Prediction of Population Growth Form in Daphnia pulex Cultures

Available data on age- and density-specific birth, death and individual growth rates, combined with new information on lags in these statistics following density change, can be applied to models of population growth of Daphnia in a constant environment. Six alternative models have been tested. They range in complexity from the logistic curve to an iterative model that takes into account the initial age distribution, and effects of age, density and density change on natality, mortality and size. This complex model is the only one that provides a fair representation of observed events in experimental Daphnia populations. Several independent areas of agreement between observations and model provide assurance that the observed fit is not merely fortuitous. The most important relations determining population growth form in this system are those between (1) natality and age, density and density change and between (2) size and individual age. Comparisons between the models emphasize that, in most organisms, the populational effects of different age and size classes can not be considered negligible in assessing population dynamics. Knowledge of the vital statistics of the younger members of a population is of paramount importance for demographic prediction.