THE GEOGRAPHIC RANGE - SIZE, SHAPE, BOUNDARIES, AND INTERNAL STRUCTURE

Comparative, quantitative biogeographic studies are revealing empirica l patterns of interspecific variation in the sizes, shapes, boundaries , and internal structures of geographic ranges; these patterns promise to contribute to understanding the historical and ecological processe s that influence the distributions of species. This review focuses on characteristics of ranges that appear to reflect the influences of env ironmental limiting factors and dispersal. Among organisms as a whole, range size varies by more than 12 orders of magnitude. Within genera, families, orders, and classes of plants and animals, range size often varies by several orders of magnitude, and this variation is associat ed with variation in body size, population density, dispersal mode, la titude, elevation, and depth (in marine systems). The shapes of ranges and the dynamic changes in range boundaries reflect the interacting i nfluences of limiting environmental conditions (niche variables) and d ispersal/extinction dynamics. These processes also presumably account for most of the internal structure of ranges: the spatial patterns and orders-of-magnitude of variation in the abundance of species among si tes within their ranges. The results of this kind of ''ecological biog eography'' need to be integrated with the results of phylogenetic and paleoenvironmental approaches to ''historical biogeography'' so we can better understand the processes that have determined the geographic d istributions of organisms.