Complex Interactions between Natural Coexploiters--Daphnia and Ceriodaphnia

This study concerns a seasonal succession of a large herbivore, Daphnia pulex, by a smaller one, Ceriodaphnia reticulata, in a Minnesota pond. Based on a description of events in the pond and results of several field experiments, one can explain this replacement in terms of a coexploitative interaction between these 2 species. The field experiments, in which pairs of cladocerans were isolated for 4 wk, gave several results which are inconsistent with the assumptions of contemporary competition theory. In the 1st experiment, Daphnia survived when solitary but went extinct when paired with Ceriodaphnia, regardless of starting densities or age structure. At the same, Daphnia was declining in the pond while Ceriodaphnia was increasing. Four wk later, Daphnia coexisted in pairs with both Ceriodaphnia and Bosmina longirostris, and at much greater densities than in its controls. Not only did the competitive abilities of these species shift over this very short time period, but the densities which they attained in control enclosures dramatically declined. Thus, static measures of competition (such as alphas) and carrying capacities are inappropriate for these species. Furthermore, these interactions can be understood only in the context of a framework which considers age structure. Generally, juveniles were more sensitive to coexploitative interactions while adults were unaffected and sometimes even benefited. Since their resources interact, coexploitative interactions between these species are not simply a result of resource depression but of resource alteration. Despite the fact that they overlap extensively in resource utilization, an appropriate modification of the resource base by 1 species can lead to enhanced conditions for a 2nd. I suggest that such facilitation may be an important process in these communities which merits further investigation.