Factor interaction influencing the distribution of the freshwater “shrimp” Gammarus

Summary The enormous variation in the environmental factors of south-eastern Norway makes this area particularly suitable for field studies on the relationship between environmental factors and fauna. Lakes are abundant, from 1 to 1,837 m a.s.l., and the geology of the surroundings as well as acid precipitation influence their chemical characteristics. In lowland areas the distribution of Gammarus lacustris is related to how rich in lime the lakes are. Populations found in mountain lakes tolerate much lower levels of calcium and higher H⁺ concentrations than populations inhabiting lowland lakes. The lowland populations probably have a higher calcium demand when stressed by high temperature. We predict that factor interaction may be used to explain the distribution patterns of the species also in other parts of Europe.