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. The northern 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 superior competitor Semibalanus balanoides. North of the cape, the high intertidal is cooler, and Semibalanus survives, so Chthamalus has no refuge. Thus, geographic variation in the strength of competition may determine the geographic limit of Chthamalus. Intolerance of cold by Chthamalus cannot account for the geographic limit: transplants of Chthamalus 80 km beyond its northern limit survived up to 8 yr in the absence of competition with Semibalanus. At the geographic limit of Chthamalus in the Cape Cod Canal there are two bridges, 5 km apart. On the southern bridge, Chthamalus is abundant and occupies a refuge above Semibalanus. On the northern bridge in 2001, only 7 individual Chthamalus were present. Despite the proximity of the bridges, their microclimates are very different. The southern bridge, where Chthamalus is abundant, is up to 8°C 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. Because of the difference in temperatures of the water masses that meet in the canal, heat storage in the rock of the bridge piers causes the temperatures to differ between the bridges. Thus, geographic change in microclimate alters the strength of competition, and determines the geographic limit.
“When we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing; and the change in climate being conspicuous, we are tempted to attribute the whole effect to its direct action. But this is a very false view: we forget that each species, even where it most abounds, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favoured by any slight change of climate, they will increase in numbers, and as each area is already fully stocked with inhabitants, the other species will decrease.”—Charles Darwin, On the Origin of Species, 1859, p. 69.
