@ 1979, by the American Society of Limnology and Oceanography, Inc. Predation, competition, and zooplankton community structure: An experimental study1’2

An experimental investigation of the zooplankton community of a small Minnesota pond was conducted for 2 years to determine the mechanisms maintaining its structure and to show that it is predictably organized. A mechanistic interpretation of the structure of this com-munity cannot be made solely on the basis of predation, but also requires evaluation of the relative competitive abilities of the herbivores. The presence of Chaoborus and fish place predictable constraints on the abundance of zooplankton species in this pond. The compet-itive dominant (Cerioduphniu reticulata) is of intermediate size and is removed when either predator is abundant. In the presence of intense Chaoborus predation, Cerioduphniu is re-placed by a larger subordinate competitor, Daphnia pulex; when fish predation is intense, smaller species (Rosminu Zongirostris and rotifers) increase. These small species are also able to maintain large populations in vertebrate-free environments when Chaoborus is rare. When these small herbivores are abundant, two additional invertebrate predators (Cyclops vernalis and Asplunchnu priodontu) arrive, neither of which seems able to reduce its prey to extinc-tion