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 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 (Cerioduphniu reticulata) is of intermediate size and is removed when either predator is abundant. In the presence of intense Chaoborus predation, Cerioduphniu is replaced 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 extinction. Recent studies provide ample evidence that predator-prey interactions are of major significance in structuring freshwater zooplankton communities (see Hall et al. 1976). Vertebrate planktivores (fish and salamanders) feed visually and remove the largest and most conspicuous zooplankton (Brooks 1968; Werner and Hall 1974; Zaret and Kerfoot 1975; O’Brien et al. 1976); invertebrate predators (predaceous copepods and rotifers, midge larvae, and Leptodora) cannot handle the large herbivores but prey extensively on small species (Dodson 1974a; Fedorenko 197%; Kerfoot 1977); when vertebrates are present the larger, more conspicuous invertebrate predators are replaced by smaller, less conspicuous ones (Dodson 1970, 1974a). These generalizations form the bases for recent conceptual models of zooplankton community structure (Dodson 1974a; Zaret in prep.). However, despite their success at predicting the distribution of different zooplankton species, these models are phenomenological in i Contribution 172 from the Limnological Research Center, University of Minnesota. 2 Work supported by National Science Foundation grant EMS 74-19490 to J. Shapiro. nature. The information they are based on does not allow a mechanistic interpretation of zooplankton community structure. For instance, while it is well documented that large herbivores disappear when vertebrate planktivores are introduced to a community, no data exist to determine whether their extinction is entirely a result of direct removal by vertebrates. It is possible that vertebrates, through their effects on the rest of the community, may impose other deleterious conditions on the large herbivores (e.g. increased competition with riewly established smaller herbivores). Furthermore, there is no direct evidence that invertebrate predation can be sufficient to cause extinction of small herbivores. Finally, the role of herbivore competition in structuring zooplankton communities is poorly understood (Lynch 1977a), and nothing is known about competition between invertebrate predators. To eliminate these ambiguities in interpreting zooplankton community structure, I examined simultaneously the predatory and competitive interactions in the community of a small Minnesota pond. Here I report on the mechanisms determining the relative abundances of species in this pond and show that the