A Síndrome das florestas vazias e a importância dos pequenos fragmentos para a conservação dos anfíbios

Abstract

The planet is undergoing an unprecedented loss of biodiversity, so much so that it is considered the sixth mass extinction event. The most drivers of species decline are habitat loss and fragmentation, affecting all natural ecosystems, especially tropical forests. Amphibians are particularly sensitive to fragmentation and habitat loss and many species are strict forest inhabitants. We evaluated 22 fragments of Atlantic Forest in southeastern Brazil, between 5 and 1570 hectares, in order to evaluate the potential of small fragments to be useful for the conservation of anurans and to determine if this group of animals has also vanished from larger areas, implying in the defaunation of forest fragments. For each fragment, we sample the number of individuals, species richness and composition in anuran assemblages in relation to the area, habitat heterogeneity and vegetation quality. The structure of the anurans composition in assemblages and guilds was tested in relation to the gradients of heterogeneity, vegetation quality and size of forest fragments, searching for patterns of species loss (nested) or to species substitutions (turnover). We not observed relation between the variables of the fragments with individuals number or structuring patterns for the species considering all anurans in the assemblages. However, when we analyzed separately in guilds, considering restricted forest species and species with high scatterability in the landscape, the effects on the structure of the assemblages were different. Our data indicate that the individuals number in the assemblages or guilds was not affected by the area, heterogeneity or quality of the habitat, suggesting that in each fragment there are resources to support the same individuals number. The all species in the assemblage and the strictly forest guild showed a decrease in number of species associated only with the habitat quality variable. The species composition in both guilds presented different patterns of structuration. The strictly forested guild presented a nested pattern in relation to vegetation quality, habitat heterogeneity and area of the fragments, while the dispersers guild presented a turnover pattern only in relation to habitat quality. We conclude that restricted forest species depend on a greater set of factors to maintain their populations in forest fragments, such as the fragment area, quality and heterogeneity of habitat. We considered, that the presence of species with wide displacement in the landscape is defined by the fragment quality. In this scenario, many small fragments supported a relatively high number of restricted forest species when compared to larger fragments, but with a greater degree of degradation, demonstrating the importance of these fragments have for the conservation of many species. In addition, some fragments presented total defaunation of strictly forest species and in some cases for all anurans assemblage, becoming empty fragments.