Body size and species richness

This pattern was first observed by Hutchinson and MacArthur (1959),[1] and it appears to apply equally well to a broad range of taxa: from birds and mammals to insects, bacteria (May, 1978;[2] Brown and Nicoletto, 1991[3]) and deep sea gastropods (McClain, 2004[4]).

Most studies focus on the distribution of taxonomic fractions of largely non-interacting species such as birds or mammals; this article is primarily based on those data.

This yields a highly right skewed body size distribution with a mode centered near species with a mass ranging from 50-100 grams.

[3] Not all geographic subsets of taxonomic groups follow this broad pattern, in contrast, Northwest European land snails exhibit a normal distribution (Hausdorf, 2006[8]).

Additionally, the terrestrial mammals of the islands of Madagascar, New Guinea and Australia do not show a right skewed body size-species richness distribution (Maurer et al. 1992[7]).

Since it does not, and if it is assumed that the observed distribution of species among body sizes is not a sampling bias, then some ecological interactions must underlie this pattern.

Explanations in the literature suggest the rates of speciation and/or dispersal ability vary with size and could lead to more small bodied organisms (May, 1978[2]).

It is important to note that the 3 sub-mechanisms: dispersal, competition and energetic restraints must in some manner feed back into either speciation or extinction rates as these are the only ultimate processes (see Tinbergen's four questions) governing the number of species on earth and hence the body size-species richness pattern.

These traits may promote speciation through increased mutation and selection events as molecular evolution scales with metabolic rate (Gillooly et al. 2005[12]) and thus with body size.

However, an analysis by Cardillo and colleagues (2003)[13] revealed that body size did not influence the speciation rates of Australia's mammals.

Similarly, a study using birds failed to demonstrate that body size and speciation rates were negatively correlated (Nee et al. 1992[14]).

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.

For example, Bowers and Brown (1982)[17] found that the number of similar sized species in a community of granivorous rodents was fewer than expected.

Additionally, smaller species may have many more ecological niches available to them and thus facilitating the diversification of life (Hutchinson and MacArthur, 1959[1]).

For example, grazing animals make up for their poor quality diet by digesting food longer and are able to extract more energy from it (Maurer et al. 1992[7]).

This mechanism likely contributes to the shift in body size-species richness distribution observed between continental and local scales.

This theory is based on data from many different terrestrial animals (mammals, birds, beetles, and butterflies) and has garnered further support from a more recent study by Rice and Gislason (1996).

[19] These new results also demonstrate a slope of negative two with respect to the right side of the body size-species richness pattern in the fish assemblage of the North Sea.

Figure 1 . The Frequency distribution of North American land mammals (n=465). The figure shows a right skewed distribution, with a mode tending to smaller species (50–100 g). The x-axis encompasses mammals ranging in size from 3 grams to 393 kilograms. Redrawn from Brown and Nicoletto (1991) [ 3 ]
Figure 2. a)
Figure 2. b)
Figure 3. The Frequency distribution of North American land mammals (n=465). The figure shows a right skewed distribution, with a mode tending to smaller species (50–100 g). Redrawn from Brown and Maurer (1989). [ 5 ]