(1) The importance of interspecific competition relative to other factors in affecting the distribution and local coexistence of three rockpool Daphnia species is evaluated using experimental data on life histories and competitive abilities. (2) D. longispina showed life-history traits commonly believed to have evolved in response to size-selective predation on larger prey individuals. D. pulex and D. magna had comparatively high rates of increase, which is of special importance in, for example, temporary rockpools. D. magna is presumably adapted to more eutrophic conditions than the other two species. (3) In laboratory experiments, D. pulex was the best competitor, whereas the outcome between D. magna and D. longispina depended on temperature and food conditions. The larger D. magna did better at the lower temperature and in the higher food density. Body size alone could not be used to predict the outcome of competition experiments. In field experiments lasting for two summers, competitive exclusion did not occur. (4) This study supports the view that closely related species are often unable to exclude each other, even though competition may be intense. Local distribution patterns of rockpool Daphnia probably depend more on species-specific responses to abiotic factors, food density and predation, rather than on colonization, extinction and competitive exclusion.
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Jan Bengtsson (1986) studied this question.