Zooplankton Competition and Predation: An Experimental Test of the Size‐Efficiency Hypothesis

Abstract

Twelve 42—liter plankton cages were used in an alpine Colorado pond to test a size—efficiency hypothesis: to determine why small herbivorous zooplankton species tend not to coexist with large species. The size—efficiency hypothesis, that large species exclude the smaller ones through competition for food, was not substantiated. An alternate hypothesis extends the understanding of the importance of size—selective predators to include invertebrates selecting small prey. A predaceous copepod Diaptomus shoshone excluded the small Daphnia minnehaha from an association with the large D. middendorffiana within 1 mo. By implication, the predacious copepod is responsible for the absence of the small species in ponds occupied by the large Daphnia species.