Habitat filtering differentially modulates phylogenetic vs functional diversity relationships between dominant ground-dwelling arthropods in salt marshes

While mechanisms underlying biological diversities at different scales received huge attention over the last decades, whether local abiotic factors driving functional and phylogenetic diversities can differ among ecologically and phylogenetically closely related taxa remains under-investigated. In this study, we compared correlations and drivers of functional (FD) and phylogenetic (PD) diversities between two dominant taxa of ground-dwelling arthropods in salt marshes, spiders and carabids. Pitfall trapping in two sampling sites of N-W France resulted in the collection and identification of more than 7000 individuals belonging to 67 species. Morphological and behavioral traits, as well as molecular sequences of COI gene, were attributed to all species for calculating functional and phylogenetic diversities respectively. Both taxa exhibited high correlation between FD and PD, which was even higher in carabids probably due to their lower species richness. Analyses using Bayesian framework and structural equation modeling revealed that FD and PD were positively influenced by taxonomic diversity in spiders and carabids, but abiotic factors driving FD and PD differed between taxa. Salinity especially drove the taxonomic diversity of carabids, but not that of spiders, suggesting that spiders are more plastic and less selected by this factor. Phylogenetic diversity was conversely influenced by salinity in spiders but not in carabids. This interesting result can be interpreted by different evolutionary history and colonization process of salt marshes between the two model taxa. Our study finally highlights that, even in taxa of the same phylum and occupying the same niche in a highly constrained habitat, functional and phylogenetic diversities can have different drivers, showing different filtering mechanisms and evolutionary history at small spatial and temporal scales.