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要旨

We performed a series of computer experiments with a population of Daphnia galeata. The life-history parameters of the daphnids in the model were based on experimental data. The variation in these parameters among daphnids and instars was explicitly included in the model. A model population of 1000 individuals was kept in steady state by applying thirteen different size-selective sampling regimes. The consequences of these different scenarios for the population growth rate, the production to biomass (P/B) ratio and the population structure under the different selection scenarios were studied. Compared to random selection, the population growth rate in terms of individuals per time unit increased strongly under size-selective predation on the smaller individuals, and decreased weakly to an asymptotic level under size-selective predation on the larger individuals. Conversely, the P/B ratio decreased under selection on the smaller individuals and remained almost stable under selection on the larger individuals. The overall difference in P/B ratios between different selection scenarios was markedly smaller than the difference in population growth rate. The average length of the daphnids in the model increased with selection on the smaller individuals, but, surprisingly, also increased with selection on the larger individuals. This phenomena could be explained by the demographic structure of the population under the different scenarios