Bird species diversity in the Atlantic Forest of Brazil is not explained by the Mid-domain Effect

The Atlantic Forest is an excellent case study for the elevational diversity of birds, and some inventories along elevational gradients have been carried out in Brazil. Since none of these studies explain the patterns of species richness with elevation, we herein review all Brazilian studies on bird elevational diversity, and test a geometric constraint null model that predicts a unimodal species-altitude curve, the Mid-domain Effect (MDE). We searched for bird inventories in the literature and also analysed our own survey data using limited-radius point counts along an 800 m elevational gradient in the state of São Paulo, Brazil. We found 10 investigations of elevational diversity of Atlantic Forest birds and identified five different elevational patterns: monotonic decreasing diversity, constant at low elevations, constant at low elevations but increasing towards the middle, and two undescribed patterns for Atlantic Forest birds, trough-shaped and increasing diversity. The average MDE fit was low (r² = 0.31) and none of the MDE predictions were robust across all gradients. Those studies with good MDE model fits had obvious sampling bias. Although it has been proposed that the MDE may be positively associated with the elevational diversity of birds, it does not fit the Brazilian Atlantic Forest bird elevational diversity.

We thank Pedro F. Develey and José C. Motta Jr for important suggestions on an earlier stage of this study, and Instituto de Biociências da Universidade de São Paulo. Gabriel P. Moraes and Fernanda G. Maciel helped during recent bird surveys in São Paulo. IdeaWild and United Parcel Services (UPS), through Bruno Ehlers, provided essential field equipment. VC benefited from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) scholarship during the development of this study. LFS receives a grant from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, #313818/2009-6). Léo Novaes produced all images. Three anonymous reviewers and Michael Lawes greatly improved an initial manuscript with several important contributions.