BIMODALITY IN SIZE DISTRIBUTIONS - THE RED-SEA URCHIN STRONGYLOCENTROTUS-FRANCISCANUS AS AN EXAMPLE

We use a model based on the size-structured von Foerster equation to d escribe how size-dependent growth and mortality rates, pulsed recruitm ent, and variability in growth affect the shape of a size distribution . The deterministic, equilibrium size distribution with constant recru itment increases with size when the difference between mortality rate and the rate at which growth rate decreases with size is positive (gro wth dominated), and decreases when it is negative (mortality dominated ). Pulsed recruitment causes modes whose relative amplitudes are indic ated by the corresponding constant recruitment case. For typical anima l growth patterns, the distance between pulses decreases with age. Pul ses merge and can be selectively obscured by variability in growth so that their relative amplitudes no longer correspond to the constant re cruitment case. We use this information to evaluate why bimodality occ urs in size distributions of the red sea urchin, Strongylocentrotus fr anciscanus, in some habitats, but not others. The mode at larger sizes , which occurs in all habitats, arises because the distribution is mor tality dominated and the final sizes of individuals vary. The upper ha lf of a second mode at smaller sizes is caused by higher mortality rat es at sizes greater than the peak of that mode. The lower half may be due either to a refuge from predation under the spine canopy of adults or to sampling selectivity.