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Abstract

1. Crowding ''info-chemicals'', metabolites released into the environment that act as signals to conspecifics as well as other organisms, have often been shown to cause phenotypic plasticity in cladoceran life-history traits including morphology, reproductive strategy and sex induction. Effects on population demography and diversity, however, have not been examined directly. 2. Replicate laboratory populations of the zooplankter, Daphnia magna, were started with 250 juveniles and allowed to develop for 8 weeks in 10 L flow-through aquaria with food levels of 1 mg CL-1. Two crowded water treatments from different clonal sources were compared with untreated water as a control. 3. The presence of crowding chemicals affected initial population structure, through reduction in parthenogenetic female body lengths, brood sizes and juvenile densities. However, the timing of population density maxima and subsequent population structure, clonal composition and diversity were similar among all treatments. 4. Clonal richness remained slightly higher in untreated populations because of increased prevalence of rare clones. 5. Exploitation competition had greater impact on population structure than the relative concentration of crowding chemicals, indicating that density dependent effects were mediated primarily by competition for common resources. 6. Crowding info-chemicals may play a greater role in community structure than in single species population structure because of taxonomic diversity in species'' response that may be unrelated to the source species density.