Disentangling latitudinal diversity gradients in Taxonomic, Phylogenetic, and Functional diversity of Atlantic reef fishes

Abstract

The rate of biodiversity loss in the Anthropocene have triggered a research agenda that move beyond the description of global patterns and drivers of species diversity (i.e. taxonomic diversity - TD) to the distribution of phylogenies (i.e. phylogenetic diversity - PD) and functional traits (i.e. functional diversity - FD). Understanding the complementarity between these three biodiversity facets have provided important insights in biodiversity-ecosystem functioning relationships and community assembly mechanisms, and provide a useful framework for conservation planning of both terrestrial and marine ecosystems. Yet, empirical studies for marine fishes covering broad latitudinal gradients remain scarce. Importantly, understanding regional differences in the macroevolutionary context over which these diversity patterns have arisen can provide important insights into the balance between scale-dependent community assembly processes - i.e. competition vs. environmental filtering. Here, using standardized underwater visual census techniques, we compiled an extensive quantitative dataset on eastern Atlantic shallow-water reef-dwelling fishes (148 species within 48 families, including bony and cartilaginous fishes) covering a latitudinal extent of 0° to 60° (including 66 sites within 6 marine ecoregions). We used the unique historical and biogeographic context of the eastern Atlantic basin to: i). test for congruency in latitudinal diversity gradients between TD, PD and FD, ii). investigate the level of phylogenetic overdispersion vs. phylogenetic clustering across different ecoregions. We hypothesized that environmental filtering play a major role in shaping the structure of fish assemblages at cool-temperate ecoregions (North Sea and Celtic Seas), generating assemblages in which the constituent species are more related than chance (i.e. phylogenetic clustering). Whilst, in warm-temperate (South European Atlantic Shelf, Macaronesia) and tropical ecoregions (Gulf of Guinea Islands, Cape Verde) competition have been the major structuring force, generating assemblages in which the constituent species are less related than chance (i.e. phylogenetic overdispersion or evenness). In summary, this study represents a novel approach to understand mechanisms of generation of diversity, in a relatively understudied ocean basin.