<P> It is known that extinction risk is directly correlated to the size of a species population . Small populations tend to go extinct more frequently than large ones (MacArthur and Wilson, 1967). As large species require more daily resources they are forced to have low population densities, thereby lowering the size of the population in a given area and allowing each individual to have access to enough resources to survive . In order to increase the population size and avoid extinction, large organisms are constrained to have large ranges (see Range (biology)). Thus, the extinction of large species with small ranges becomes inevitable (MacArthur and Wilson, 1967; Brown and Maurer, 1989; Brown and Nicoletto, 1991). This results in the amount of space limiting the overall number of large animals that can be present on a continent, while range size (and risk of extinction) prevents large animals from inhabiting only a small area . These constraints undoubtedly have implications for the species richness patterns for both large and small - bodied organisms, however the specifics have yet to be elucidated . </P> <P> Understanding dispersal is an important mechanism that may underlie the body size - species richness pattern . Mainly, dispersal ability can affect speciation rates in various ways: if an organism is capable of dispersing across great distances which span variable habitat, it will be more likely that individuals that settle in these new areas will be subject to different selection regimes, leading to new species through natural selection . Alternatively, great dispersal ability can act against the efforts of natural selection by facilitating geneflow . Thus, species with greater dispersal ability may in fact have lower speciation rates . Then again, if a species disperses long distances and has a large range, it may also be more likely to have its range bisected by natural barriers which could promote allopatric speciation over evolutionary time . </P> <P> Researchers have shown that body size and dispersal ability do not correlate (Jenkins et al. 2007). However, this finding rests heavily on the fact that active and passive dispersers have been grouped together in the analyses . When the same data are separated into two categories (active and passive dispersers) a pattern emerges . Jenkins and others (2007) have found that for active dispersers larger organisms will disperse greater distances . Regardless of this correlation its effects on speciation rates remain unclear . </P> <P> Could similar sized species be mitigating the effects of competition by avoiding each other? Interspecific competition is in fact strong among species that are the same size (Bowers and Brown, 1982). Researchers have proposed that in order to lessen the burden of competition among animals within the same guild (see Guild (ecology)), they must vary in size (Brown and Maurer, 1989; Brown and Nicoletto, 1991). For example, Bowers and Brown (1982) found that the number of similar sized species in a community of granivorous rodents was fewer than expected . They believe that these results suggest that competition between same - sized species is prohibiting co-existence in this community . </P>

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