Vertical Migration of Zooplankton as an Evolutionarily Stable Strategy

By applying evolutionary game theory, we calculate the differences in payoffs between diel vertical migrating and nonmigrating zooplankton. The results depend on the abundance and growth rate of the algae, the filtration rate and density of the zooplankton, the strategy-dependent efficiency of converting food into reproduction, temperature-dependent egg developmental time, day length, and predation pressure. In addition to regions where the two strategies exclude each other, it is also possible for a population to reach an evolutionarily stable equilibrium mixture of strategies. At high food concentrations, equilibrium is unlikely because it would require a predation risk increasing with prey abundance; but at low food concentrations, strategy mixtures can be stable for many combinations of realistic model parameters, even with density-independent mortality caused by fish predation. Temporal variation in the selective forces shifts this equilibrium point. Therefore, the model can explain seasonal changes and long-term trends in the pattern of diel vertical migration.