Population and Species Level Patterns

Range-Size Patterns

The range of a species is its natural area of geographic distribution. Considering the overall range ofspecies and body sizes occurring in the biosphere, there does not seem to be any simple and deterministic relationship between body size and species range size: very large species, such as some cetaceans, and very small species, such as many microorganisms, can have very wide natural ranges. However, within much more restricted taxonomic groups, small-bodied species tend to have smaller minimum geographic ranges than large-bodied species. The interspecific relationships of body size to geographic range size commonly exhibit an approximately triangular form, where species of all body sizes may have large geographic ranges while the minimum range size of a species tends to increase with body size.

The relationship between body size and home-range size (i.e., the minimum space needed by an individual to successfully complete its life cycle) can help to account for patterns of natural range size. Since home-range size (H) scales with individual body size (BS) according to an allometric equation (H = aBSb), in which the slope (b) is significantly larger than 1, large-bodied species may require a larger total geographic range than small species in order to maintain minimum viable population sizes in all local areas. This results in the triangular relationship between body size and range size, because there is not necessarily an upper limit on the range size of small-bodied species.

The dependence of a species' fundamental niche space and dispersal ability on body size may also help to explain range-size patterns, since species oflarge body size are potentially able to maintain homeostasis in a wider range of conditions and to successfully colonize a larger proportion of their potential range than small-bodied species.

These mechanistic explanations of the relationship between range size and body size are not mutually exclusive and may be reinforcing.

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