Population-level responses to temperature, density and clonal differences in Daphnia magna as revealed by integral projection modelling

Raising global temperatures are predicted to have strong consequences for ectotherms, as metabolic rates depend directly on external temperatures. To understand consequences for population fitness, a full life cycle approach is important because (a) temperature can have opposite effects on different vital rates (growth, survival, reproduction) and (b) sensitivities of population growth rate to changes in vital rates can vary in magnitude. As vital rates are concurrently influenced by other factors, adequately predicting temperature effects requires factors such as body size, population density and genetics to be taken into account. The aim of this study was to quantify the role of temperature on all vital rates of Daphnia magna individuals and their integrated effects on population dynamics. In addition, we evaluated how clonal lineages differed in their temperature response, both on the vital rate and population level. We performed a laboratory experiment, in which we followed 40 populations (five clonal lineages × eight temperatures) during 80 days. Due to our novel set‐up, we were able to quantify vital rates of individuals within those populations. We identified relations between vital rates and body size, lineage, temperature and population density and used a size‐structured integral projection model to integrate the experimental effects over all vital rates. We found negative density dependence in growth and reproduction, resulting in lineage‐specific carrying capacities. Population fitness showed a thermal optimum that differed among genotypes. It is interesting that we found that clones had different life‐history strategies, optimizing population fitness via different routes. As no lineage outperformed the others in all vital rates, we identified trade‐offs between vital rates, which had strong effects on the dynamics of the population. Moreover, simulations suggest that the genetic composition of mixed populations is temperature‐dependent. Our results underscore the importance of studying individuals within their population when predicting responses to environmental change. The observed density effects, which were as strong as temperature effects but explained considerably more variation in population growth, would have been overlooked in life table experiments. Furthermore, differential temperature responses emphasize the importance of genetic variation in the ability of ectotherm species such as Daphnia magna to respond to climate change. A plai