Structured Population Models of Herbivorous Zooplankton

In this paper, we investigate whether a stage-structured population model can explain major features of dynamics of the herbivores Daphnia galeata and Bosmina longirostris reared under controlled laboratory conditions. Model parameters are determined from independent individual-based information gleaned from the literature on feeding, growth, reproduction, and survivorship of these herbivores. We tested predictions of our model against published observations on the dynamics of laboratory populations. The feeding protocols used in these experiments present a highly dynamic food environment that rigorously challenges the ability of stage-structured models to predict the dynamics of populations as they approach equilibrium. For both herbivore species, the models correctly predict feasible equilibria and some features of their dynamics (e.g., periodicity, cycle amplitude, demography, and fecundity) for experiments in which the species were raised in isolation and food transfers were relatively frequent (at least one transfer per instar). With frequent food transfers, the model also correctly predicts coexistence of the herbivores during competition experiments and suggests a novel mechanism for coexistence. The model fails to predict correctly single-species dynamics and the outcome of competition in experiments where food transfers were infrequent and utilization of internal reserves by individuals in the populations must have been high.