What individual life histories can (and cannot) tell about population dynamics

We present an overview of a long-term research programme that is aimed at revealing the relations between individual feeding, growth, reproduction and mortality in Daphnia pulex and the state and dynamics of the population. We analyse a physiologically structured population model, in which individual performance is described using an energy budget model that incorporates a food dependence. The model predictions are shown to be at odds with experimental observations on populations of Daphnia. We argue that these discrepancies are primarily due to insufficient knowledge about the precise size-scaling of the food ingestion rate, which plays a central role in the competitive interaction among individuals. To a lesser extent, the discrepancies arise because details about the energy budget of individual Daphnia are not sufficiently known for the food conditions prevailing in population experiments.