Body size and reproductive investment of Daphnia galeata under predation by cyprinid fishes: A mesocosm study

Abstract

Demographic and life-history responses of Daphnia galeata to predation by cyprinid fish were studied in a mesocosm experiment where four levels of predation were repeated at two inorganic nutrient loads. No clear trend was observed in response to nutrient enrichment, so the competition hypothesis, as shaping the demographic profile of the studied population was discarded and only the predation effects were analysed. The changes in body size distribution and reproductive effort estimated through the proportion of ovigerous females and brood sizes were analysed and compared to predicted patterns. Both a significant decrease in body size and an increase in proportion of egg-carrying females in the population could be attributed to increasing vertebrate predation. The direct effect of selective predation resulted in a decrease in population size despite an increased reproductive effort and high food abundances. An analysis of egg size vs. ovigerous female body size revealed treatment related clusters with little or no overlap which indicates that not only the direct size-selective effects take place in natural populations but that a phenotypic response takes part. The increase in proportion of ovigerous females under predation can provide a qualitative estimation of phenotypic response if size-selectivity of predators is assumed.