The interplay of past and current stress exposure on the water flea "Daphnia"

1. Natural populations are exposed to multiple stressors, including both anthropogenic challenges such as xenobiotics and natural stressors associated with exposure to parasites and predators. While there is increasing concern and interest in the combined impact of current exposure to multiple stressors, little attention has been given to how past exposure to a stressor and its evolutionary response shapes the effects of current stressors. 2. Here, we performed a life-table experiment using the water flea Daphnia magna to study combined effects of current exposure to the pesticide carbaryl, parasite spores and fish predation risk and how these effects depend upon past exposure to carbaryl using clones obtained from a previous carbaryl selection experiment. 3. The current exposure to all three treatments affected life-history traits. Exposure to fish kairomones increased intrinsic population growth rate, while carbaryl and parasite exposure decreased this fitness measure. The three treatments interacted only in a few cases: carbaryl and fish kairomone exposure interacted in shaping intrinsic population growth rate and its component individual reproductive performance, yet the latter only in the animals not exposed to carbaryl stress in the past. 4. Our data revealed not only adaptive evolution of carbaryl resistance but also associated evolutionary costs in terms of reduced resistance to parasites, corroborating results of an earlier study. Importantly, both the evolutionary benefits and costs of past exposure to carbaryl stress were conditional on current environmental conditions, exposure to predation risk and parasites, respectively. 5. The emerging pattern showed that past stress interacted with current stress in shaping life history. Such evolution-driven carry-over effects across generations have been often ignored and may complicate the prediction of effects of current exposure to single and combined stressors even long after the past stress has disappeared.