Environmental stress and local adaptation in Daphnia magna

The effects of fish kairomones, crowding chemicals, and day length on the life–history traits of a set of 16 Daphnia magna clones, derived from four populations that differ in fish‐predation pressure, were studied. Significant among‐population differences were observed, the differences being in concordance with the hypothesis of local adaptation. The among‐population genetic differences were not mediated through a change in response to fish kairomones, but through an overall smaller body size, smaller eggs, and a higher number of eggs in clones derived from habitats in which fish are present. Using a model, we show that the observed changes in life‐history characteristics may lead to differences in fitness under different predation regimes, such that populations from habitats with fish have highest fitness under fish‐predation regimes and populations without a fish background have higher fitness values under invertebrate predation regimes.