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

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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 non-overlapping 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 predications 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.