Determinants of different facets of beta diversity in Mediterranean marine amphipods

Abstract

Relying on a purely taxonomic view of diversity may ignore the fact that ecological communities can be constituted of species having both distinct evolutionary histories and functional characteristics. Thus, considering how the multiple facets of diversity vary along environmental and geographic gradients may provide insights into the role of historic processes and current environmental changes in determining the divergence or convergence of lineages and functions, ultimately influencing the way species assemble across space. However, analyses can be somehow flawed by the choice of traits being analysed, as they should capture the whole functional variability of species in order to assess the relationship between phylogenetic and functional diversity along a gradient. When continuous measures of functional diversity based on a variety of different traits are absent, the use of functional traits known to show strong phylogenetic signal can help elucidating such relationship. By using distributional, traits and taxonomic-distance information, we explored how the taxon, functional and phylogenetic community composition (co)vary along spatial and environmental gradients in seagrass amphipod metacommunity within the Mediterranean Sea. We used beta diversity partitioning and null models to determine the role of deterministic and stochastic processes on the replacement and the net loss/gain of species, lineages and highly conserved beta-niche traits. We showed that dispersal-based processes are the main determinants of the high taxonomic and phylogenetic beta diversity, while niche-based processes explain the low functional dissimilarity among assemblages. Moreover, phylogenetic and functional beta diversity showed contrasting patterns when controlling for the underlying taxonomic composition, with the former being not significantly different and the latter significantly lower than expected. Our results suggest the key role of historical and biogeographic processes in determining the present-day patterns of community assembly and species turnover, providing also evidence for parallel assemblage structures in Mediterranean seagrass amphipods.