Population dynamics based on individual size rather than age: a general model with a reef coral example

Models of population dynamics based on age-related parameters do not provide accurate predictions if within-age-class variance in those parameters is high. Often the demographic fate of an individual or population may be predicted more accurately using size rather than age classifications. I describe here a general model of size-related population dynamics, based on a modified Leslie matrix. The model is designed to include demographic processes, such as fragmentation and shrinkage, which are excluded from conventional age-related demographic analysis. The parameters of the model were measured during a calm and a stormy year in a field study of a colonial, sessile coral. These data were used in computer simulations to predict the fate of a cohort over long periods of time. Measured differences in individual rates of growth and shrinkage resulted in huge long-term variations in calculated colony size, fecundity, and mortality rates, between corals of equal age. Therefore, individuals of the same age will have widely differing probabilities of obtaining representation in subsequent generations. Population growth was simulated by adding a constant number of juvenile corals every year to the smallest size class. Although the model has no density-dependent parameters, population growth stops. The population size attained depends on the number of settling juveniles. Local populations of sessile organisms may be regulated by larval input, as well as by recurring disturbances or density-dependent interactions