WETHEY, D. S.: Microclimate and biogeography: The barnacle Chthamalus fragilis in New England

Geographic limits of species are commonly associated with climatic or physical boundaries, but the mechanisms of exclusion at the limits of distribution are poorly understood. In some intertidal populations, the strengths of interactions with natural enemies are mediated by microclimate, and determine geographic limits. For example, the northern geographic limit of the barnacle Chthamalus fragilis in New England is the south side of Cape Cod, Massachusetts. South of the cape, Chthamalus has a refuge from competition in the high intertidal which is too hot for survival of its competitor Semibalanus balanoides. North of the cape, the high intertidal is cooler, and Semibalanus survives, so there is no refuge for Chthamalus. Thus, geographic variation in the strength of competition may determine the geographic limit of Chthamalus. The distributions of the two species, and climatic conditions were measured on both sides of the geographic limit of Chthamalus, in the Cape Cod Canal. Spanning the canal are two bridges, 5 km apart. Temperature loggers and photo quadrats were established in the barnacle zone on the granite block piers of the bridges. On the southern bridge, Chthamalus is abundant and occupies a refuge above Semibalanus. On the northern bridge in 2001, only 7 live Chthamalus are present. Despite the proximity of the bridges, their microclimates are very different. The southern bridge, where Chthamalus is abundant, is up to 8^oC hotter than the northern bridge. This higher temperature creates a refuge in the high intertidal for Chthamalus. On the cooler northern bridge, there is no refuge for Chthamalus. The geographic change in microclimate alters the strength of competition, and determines the geographic limit.