RESOURCE DEPRESSION IN DAPHNIA-GALEATA, DAPHNIA-CUCULLATA AND THEIR INTERSPECIFIC HYBRID - LIFE-HISTORY CONSEQUENCES

In this study, the effect of the food concentration on several life history parameters of Daphnia galeata, Daphnia cucullata and the hybrid between these two species was examined. The food concentration had a significant effect on both growth and reproduction of all three taxa. Mortality was highly species specific, and to a lesser extent dependent on the food concentration. Changes in life history parameters resulted in changes in the intrinsic rate of population increase (r). The food dependence of the different species was found to be different. Daphnia cucullata had a relatively constant and low v. The r values for the hybrid varied most, with low values compared with the parental species when grown at low food levels and relatively high values at higher food concentrations. Our results suggest that when food concentrations are high the hybrid will have a selective advantage over both parental species, but that at lower food levels D.galeata will have an advantage over both D.cucullata and the hybrid.