Habitat and Community Modification by An Introduced Herbivorous Snail

Experimental removal of the introduced herbivorous snail Littorina littorea from a protected New England rocky beach resulted in rapid habitat and community changes. At normal snail densities, L. littorea grazing bulldozes sediments from hard substrate and precludes the presence of an algal canopy. Snail removal resulted in rapid sediment accumulation and the development of an algal canopy, which accelerated sedimentation and bound sediment to hard substrate. These changes led to the increased success of organisms characteristic of soft—sediment habitats, such as polychaetes, tubiculous amphipods, mud crabs, and mud snails, and decreased success of organisms characteristic of hard—substrate habitats, such as barnacles and encrusting algae. Snail removal also significantly influenced the success of the marsh grass Spartina alterniflora. L. littorea consumes the shoots and rhizomes of marsh grass, as well as mediating sediment accumulation, which is necessary for vegetative expansion of root mat. Removal of L. littorea resulted in expansion of the littoral area dominated by S. alterniflora, as well as increased productivity of the marsh grass. These results suggest that the North American invasion of the European periwinkle has altered habitats and communities in protected littoral waters. Prior to the introduction of L. littorea, soft—bottomed littoral habitats and fringing salt marsh environments may have been more common that they are presently. Herbivorous snails in general may have an important habitat—modifying effect in protected marine communities that has not been appreciated previously