Large-amplitude cycles of Daphnia and its algal prey in enriched environments

Ecological theory predicts that stable populations should yield to large-amplitude cycles in richer environments1±3. This does not occur in nature. The zooplankton Daphnia and its algal prey in lakes throughout the world illustrate the problem4±6. Experiments show that this system its the theory's assumptions7±9, yet it is not destabilized by enrichment 6. We have tested and rejected four of ive proposed explanations 10. Here, we investigate the fifth mechanism: inedible algae in nutrient-rich lakes suppress cycles by reducing nutrients available to edible algae. We found three novel results in nutrient-rich microcosms from which inedible algae were excluded. First, as predicted by theory, some Daphniaedible algal systems now display large-amplitude predator-prey cycles. Second, in the same environment, other populations are stable, showing only small-amplitude demographic cycles. Stability is induced when Daphnia diverts energy from the immediate production of young. Third, the system exhibits coexisting attractors -a stable equilibrium and large-amplitude cycle. We describe a mechanism that flips the system between these two states.