The results of the investigations on environmental factors, standing crop and migration patterns of Gammarus pulex pulex (Linnaeus, 1758), G. fossarum Koch in Panzer, 1836, and Echinogammarus berilloni (Catta, 1878) in the river Slack (Pas-de-Calais, France) are linked to get a picture of gammarid life in a small chalk stream. Gammarids appear to live in clusters, here called microgeographic races, with more or less unpopulated areas between them. By non-accidental migration they are able to colonize new areas. The locomotory activity necessary to feed and reproduce causes (and is reflected in) accidental migration. The different ways in which the three species respond to environmental factors can be explained by their different origin. G. p. pulex and G. fossarum are both species originating from eastern Europe, the first one euryecious, the second cold stenothermous; E. berilloni is an euryecious animal with a distribution centre in southwestern Europe. The concept of ecological zonation is strongly supported by our investigations. The three species can be used as a helpful tool in bioassays, since they demand different environmental conditions. The three freshwater species compete for the same limited resource: space. They often occupy the same ecological niche. This invalidates the competitive exclusion principle (s.l.: competitors can not coexist permanently): competitive exclusion is only one of the solutions for animals sharing resources.