Predation, competition, and zooplankton community structure: An experimental study1,2

Abstract

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 community 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 competitive dominant (Ceriodaphnia reticulata) is of intermediate size and is removed when either predator is abundant. In the presence of intense Chaoborus predation, Ceriodaphnia is replaced by a larger subordinate competitor, Daphnia pulex; when fish predation is intense, smaller species (Bosmina longirostris 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 Asplanchna priodonta) arrive, neither of which seems able to reduce its prey to extinction.