Relação entre a diversidade funcional e área em assembleias de peixes recifais sujeitas a um gradiente de riqueza de espécies


Understanding the patterns and processes that govern the functional structuring of communities along spatial scales is still a challenge in community ecology. Functional diversity is the result of the action of abiotic and biotic filters that regulate the presence and contribution of functional attributes inside the assemblages. Large-scale studies demonstrate that reef fish fauna has a high degree of functional redundancy, where the addition of species does not normally result in new functions in local assemblages. However, niche theory predicts that species tend not to overlap in resource use, a pattern that can be exacerbated at small scales. In this context, the aim of this study is to investigate how the functional structure of reef fish assemblages respond to area increase in three locations in the western Atlantic Ocean subject to a wide gradient of species richness: Curaçao, Abrolhos and Atol das Rocas. More specifically, the relationships between the indices of functional richness (FRic), functional evenness (FEve) and area were evaluated. Based on niche theories and efficient use of resources, we hypothesized that as area increases, new niches become available, and therefore new species are sampled. Likewise, it is expected that functional richness will increase as the sample area increases. On the other hand, evenness may decrease, since larger areas can accommodate more functionally similar species and concentrate greater accumulations of functional entities in certain areas of the functional space, decreasing the FEve. For this purpose, species occurrence data were gathered from checklists and occurrence and abundance data from 40 m² visual censuses. Species were classified into eight functional attributes. For each metric, FRic and FEve accumulation curves were constructed using an extension of the Gower dissimilarity matrix and Principal Coordinate Analysis (PCoA). Functional indices were calculated from randomization of sample units (visual censuses) and contrasted with null models. We found that increases in the sample area lead to an increase of FRic and a decline of FEve, regardless of local species richness. Also, small sample areas are characterized by high FEve (i.e., functional entities are more evenly distributed in the functional space). Regardless of the species richness, the functional spaces are relatively similar between the three locations, suggesting that even isolated reefs have communities that present a minimal functional structure to maintain the functioning of the ecosystem. These patterns may be related to the optimization of resource use in isolated and poorly connected areas, a mechanism to reduce local competition. The combination of limiting similarity and the best use of available resources result in a high functional equitability ? when the functional entities are more evenly distributed in space. These processes, therefore, may have shaped the functional structure of communities in isolated reef systems sampled.

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