Abstract
Gradients in the composition and diversity (e.g. number of species) of faunal assemblages are common at ecotones between juxtaposed habitats. Patterns in the number of species, however, can be confounded by patterns in abundance of individuals, because more species tend to be found wherever there are more individuals. We tested whether proximity to reefs influenced patterns in the composition and diversity (‘species density’ = number of species per area and ‘species richness’ = number of species per number of individuals) of prosobranch gastropods in meadows of two seagrasses with different physiognomy: Posidonia and Amphibolis. A change in the species composition was observed from reef-seagrass edges towards the interiors of Amphibolis, but not in Posidonia meadows. Similarly, the abundance of gastropods and species density was higher at edges relative to interiors of Amphibolis meadows, but not in Posidonia meadows. However, species 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 results suggest that patterns in the composition and diversity of fauna with proximity to adjacent habitats, and the underlying processes that they reflect, likely depend on the physiognomy of the habitat.
Highlights
Habitats are connected within landscapes by multiple processes that influence the number of species and the abundance of individuals at ecotones where habitats are juxtaposed
We selected 6 reefs, generally separated by .500 m, within each location: 3 adjacent to meadows dominated by Posidonia and 3 adjacent to meadows dominated by Amphibolis
A total of 15,368 individuals and 43 species of gastropods were counted on reefs and adjacent seagrass meadows
Summary
Habitats are connected within landscapes by multiple processes that influence the number of species (diversity) and the abundance of individuals at ecotones where habitats are juxtaposed. Ecotones can host higher productivity or receive more propagules, processes which tend to increase the number of individuals, and this may enhance diversity [11]. Ecotones can be subjected to elevated predation rates, which tend to decrease the number of individuals, and so reduce diversity [6,12] The counterbalance of these opposing forces can produce unexpected patterns in species diversity. We sought to determine whether gradients in species richness existed independently from changes in composition and gradients in species density We tested these predictions separately in meadows constituted by one of two species of seagrasses with contrasting physiognomy (Posidonia and Amphibolis) to (iii) determine whether differences in physiognomy can alter patterns in the composition and diversity of gastropod assemblages from reef-seagrass edges to interiors of seagrass meadows
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