Background for E122

Biological diversity, population size and body size are interdependent(Basset 1991, Blackburn and Glaston 1994, Brown 1995, Hutchinson 1959, Lawton 1990, May 1986 1988, Morse et al 1988), but there is little consensus on the nature or causes of these relations.

Local diversity comes from the balance between immigration of new species and local extinction (MacArthur and Wilson 1967). Immigration rate depends on the number of species already present (MacArthur and Wilson 1967) and dispersal ability, which is likely to be dependent on body size (Peters 1983). Extinction rate depends on population abundances and their distribution, with rare species more likely to become extinct as a result of population fluctuations (Pimm et al 1988). Because of this, species richness (S) should depend on the number of individuals (I) within a group of interacting species, here assumed to be species of similar body size, especially within taxonomic orders (Tilman and Pacala 1993, Cousins 1989). The number of rare species also depends on the abundance distribution (for example, log-normal [Preston 1962], broken-stick [MacArthur 1960]), as will the precise relation between species richness and the number of individuals.

Assuming a minimum population size for population persistence, and abundances following a broken-stick model (MacArthur 1960), it has been shown (May 1975) that S = cI^0.5, where c is a case-specific constant. The relation S = cI^0.25 has previously been derived (Preston 1962) for a canonical log-normal distribution. This study can investigate the effect on species richness of the total number of individuals in each size class (Lawton 1986) and examine the relations among species richness, number of individuals, and body size.

[Modified from: Siemann, E.; Tilman, D.; Haarstad, J. 1996. Insect species diversity, abundance and body size relationships. Nature 380:704-706.]