Eutrophication of the Baltic Sea- consequences for stickleback populations
In this project we investigate the effects of the eutrophication of the Baltic Sea on sexual selection and natural selection in a common littoral fish, the three-spined stickleback Gasterosteus aculeatus. Sexual selection is one of the main forces of evolution and gives rise to the evolution of characters that increase mating and fertilisation success. It is opposed by viability selection and its intensity varies in time and space, depending on environmental heterogeneity. Thus, while equilibrium between sexual and viability selection may be achieved in one environment, changes in conditions can change the relative intensity of the selection pressures and influence the fitness of populations. Despite these obvious connections between ecological conditions, selection and population viability, the effects of environmental changes on sexual selection and the consequences this may have for populations, species and ecosystems are poorly understood.
Eutrophication is a serious environmental problem along the coasts of Europe, with notorious effects in the Baltic Sea. It alters the species composition of the ecosystem by increasing the growth of filamentous algae at the expense of slow-growing macro algae, and by altering the phytoplankton community and increasing turbidity. This changes ecological condition, but the long-term effects through evolutionary processes are unknown.
Our results show that habitat changes caused by eutrophication relax sexual selection on several reproductive traits. Interestingly, the changes result in an increased cost of sexually selected traits and apparently a relaxation of the good genes process. Thus, sexual selection is maladaptive under environmental change and could potentially reduce population viability. However, the reproductive output of the populations is increased under eutrophication. A relaxation of sexual selection could therefore be compensated by strengthened natural selection at later life-history stages, such as the juvenile stage. This could enhance selection for populations adapted to eutrophied conditions and ensure the success of the stickleback in the changing environment.
Currently, we are investigating the degree to which stickleback populations have phenotypically and genetically adapted to different degrees of eutrophication (at different time scales) when it comes to reproductive behaviours, parasite resistance and body condition. The results show that sticklebacks express a high degree of phenotypic flexibility and that populations separated by only a few kilometres exhibit local adaptations. This suggests that sticklebacks have the potential to relatively quickly adapt to different environmental conditions. High plasticity could be one explanation for why sticklebacks have been able to rapidly adapt to a range of different freshwater conditionsfrom their marine origin. Plasticity may initially have allowed the species to survive and reproduce in a new environment and thereby given the population time to accrue genetic changes and genetically adapt to the new conditions. This plasticity could now help the species to adapt to anthropogenic disturbances.