Original paper

Competitive ability and life history strategies in four species of Daphnia: D. obtusa, D. magna, D. pulex and D. longispina

Milbrink, Göran Kruse


To examine how life history traits influence community composition, we tested three hypotheses for competitive dominance among zooplankton: The "r-max hypothesis" stating that the species with the highest intrinsic rate of increase wins in competition, the "size-efficiency hypothesis" saying that larger species are superior competitors, and the "small body size hypothesis" suggesting that small species are better competitors. The latter two hypotheses are versions of Tilman's R*-hypothesis, which suggests that the species that is able to survive under the lowest resource concentrations will win in competition, but they differ in the role of body size determining R*. We used a laboratory model system of four species of Daphnia commonly occurring in ponds and rock-pools in the northern temperate zone, D. magna, D. pulex, D. obtusa and D. longispina (in order of decreasing adult body size), and the green alga Scenedesmus quadricauda as food. Life-table and competition experiments were focused on the factors explaining the good competitive ability of D. obtusa. One-, two and four-species experiments were run without predation and under different predation profiles mimicking vertebrate and invertebrate predators. Life-history traits of D. obtusa under high food conditions at 20 °C were measured and compared with previous measurements on the other three species. D. obtusa was the superior species in most situations, except under high invertebrate predation when the large D. magna was superior. The outcomes did not depend on the specific composition of the zooplankton community (two or four species present initially). Mean population sizes of all four species were higher in experiments with fewer species, indicating strong interspecific competition also under predation. D. obtusa had a higher intrinsic rate of increase (r) than the other three species studied. Therefore, our results mainly support the r-max-hypothesis. We suggest that a competitive hierarchy among the four Daphnia species may exist, but the relative positions of the species in the hierarchy may vary with environmental conditions like the type of predator present, food quality and quantity, and temperature. Competitive ability did not seem to be strongly related to body size.


competitiondaphnialaboratorylife-history strategiespredation