Gradients in the number of species at reef-seagrass ecotones explained by gradients in abundance

Abstract

Gradients in the composition and diversity (e.g. number of species) of faunal assemblages are common at ecotones\nbetween juxtaposed habitats. Patterns in the number of species, however, can be confounded by patterns in abundance of\nindividuals, because more species tend to be found wherever there are more individuals. We tested whether proximity to\nreefs influenced patterns in the composition and diversity (‘species density’ = number of species per area and ‘species\nrichness’ = number of species per number of individuals) of prosobranch gastropods in meadows of two seagrasses with\ndifferent physiognomy: Posidonia and Amphibolis. A change in the species composition was observed from reef-seagrass\nedges towards the interiors of Amphibolis, but not in Posidonia meadows. Similarly, the abundance of gastropods and\nspecies density was higher at edges relative to interiors of Amphibolis meadows, but not in Posidonia meadows. However,\nspecies richness was not affected by proximity to reefs in either type of seagrass meadow. The higher number of species at the reef-Amphibolis edge was therefore a consequence of higher abundance, rather than species richness per se. These\nresults suggest that patterns in the composition and diversity of fauna with proximity to adjacent habitats, and the\nunderlying processes that they reflect, likely depend on the physiognomy of the habitat.