A Model of Population Dynamics for the Fruit Fly Drosophila melanogaster with Density Dependence in More than One Life Stage and Delayed Density Effects

Abstract

(1) Effects of density on various growth parameters of Drosophila melanogaster were studied. Pre-adult survival decreased with pre-adult density, and adult fecundity decreased with both pre-adult density and adult crowding. (2) These three effects were used to construct a model of density-dependent population dynamics. The model predicts the number of pre-adults for each generation in a population with two life stages and non-overlapping generations. (3) Twenty-five experimental populations of D. melanogaster of small size and nonoverlapping generations were maintained in vials for nine generations. The numbers of eggs and adults were counted each generation. These numbers showed sustained and erratic fluctuations. (4) A deterministic version of the dynamical model predicts rapid approach to equilibrium and cannot explain such fluctuations. (5) The predictions of a stochastic version of the dynamical model, however, agree with the observed behaviour of the experimental populations: the frequency distribution of the mean population size (MPS), and the square root of the mean squared deviation of population size from MPS, do not show a statistically significant deviation from those predicted by the stochastic model. Thus, the oscillations in numbers of the experimental populations are interpreted as a consequence of random fluctuations.