Habitat variation, species diversity and ecological functioning in a marine system

The expectation that long-term, broad-scale changes in the relative abundance of species, homogenisation of habitats and decreases in diversity will affect ecosystem function has led to an increasing number of studies on functional diversity and composition. Such studies frequently consider the effect of environmental gradients and anthropogenic impacts, but rarely the effect of biogenic habitat variation. In marine soft-sediment systems, habitat variability is likely to be of particular importance because of the strong link between habitat and species diversity. In this study we examine the link between functional trait diversity (as richness and evenness) and composition, and habitat variation in two locations with different regional species pools. We found similar functional traits occurring in the two locations, but differences between habitats within the locations. High evenness within traits was apparent (across both locations and habitats) reflecting the potential for the maintenance of function with the loss of individual species. Between-habitat differences in functional traits were driven by differences in organism densities rather than the presence/absence of individual traits, emphasising the importance of density shifts in driving function. Furthermore, our demonstration of habitat variation as a driver of functional composition and diversity suggests that habitat heterogeneity should be explicitly included within studies trying to predict the effect of species loss on ecosystem function.