Body size and abundance in a dung beetle assemblage: Optimal mass and the role of transients

It has been argued that the strength of the relationship between body size and abundance in local assemblages can be predicted from both the number of species included in a study and the extent of their size range; that transient species do not alter the statistics of this relationship; and that the optimal or modal size for a given assemblage can be predicted from a model using the constants and exponents of the allometric relationships between body mass and resource acquisition, and body mass and resource conversion to offspring. Data on the body size and abundances of scarabaeine dung beetles from an arid, southern African savanna habitat, sampled over a 16-month period, and on the scaling of metabolic rate and population growth in insects, were used to examine these hypotheses. Coefficients of determination for the abundance:body size relationship were within the range expected from the sample size and size range of the assemblage when transient species were excluded. When transients were included the fit was poorer. Transient species had a lower abundance than the resident species, but no difference in body size between the two groups was found. The mode of the species-body size frequency distribution was not predicted by the model when the required parameter values were obtained from the relationship between exact rates of increase (r(m)) and body mass (resource acquisition), and metabolic rate and body mass.