The importance of the standardizing sampling methodology to detect altitudinal gradients in mountains: A study case for the resident bird community in a hotspot (Atlantic forest) and the Middle Domain Effect

Abstract Among the theories that attempt to explain the elevational distribution of metazoan species along elevational gradients, the Mid-Domain Effect (MDE) is one of most debated and criticized. Here, we test whether the diversity of birds along elevational gradients in the Serra do Caparao, a mountain of the Brazilian Atlantic Forest, is explicable by the MDE. We sampled an elevational gradient between 970 m.a.s.l. and 1970 m.a.s.l. of forest vegetation, with elevational bands every 100 m. We observed a hump-shaped richness pattern of birds along the gradient, with more species in intermediate elevations. It decreased at lower and higher elevations, thereby following a unimodal distribution. Distribution of species richness indicated a high correlation with the tested MDE null model. The result was the same for endemic species. Considering the total bird species registered, up to 30% are endemic and 9 are endangered. Threatened species richness followed a different elevational distribution along the gradient: three species occurred at the lower elevations, two at the interquartile elevations and four at the highest elevations. Despite finding more species in the intermediate elevations, the areas of low and high elevations are also important for the maintenance of endemic and threatened species. To test the MDE theory and compare our results with other studies, we highlight the importance of standardized samplings and habitats. Moreover, we focus on the characterization of the surrounding landscape, which can also influence the richness of bird species diversity.