Data from: Patterns of understorey bird diversity across Amazonian forests: survey effort and range maps predict local species richness

Species diversity typically increases from higher to lower latitudes, but
the regional-scale variation along this geographic gradient remains
unclear. It has been suggested that species diversity throughout Amazonia
generally increases westward toward the Andes, but this pattern and its
environmental determinants require further investigation for most taxa.
Using mist-net data on understorey birds, we evaluated patterns of species
richness using two approaches by addressing methodological issues that
influence local species richness and the determinants of species richness
across Pan-Amazonia. Specifically, we examined (i) the disparity between
observed and expected species richness obtained from geographic range
maps; (ii) how species' eco-morphological traits influence their
detection and relative abundance; (iii) the spatial variation in estimated
local species richness after controlling for sampling effort; and (iv) the
environmental determinants of estimated richness. We found no evidence for
a longitudinal westward increase in estimated species richness, but there
was a marked difference between the northern and southern banks of the
Amazon river. Species detection and abundance were modestly explained by
species traits, and estimated richness was weakly associated with
latitude, aboveground biomass and climatic aridity. We found that observed
variation in the local species richness was primarily driven by
differences in sampling effort, while estimated species richness showed
modest variation across large spatial scales and was poorly explained by
environmental and spatial gradients. Despite wide variation in local
species richness, we conclude that at broader scales, species richness of
understorey bird assemblages was surprisingly stable across Pan-Amazonia,
suggesting that evolutionary processes may be important in determining
these patterns at larger scales.