Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 29208.
Integr Comp Biol. 2002 Aug;42(4):872-80. doi: 10.1093/icb/42.4.872.
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.
物种的地理分布范围通常与气候或物理边界有关，但分布范围限制处的排斥机制还了解甚少。在一些潮间带种群中，与天敌的相互作用强度受小气候调节，并决定了地理分布范围。新英格兰藤壶 Chthamalus fragilis 的北部界限是马萨诸塞州科德角的南侧。在海角以南，藤壶在高潮带拥有一个免受竞争的避难所，那里的温度对于其优势竞争者半蹼滨螺 Semibalanus balanoides 来说太热而无法生存。在海角以北，高潮带较凉爽，半蹼滨螺得以存活，因此藤壶没有避难所。因此，竞争强度的地理变化可能决定了藤壶的地理分布范围。藤壶对寒冷的不耐受不能解释其地理分布范围的限制：在没有与半蹼滨螺竞争的情况下，将藤壶向北移植 80 公里，其在 8 年内仍能存活。在科德角运河藤壶的地理分布范围的极限处，有两座相距 5 公里的桥梁。在南侧的桥上，藤壶大量存在，并在半蹼滨螺上方占据了一个避难所。在 2001 年的北侧桥上，仅存在 7 个藤壶个体。尽管两座桥相距很近，但它们的小气候却有很大差异。藤壶大量存在的南侧桥的温度比北侧桥高 8°C。这种较高的温度为藤壶在高潮带创造了一个避难所。在较凉爽的北侧桥上，藤壶没有避难所。由于在运河中相遇的水体的温度不同，桥墩岩石中的热量储存导致了两座桥之间的温度差异。因此，小气候的地理变化改变了竞争的强度，并决定了地理分布范围的极限。“当我们从南向北旅行，或者从潮湿地区到干燥地区时，我们总是会看到一些物种逐渐变得越来越稀少，最终消失；由于气候变化引人注目，我们倾向于将整个影响归因于其直接作用。但这是一种非常错误的观点：我们忘记了即使在物种最丰富的地方，在其生命的某个阶段，它也会不断受到来自敌人或同一地点和食物竞争者的巨大破坏；如果这些敌人或竞争者受到任何小气候变化的稍微有利的影响，它们的数量就会增加，而且由于每个区域都已经挤满了居民，其他物种的数量就会减少。”-查尔斯·达尔文，《物种起源》，1859 年，第 69 页。
