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

Ecological theory predicts that stable populations should yield to large-am plitude cycles in richer environments(1-3). This does not occur in nature. The zooplankton Daphnia and its algal prey in lakes throughout the world il lustrate the problem(4-6). Experiments show that this system fits the theor y's assumptions(7-9), yet it is not destabilized by enrichment(6). We have tested and rejected four of five proposed explanations(10). Here, we invest igate the fifth mechanism: inedible algae in nutrient-rich lakes suppress c ycles by reducing nutrients available to edible algae. We found three novel results in nutrient-rich microcosms from which inedible algae were exclude d. First, as predicted by theory, some Daphnia-edible algal systems now dis play large-amplitude predator-prey cycles. Second, in the same environment, other populations are stable, showing only small-amplitude demographic cyc les. Stability is induced when Daphnia diverts energy from the immediate pr oduction of young. Third, the system exhibits coexisting attractors-a stabl e equilibrium and large-amplitude cycle. We describe a mechanism that flips the system between these two states.