Enhanced anti-predator defence in the presence of food stress in the water flea Daphnia magna

1. Many prey organisms show adaptive trait shifts in response to predation. These responses are often studied under benign conditions, yet energy stress may be expected to interfere with optimal shifts in trait values. 2. We exposed the water flea Daphnia magna to fish predation and food stress and quantified both life history responses as well as physiological responses (metabolic rate, stress proteins, energy storage and immune function) to explore the architecture of defence strategies in the face of the combined stressors and the occurrence of trade-offs associated with energy constraints. 3. All traits studied showed either an overall or clone-dependent response to food stress. The chronic response to predation risk was less strong for the measured physiological traits than for life history traits, and stronger under food stress than under benign conditions for age at maturity, intrinsic population growth rate and offspring performance (measured as juvenile growth). Immune function (measured as phenoloxidase activity) was lower under predation risk but only at high food, probably because minimum levels were maintained at low food. 4. Overall, food stress induced stronger adaptive predator-induced responses, whereas more energy was invested in reproduction under benign conditions at the cost of being less defended. Our results suggest that food stress may increase the capacity to cope with predation risk and underscore the importance of integrating responses to different stressors and traits, and show how responses towards one stressor can have consequences for the susceptibility to other stressors.