Resource‐mediated demographic variation during the midsummer succession of a cladoceran community

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SUMMARY. 1. The midsummer succession of cladoceran zooplankton in Lake Texoma was examined by demographic characterization of lake populations and a parallel set of life table experiments in which the dominant cladoceran species were raised on naturally occurring lake phytoplankton at lake temperatures. Diaphanosoma leuchtenbergianum occurred throughout the study period (mid June‐mid August 1981) but Daphnia parvula, D. galeata mendotae and Ceriodaphnia lacustris were rarely detected after 15 July.2. Analyses of size structure and size‐specific fecundity of the cladoceran populations showed that (1) size structure shifted to smaller individuals in mid July, (2) size‐specific fecundity was relatively constant in the smaller size classes and was constant or declined in larger females of each species, and (3) size at maturation did not vary significantly during the study of any population.3. Individual growth rates of the two Daphnia species and Ceriodaphnia in the life table experiments were significantly lower in mid July when populations were declining and when size structure of each population was more dominated by smaller Individuals. The reduction in individual growth rates was short‐lived in the life table experiments; growth rates returned to pre‐decline levels by the end of July.4. Two effects of temporary reductions in individual growth rates when size‐specific fecundity and size at maturation are constant are (1) that size structure is temporarily shifted to smaller individuals, and (2) the potential rate of population growth is reduced because of delays in reproduction. Because data on size structure and size‐specific fecundity are commonly used to evaluate the relative effects of predators and resource changes on populations it is noteworthy that erroneous conclusions about the mechanism of population decline may be reached when individual growth rates vary.