Multiple dynamics in a single predator-prey system: experimental effects of food quality

Recent work with the freshwater zooplankton Daphnia has suggested that the quality of its algal prey can have a significant effect on its demographic rates and life-history patterns. Predator-prey theory linking food quantity and food quality predicts that a single system should be able to display t wo distinct patterns of population dynamics. One pattern is predicted to ha ve high herbivore and low algal biomass dynamics (high HBD), whereas the ot her is predicted to have low herbivore and high algal biomass dynamics (low PIED). Despite these predictions and the stoichiometric evidence that many phytoplankton communities may have poor access to food of quality, there h ave been few tests of whether a dynamic predator-prey-system can display bo th of these distinct patterns. Here we report, to the authors' knowledge, t he first evidence for two dynamical patterns, as predicted by theory, in a single predator-prey system. We show that the high HBD is a result of food quantity effects and that the low HBD is a result of food quality effects, which are maintained by phosphorus limitation in the predator. These result s provide an important link between the known effects of nutrient limitatio n in herbivores and the significance of prey quality in predator-prey popul ation dynamics in natural zooplankton communities.