Competition and the relative abundance of two cladocerans

Two cladocerans, Daphnia parvula and Holopedium gibberum, were studied to determine (a) whether the species were competing and (b) whether the observed coexistence was predictable from competitive relationships alone. Experimental manipulations of intra— and interspecific competition using 14C—labelled algae, bacteria, and detritus revealed clear competitive effects only with algae. Some resource partitioning was demonstrated. A model was proposed specifying Holopedium to be the poorer competitor for mutually shared resources, but also to be less reliant on those mutual resources, owing to its larger niche. This model predicts coexistence and a higher relative abundance for Holopedium. However, the dynamics of field populations did not correspond to this prediction. Daphnia was rare and declining throughout the study whereas Holopedium exhibited a pronounced burst of growth. Analysis of birth and death processes revealed that Holopedium was relatively free from mortality throughout most of the study, while Daphnia suffered death rates of 20% per day or higher. This differential mortality explains the discrepancy between the competition model and observed densities. Finally, field studies indicated that mortality of Daphnia owing to predation by Chaoborus larvae is more than twice that of Holopedium. It is concluded that Dalphnia and Holopredium could coexist in a purely competitive system; however, because of selective predation, their observed abundances correspond little to their competitive status.