Spatial, environmental, and anthropogenic effect on the taxon composition of hybridizing Daphnia

In general, hybrids are known to perform poorly compared with parental species. Under specific conditions, however, hybrids can be equally or more fit than their parents. For example communities of the Daphnia galeata-hyalina-cucullata species complex show often hybrid dominance. By studying habitat characteristics (e.g., size descriptors, trophy level) of 34 43 Swiss and North Italian lakes we found that D. galeata more frequently occurring in southern, whereas D. hyalina and D. cucullata are more present in northern lakes, a pattern which can not be explained by formerly described habitat preferences alone. In a subset of 28 lakes, we studied the impact of eutrophication history on F1-hybrid success. Lakes with the highest trophic stages in the past tended to be dominated by F1-hybrids. Our data demonstrates that human-mediated habitat disturbance (eutrophication) has facilitated hybrid success and altered the Daphnia taxa composition across lakes. At the same time, specific habitat conditions might provide a refuge from hybridization for native genotypes.