Increased fish diversity over day and night in structurally complex habitats of artificial reefs
A major goal of ecology is to understand how spatial heterogeneity determines patterns of species diversity and composition. Studies have demonstrated positive relationship between environmental heterogeneity and diversity, but evidence from marine ecosystems is controversial and scarce in terms of how spatial heterogeneity and diel period mediate this relationship. We used fish communities from four Southwestern Atlantic vessel reefs to assess whether positive heterogeneity-diversity relationships (HDR) hold for these mobile organisms and whether the relationships weaken with nightfall. We sampled fishes in three habitats of contrasting structural complexity (high, low and control), over day and night, and employed two complementary diversity frameworks: partitioning of gamma diversity into independent alpha and beta components (Jost's approach) and partitioning of beta diversity into turnover and nestedness components (Baselga's approach). We recorded 5005 fishes belonging to 76 species and 31 families. As expected, the mean alpha diversity of rare species (0D) doubled from control to high complexity areas and decreased by half from day to night. The diversity of typical species (1D) also doubled from control to high complexity areas, but did not reduce at night. Complexity and diel period did not have significant effect on the diversity of dominant species (2D). No relationship between complexity and alpha diversity was weakened at night. Beta diversity of the three diversity orders significantly differed from 1 (totally homogeneous vessel reef), indicating that complexity underlies patterns of beta diversity. This effect was consistent in both diel periods, contradicting expectations of weaker influence of complexity at night. The turnover component of beta diversity was consistently greater than nestedness at day and nigth (2.8 and 1.9-fold, respectively). Our findings support positive HDR for the diversity of rare and typical species. Dominant species also respond to heterogeneity by replacing each other across the complexity gradient, but not by becoming more numerous in high complexity areas. Diel changes did not affect the strenght of HDR, revealing an uninterrupted role of environmental heterogenity on fish communities. Conserving heterogeneous, structurally complex habitats is crucial for conserving marine fish diversity.