Importance of habitat representativeness and complexity for bird species richness in a humid forest embedded in the Dry Chaco

Understanding the mechanisms determining species richness at the local scale is important in ecology and conservation. Previous studies about bird richness in isolated forest focused on the effects of fragment ation-isolation. However, other mechanisms can be equally important. Here, we evaluated two hypotheses: (i) more complex habitats hold a higher bird richness because they provide more ecological niches; (ii) the occurrence of birds associated to the region is determined by the composition of the bird regional pool combined with habitat representativeness acting as a species filter. The study area comprised three contiguous habitats (dry slopes, humid slopes, and piedmont) located in the Guasayan mountain range, Dry Chaco ecoregion, northern Argentina. The humid slopes are characterized by the presence of a humid forest, which is isolated from other humid forests because it is embedded in a semiarid region with prevailing dry forests. The results showed the humid forest as the most complex habitat and also as the least representative of the region, with the lowest bird species richness and the lowest occurrence of birds associated to the Dry Chaco. We conclude that the occurrence of birds associ-ated to the Dry Chaco is determined by the combination of the regional bird pool and habitat representativeness acting as a species filter. This combination contributes to the lowest bird richness in the isolated humid forest, despite its greatest habitat complexity. These results are relevant to the management of these forests, especially if management implies a substantial modification of habitat composition and structure.