The relative role of rivers, environmental heterogeneity and species traits in driving compositional changes in Southeastern Amazonian bird assemblages

Amazonian rivers have been proposed to act as geographic barriers to
species dispersal, either driving allopatric speciation or defining
current distribution limits. The strength of the barrier varies according
to the species ecological characteristics and the river physical
properties. Environmental heterogeneity may also drive compositional
changes, but have hardly been assessed in Amazonia. Aiming to understand
the contributions of riverine barriers and environmental heterogeneity in
shaping compositional changes in Amazonian forest bird assemblages, we
focus on the Tapajós River. We investigate how spatial variation in
species composition is related to physical barriers (Tapajós and Jamanxim
rivers), ecological characteristics of the species (distinct guilds) and
environmental heterogeneity (canopy reflectance, soils and elevation). We
sampled birds through point counts and mist nets on both banks of both the
Tapajós and Jamanxim rivers. To test for relationships between bird
composition and environmental data, we used Mantel and partial Mantel
tests, NMDS and ANOVA + Tukey HSD. The Mantel tests showed that the
clearest compositional changes occurred across the Tapajós River, which
seems to act unequally as a significant barrier to the bird guilds. The
Jamanxim River was not associated with differences in bird communities.
Our results reinforce that the Tapajós River is a biogeographical boundary
for birds, but environmental heterogeneity determines compositional
variation within interfluves. These results contrast with diversity
patterns described for other vertebrates, suggesting that upland forest
birds singularly respond to large rivers as barriers in Amazonia, leading
to erroneous extrapolations for interpreting biogeographic results for
other Amazonian organisms.