Predation, Competition, and Zooplankton Community Structure: An Experimental Study

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 strncture o f this community cannot be made solely on the basis of predation, but also requires evaluation o f 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 o f which seems able to reduce its prey to extinction. Recent studies provide ample evinature. The information they are based dence that predator-prey interactions are on does not allow a mechanistic interpreof major significance in structuring freshtation of zooplankton community strucwater zooplankton communities (see Hall ture. For instance, while it is well docue t al. 1976). Vertebrate planktivores (fish mented that large herbivores disappear and salamanders) feed visually and rewhen vertebrate planktivores are intromove the largest and most conspicuous duced to a community, no data exist to zooplankton (Brooks 1968; Werner and determine whether their extinction is enHall 1974; Zaret and Kerfoot 1975; tirely a result of direct removal by verO'Brien e t al. 1976); invertebrate predatebrates. It is possible that vertebrates, tors (predaceous copepods and rotifers, through their effects on the rest of the midge larvae, and Leptodora) cannot community, may impose other deleterihandle the large herbivores but prey exous conditions on the large herbivores tensively on small species (Dodson (e.g. increased competition with newly 1974a; Fedorenko 1 9 7 5 ~ ; Kerfoot 1977); established smaller herbivores). Furtherwhen vertebrates are present the larger, more, there is no direct evidence that inmore conspicuous invertebrate predators vertebrate predation can be sufficient to are replaced by smaller, less conspicuous cause extinction of small herbivores. Fiones (Dodson 1970, 1974~) . nally, the role of herbivore competition These generalizations form the bases in structuring zooplankton communities for recent conceptual models of zoois poorly understood (Lynch 1977a), and plankton community structure (Dodson nothing is known about competition be1974a; Zaret in prep.). However, despite tween invertebrate predators. their success at predicting the distribuTo eliminate these ambiguities in intion of different zooplankton species, terpreting zooplankton community structhese models are phenomenological in ture, I examined simultaneously the predatory and competitive interactions in the community of a small Minnesota Contribution 172 from the Limnological Repond. Here I report on the mechanisms search Center, University of Minnesota. Work supported by National Science Foundadetermining the relative abundances of tion grant EMS 74-19490 to J . Shapiro. species in this pond and show that the