Invasive plants induce the taxonomic and functional replacement of dune spiders

Understanding the impacts caused by invasive plants on higher trophic levels is fundamental to predict future changes to native communities. Invasive plants like Acacia longifolia can change habitat structure and thereby alter the taxonomic and functional diversity and composition of predator communities. This work aims to evaluate the effect of this invasive plant species on spider assemblages, at both taxonomic and functional levels, in grey dunes. Spiders were sampled in six native and six invaded sites, in northwestern Portugal. Individuals were identified to species level and their functional features were quantified. Generalized Linear Models (GLM) were used to compare differences of species richness, abundance, functional diversity (FDQ) and trait proportions between native and invaded dunes. We also assessed the relative contribution of replacement and richness differences to overall taxonomic and functional dissimilarity between native and invaded dunes. GLMs showed that there was no change in species richness between dune types, but there was a significant increase in FDQ in invaded dunes. Significant shifts in the proportion of trait categories were observed. Native dunes favoured ambush hunters and myrmecophagous species. On the contrary, invaded dunes favoured species that inhabit leaf litter, web builders and crustaceophagous species. The proportion of diurnal species was higher in native dunes, contrary to invaded dunes, where both diurnal and nocturnal species dominated. Taxonomic and functional dissimilarity was mainly determined by the replacement, not the net loss, of species and traits. These results showed that functionally different spider species were favoured by the invasion of Acacia longifolia. The invasion of Acacia longifolia certainly attenuated the extreme habitat conditions normally found in grey dunes, allowing the immigration of more generalist species from neighbouring habitat types into invaded dunes. Thus, it can be concluded that there was no homogenization, taxonomic or functional, but changes in the composition of spider assemblages and the replacement of functions after dune invasion.