Freshwater Zooplankton Community Structure: Introduction of Large Invertebrate Predators and Large Herbivores to a Small Species Community

Two hypotheses have been proposed to explain the scarcity of small zooplankton species in fishless lakes, in which large zooplankton species dominate: (1) selective predation on small species by large invertebrate predators and (2) competitive suppression of small species by large herbivores. These hypotheses were tested at the Experimental Lakes Area (ELA) by introducing a large invertebrate predator, Chaoborus americanus, and a large herbivore, Daphnia catawba, both common in fishless ELA lakes, into a zooplankton community typical of ELA lakes with fish (small zooplankton species present). Chaoborus had much more of an impact than D. catawba on resident zooplankton, although both introduced species significantly reduced the abundance of some resident species. Daphnia galeata mendotae, the dominant species in the absence of introduced species, was reduced to virtual extinction by Chaoborus. The cladocerans Bosmina and Diaphanosoma, the copepods Diaptomus spp., and the rotifer Conochilus also were reduced in abundance by Chaoborus. Daphnia galeata mendotae and Bosmina were the only resident species consistently reduced in abundance by D. catawba. These results support current conceptual models that depict invertebrate predation as an important factor responsible for the relative scarcity of small zooplankton species in fishless lakes.