Adaptive evolution can take place at ecological timescales and can lead to changes in population dynamics, alter community structure and even impact ecosystem processes. One expectation is that the impact of evolutionary change decreases with increasing ecological complexity, with a stronger impact on population characteristics than on community and ecosystem responses. However, changes in traits tightly coupled to ecosystem functioning (effect traits) may impact ecosystems more than communities and populations. The purpose of our study was to determine the impact of genetic adaptation in response to temperature and eutrophication in the water flea Daphnia on attributes of its own population, the entire grazer community, and top-down control of algae. We carried out a selection experiment in outdoor mesocosms with natural zooplankton and phytoplankton communities exposed to gradual eutrophication (from 0.4 to 6.0 mg tP/L) and/or a temperature increase of 4°C for 67 days. We isolated clonal lineages from the resulting Daphnia magna populations and purged them of maternal effects. These lineages were used in a laboratory grazing experiment and in a full factorial outdoor reciprocal transplant experiment to determine whether or not genetic divergence among populations exposed to different conditions impacts population and community structure and top-down control of algae.
Results/Conclusions
The laboratory grazing experiment revealed significant differential evolution of D. magna grazing rates after a two-month exposure to factorial combinations of ambient and elevated nutrients and temperature. In the transplant experiment, these genetic differences translated in a moderate but significant change in D. magna population biomass and structure but no change in community structure. Traditional ecological drivers such as nutrient concentrations and temperature explained most of the variation in community structure. While the impact of evolution on community structure was insignificant, we did observe a strong impact of evolution on top-down control of algae in the transplant experiment. The effect sizes of the main and interacting effects of nutrients, temperature, and adaptation to nutrients and temperature indicate that evolutionary responses that occurred during the short time frame of our experiment can alter responses at the population level, were unimportant for community level properties, but did significantly alter ecosystem level responses. Our results show that eco-evolutionary dynamics do not solely take place in simplified systems but were operational in semi-natural experimental systems that involved natural communities of grazers and phytoplankton, and that evolutionary responses in effect traits may impact ecosystem features in the absence of changes at the community level.