r-Curves and the Cost of the Planktonic Stage

When it is impossible to calculate the intrinsic rate of increase of a population because survivorship probabilities of prerecruitment stages are not available, it may be instructive to study the behavior of r as a function of prerecruitment survivorship probabilities along a reasonable range of such hypothetical probability values. When such functions, called r-curves, are constructed for benthic organisms with planktonic larvae, the true value ofr for a given larval survivorship probability lies within a range bounded from below by a value generated by Laughlin's equation for r of a cohort, and from above by a value generated by Lotka's equation for r of a population with overlapping generations and a stable age distribution. The r-curves not only enable a meaningful comparison of closely related species with regard to their capacity for increase, but also enable comparisons to be made between populations within species, with regard to the quality of their habitats. They also provide a powerful tool for investigating the adaptive significance of the planktonic larva versus the cost of direct development; their analysis in species of Cerithium snails provides evidence that the significance of the planktonic larva lies in its potential for maximizing dispersal.