Environmental changes impose selection pressures that may lead to changes in species composition as well as micro-evolutionary changes in trait values of resident species. The relative importance of changes at the community and at the population level will depend on the degree to which populations can genetically track environmental changes. In the face of competition, there may be selection for changes in competitive ability (the power to dominate) and ecological combining ability (character displacement). Using a resurrection ecology approach combined with competition trials, we reconstructed the changes in both competitive ability and combining ability with respect to both intra- and interspecific interactions in a natural Daphnia magna population inhabiting a lake that experienced pronounced environmental changes during the past 100 years. Given that selection pressure exerted by competition is density dependent, we hypothesized that genotypic trait values for both competitive ability and ecological combining ability may track changes over time in population densities and relative abundance of competitors.   

We hatched dormant eggs of the water flea Daphnia magna from different time periods of a layered sediment core of Lake Ring (DK), a lake that experienced strong changes during the past 100 years because of habitat deterioration caused by eutrophication followed by habitat restoration and recovery (1980s). Associated with these human-induced environmental changes, there was a shift in the relative abundance of D. magna and its congener, D. pulex. A total of 21 clones were used in competition trials against tester clones of both D. magna and D. pulex. Relative abundances of clones in mixed populations were determined by genetic markers.

Results/Conclusions

We observe genetic variation for intra- and interspecific competitive ability, and for interspecific ecological combining ability. There is a significant positive association between competition traits of experimental clones and Daphnia biomass and species composition of the corresponding time period as reconstructed from the layered egg bank. More specifically, interspecific combining ability is associated with D. pulex biomass, whereas intraspecific competitive ability is positively associated with the relative abundance of D. magna to D. pulex. Our results suggest that Daphnia populations are able to genetically track changes in their competitive environment in a subtle way, with intraspecific interactions leading to evolution of competitive ability, and interspecific interactions leading to evolution in ecological combining ability.