Effects of whitefish speciation on piscivorous birds : A dietary study of piscivorous birds in central and northern Sweden

Sammanfattning: The ecological communities we observe today are a product of the bidirectional interactions between ecological and evolutionary processes. Although the effects of ecological processes on population divergence and speciation have been studied extensively, far less is known about the effects of divergence and speciation on ecological dynamics. This is especially true for effects of ecological speciation processes on higher trophic levels. In this thesis I focus on how divergence in the European whitefish (Coregonus lavaretus) affects a guild of six piscivorous bird species. Previous studies have indicated that population densities of these species are higher on lakes with polymorphic whitefish than on lakes with monomorphic whitefish. Here I test the hypothesis that the high densities of piscivores is a response to the rich food resource provided by dwarf whitefish ecotypes, which are of suitable size and occur in very high abundance.To test this hypothesis I analyzed fecal samples from piscivorous birds in lakes with polymorphic whitefish, using samples from lakes with monomorphic whitefish as controls. With the method of ddPCR (digital droplet Polymerase Chain Reaction) the amount of DNA from different prey fish species in the droppings of six fish-eating birds was quantified and converted to proportional abundances. The results shows that the proportion of whitefish in the diet of the entire fish-eating guild was significantly higher in lakes with polymorphic whitefish (44%) than in lakes with monomorphic whitefish (18%). Species-level analyses showed that this result also holds for both black-throated loon (Gavia artica) and red-throated loon (Gavia stellata). Common merganser (Mergus merganser), red-throated merganser (Mergus serrator) and terns (Sterna paradisaea and Sterna hirundo) did not show any difference between the two lake categories. Thus, my study supports the idea that the evolution of small-sized whitefish ecotypes provides a profitable food source for piscivorous birds. However, the finding that only some species of piscivorous bird populations seem to rely heavily on dwarfed whitefish as food suggests that also some other aspect of the speciation process may favor these species. Thus, more studies are needed to further assess what effects polymorphic whitefish have on piscivorous bird populations.