Geographical drivers of altitudinal diversity of birds in the Atlantic Forest.

We measured bird diversity over an 800 m range in altitude to test the factors that influence changes in diversity, in the Atlantic Forest in the state of São Paulo, Brazil. We counted forest birds in nine 100 m transects comprising three-point counts, the center of each separated by 200 m. A total of 108 species were found, and diversity was distributed among sampling points and altitude formed a trough-shaped relationship of number of species by area, altitude, and, most strongly, slope. In Principal Coordinate Analysis we found that grouping points by similarity formed three distinct units that were united by altitude: the lowest at near sea level, 100–400 m, and 500–800 m. Thus, we demonstrate that, in the Atlantic Forest, turnover of species with altitude occurs, but unlike higher, often steeper, tropical gradients in Andean forests, where vegetation more clearly changes with elevation, species turnover is more likely to be associated with variables other than changes in vegetation, and which are often not included in studies of the elevation gradient in species richness. The mid-domain effect did not explain altitudinal gradient in diversity, while a quadratic relationship with altitude did, suggesting that reduced abundance and diversity are associated with steeper slopes. [ABSTRACT FROM AUTHOR]