Stability and Extinction of Laboratory Populations of Zooplankton Preyed on by The Backswimmer Notonecta

Growth of the backswimmer Notonecta hoffmani is better on a diet of Daphnia pulex than on a diet of Ceriodaphnia reticulata. Offered a choice of these two zooplankton species, Notonecta selects Daphnia or large individuals of either species. To test whether this preference affects prey species and size—classes differently, we measured population changes when the predator and one prey species or the other were placed together in plastic tubs for 7—8 wk. Roughly half of the replicated laboratory populations of Daphnia pulex that were exposed to predation by a constant number and age—class of Notonecta were driven extinct within 8 wk. The remainder of these populations maintained densities similar to those of control populations without predators. Populations surviving at each sampling period partially compensated to mortality caused by predators by small increases in individual fecundity, which maintained their population birth rates at the same level as those of control populations. Fecundity was inversely related to the density of adult Daphnia and to the total biomass of Daphnia. In populations exposed to predation, the other compensatory mechanism was decrease in the mortality of young from factors (probably starvation) other than predation. Predation reduced the average size of individuals in Daphnia populations and consequently markedly reduced the average biomass. This presumably increased the food per capita available to juveniles and small adults in predation treatments over the level available to their counterparts in controls, and hence both reduced mortality from starvation and increased size—related fecundity. Populations exposed to predation appeared to have stable dynamics for varying periods. However, as the experiment progressed, apparently random events in some populations caused the predation rate to exceed Daphnia's ability to compensate, and in each successive intersampling period a few populations were driven extinct. Extinction was probabilistic. Thus both a deterministic and a stochastic framework are required to explain the dynamics of these laboratory populations. Only two of 18 Ceriodaphnia populations were driven extinct. Notonecta instar II had the largest effect on the density of this species and also caused its temporal variability to increase. We have less information on the dynamics of this zooplankter than on D. Pulex.