Body size and species richness

The body size-species richness distribution is a pattern observed in the way taxa are distributed over large spatial scales. The number of species that exhibit small body size generally far exceed the number of species that are large-bodied. Macroecology has long sought to understand the mechanisms that underlie the patterns of biodiversity, such as the body size-species richness pattern. The body size-species richness distribution is a pattern observed in the way taxa are distributed over large spatial scales. The number of species that exhibit small body size generally far exceed the number of species that are large-bodied. Macroecology has long sought to understand the mechanisms that underlie the patterns of biodiversity, such as the body size-species richness pattern. This pattern was first observed by Hutchinson and MacArthur (1959), and it appears to apply equally well to a broad range of taxa: from birds and mammals to insects, bacteria (May, 1978; Brown and Nicoletto, 1991) and deep sea gastropods (McClain, 2004). Nonetheless, its ubiquity remains undecided. 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. The body size-species richness pattern (see fig. 1) has been documented for land mammals across numerous continents such as North America, South America, Eurasia and Africa (Brown and Maurer, 1989; Bakker and Kelt, 2000; Maurer et al. 1992). Traditionally the pattern has been explored by plotting the number of species on the y-axis and the logarithm to the base two of the species body mass (g) on the x-axis. This yields a highly right skewed body size distribution with a mode centered near species with a mass ranging from 50-100 grams. Although this relationship is very distinct at large spatial scales, the pattern breaks down when the sampling area is small (Hutchinson and MacArthur, 1959; Brown and Maurer 1989; Brown and Nicoletto 1991; Bakker and Kelt 2000). At small spatial scales (e.g. a dozen hectares or a local community) the body size-species richness pattern dissolves and the number of species per body size class is almost uniform (i.e. there is an equal number of small and large bodied species in the community (see fig. 2 b)). Researchers have documented this transition by sampling species at different spatial scales; at intermediate spatial scales (e.g. a thousand hectares or a biome) the body size-species richness distribution begins to shift from being right skewed (from fewer large-bodied species) towards a normal distribution. Finally, when macroecologists compare the body size-species richness distributions of continents to those of communities the distributions are significantly different (Hutchinson and MacArthur, 1959; Brown and Maurer, 1989; Brown and Nicoletto, 1991; Bakker and Kelt, 2000) (see fig. 1 vs. fig. 2 b)). Figure 2. The Frequency distribution of land mammals at two different spatial scales. The figure shows a shift from a slightly right skewed distribution at the biome scale (a) to a normal distribution at the community level (b). The x-axis ranges from 0 to 20 on a log-scale in both graphs. Redrawn from Brown and Nicoletto (1991). Not all geographic subsets of taxonomic groups follow this broad pattern, in contrast, Northwest European land snails exhibit a normal distribution (Hausdorf, 2006). 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). Given the limited amount of non-conforming data it is not possible to determine if this phenomenon is universal or is simply novel in certain environments. Underreporting of taxonomic and geographic subsets that do not conform to this pattern may be partially responsible for this unresolve. If it were possible to randomly sample a subset of the known continental species pool for mammals, one would expect the body size-species richness distribution of this sample to roughly mirror that of the entire continent. 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). Additionally, extinction risk, competition and energetic restraints have also been proposed as critical mechanisms that may drive this pattern (Brown and Maurer, 1989; Brown and Nicoletto, 1991). Finally, the metabolic theory of ecology explains how there is a negative relationship between number of individuals (N) and size (M) ( N = M − 3 / 4 {displaystyle N=M^{-3/4}} ), but it is silent on species richness. Remember, while there are fewer individuals in the largest size classes, energy availability is equal across all classes of interacting organisms (i.e. they share the same energy pool and are thus part of the same ecosystem) (energetic equivalence) (May, 1988). 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. Small organisms generally have shorter generation times and are quick to mature and reproduce (May, 1978; Denney et al. 2002; Savage et al. 2004). These traits may promote speciation through increased mutation and selection events as molecular evolution scales with metabolic rate (Gillooly et al. 2005) and thus with body size. For example, bacteria can evolve a degree of antibiotic resistance in a very short time period. This evolution is facilitated by a short generation time and presumably would not be possible for species with slower life histories. Thus, one can think of it as if the 'evolutionary clock ticks faster' (May, 1978) for small organisms. If this were true then a simple explanation of why there are more small species on earth would be because they speciate faster. However, a recent analysis by Cardillo and colleagues (2003) 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). 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.