Demographic Models Based on Size, Not Age, For Organisms with Indeterminate Growth

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Abstract
In conventional demographic theory fecundity and survival are determined by the age of the individual, but for many organisms size is more important than age in determining life history characteristics. Continuous—time demographic models presented here explicitly account for growth, including individual variability in growth rate, and assume that size alone determines the birth and death rates. Growth and mortality processes are introduced that result in a normal distribution (on some scale of measurement) of the sizes of individuals within a cohort; this permits the cohort to be described in terms of only three variables; mean size, variance in size, and proportion of individuals surviving. Using general formulae for the intrinsic rate of population increase and the stable size distribution, applications of the models are presented illustrating the effects of individual growth rate variance and size—specific mortality on characteristics of the population.