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COMPETITIVE DISPLACEMENT OF NATIVE MUD SNAILS BY INTRODUCED PERIWINKLES IN THE NEW ENGLAND INTERTIDAL ZONE.

During the nineteenth century the mud snail Ilyanassa obsoleta was abundant on sand and mud flats, wood works, sea walls, salt marshes, eel grass beds, and cobble beaches in New England. With the exception of sand and mud flats, these habitats are now largely occupied by the introduced periwinkle, Littorina littorea. To determine whether Littorina competitively displaces Ilyanassa, an experimental study was conducted at a site in Barnstable Harbor, Massachusetts where the observed distributions overlapped by 3% by Morisita's index. Mark-recapture studies suggested that the distribution of Littorina was limited by an abiotic factor, currents, through which this species realized its fundamental niche. In contrast, density manipulations demonstrated that Ilyanassa emigrated from areas where Littorina exceeded densities of 2 to 5 per 0.25 m2. Littorina limited the upper and lower distribution of Ilyanassa and affected its microhabitat distribution in the mid intertidal zone. Habitat displacement was 70% for Ilyanassa, calculated as the difference between Ilyanassa's observed distribution and its distribution during littorinid removal experiments. The two species display reciprocal niche overlap with each possessing an exclusive region from which the other is physically restricted. The results suggest that the historical change in the distribution of Ilyanassa was due to competitive exclusion by introduced Littorina.