Habitat destruction and metacommunity size in pen shell communities.

1. In spatially structured communities, habitat destruction can have two effects: first, a main effect that occurs because of the loss of habitat area within a larger region, and a secondary effect due to changes in the spatial arrangement of local communities. Changes to the spatial arrangement can, in turn, affect the migration and extinction rates within local communities. 2. Our study involved the experimental destruction of entire local communities within larger regions in natural marine microcosms. Large and small arrays of dead pen shells were created in a shallow bay in north Florida, and the colonization by both encrusting and motile species on this empty substrate were followed through time. After most species had become established, half of the large arrays were perturbed to create small arrays by removal of half the shells, simulating habitat destruction. 3. After 48 days of further community development, comparisons of the large arrays, reduced arrays and original small arrays suggested that the mechanisms by which habitat destruction affects diversity could depend upon the size of the region affected and the natural history of the species being studied. 4. Habitat destruction reduced the diversity of motile species to a level lower than that found in the undisturbed small arrays, suggesting that the species that assembled in the original large metacommunities negatively influenced the species that occurred ultimately in the converted small arrays. 5. With sessile species, habitat destruction created richness levels that were intermediate to those of small and large arrays. The initial predestruction richness appears to have had a positive effect; because sessile species cannot disperse as adults, they may not respond to significant shifts in metacommunity size later in succession. Initial metacommunity size may be important for allowing individuals to select appropriate habitats before they settle.