effects. We make ecological and historical interpretations of dissimilarity in avian species composition across a riverine dispersal boundary in the light of environmental characteristics, species and subspecies range limits, and geographical distances. Location Lowland rain forest, western A mazon R iver B asin, P eru. Methods We surveyed all birds and one plant family, collected soil samples and measured forest structural characteristics and fragmentation in surrounding landscapes, at sites to the north and south of the A mazon R iver flood plain. We used Mantel tests, multiple regression on distance matrices, indicator species analysis and ordination methods to assess the relationships among dissimilarities in species composition, geographical distance, position relative to the river and environmental characteristics. We examined compositional variation for all bird species, for only species without range limits between sites, and for species with and without subspecies limits at the A mazon R iver. Results Dissimilarity in avian species composition across the river was large, despite a lack of environmental differences. Most of this dissimilarity was accounted for by species and subspecies range limits at the river. Plant species composition did not show any dissimilarity across the river. Plant species composition and forest fragmentation explained additional components of avian compositional dissimilarity not associated with the riverine boundary and involving different bird species. Main conclusions The riverine dispersal boundary, floristic heterogeneity and forest fragmentation were associated with distinctive components of avian species compositional dissimilarity, collectively explaining three‐quarters of the total dissimilarity among sites. Compositional dissimilarity was consistent with historical and continuing isolation of avian populations on opposite sides of the river, and may be partly driven by niche differentiation between subspecies. Geographical distance as a measure of dispersal limitation would not have accounted for these relationships. The use of rivers in biogeographical region delineation should address their variable importance for different taxa.