Influence of Coral-reef Complexity on Species Richness and the Hierarchical Structuration of Species Abundances in Reef fish Communities: A Case Study in South-east Brazil

Growing complexity of coral habitat is expected to increase resource partitioning among co-occurring reef fish and, thereby, reduce to some extent the mean competitive intensity. This will have associated consequences on the internal structuring of species in reef fish communities, in particular regarding species richness and evenness of species abundances. Accumulating dedicated case studies are necessary, however, to get further empirical confirmations. The present analysis aims to contribute in this respect, comparing reef fish communities associated to two coral-reef settings that markedly differ in their degree of morphological complexity, at Itaipu Sound, Brazil. As the available samplings of these communities remained incomplete, numerical extrapolations were implemented, thereby providing least-bias estimates for both total species richness and the exhaustive distribution of species abundances in both compared reef fish communities. As expected, total species richness increases with greater degree of coral habitat complexity, while the unevenness of species abundances decreases. This decrease in abundance unevenness – reflecting the corresponding relaxation of the mean level of competitive intensity – is partly due to the direct, negative influence of species richness on abundance unevenness, as an overall trend. Beyond that, however, the relaxation is further strengthened by an additional “genuine” contribution – this time independent from the variation in species richness – and, as such, directly and idiosyncratically attached to the improvement in habitat complexity.