Amphibian distribution patterns in western Europe

Mechanisms controlling the distribution of amphibians in western Europe have been studied in France where related species, isolated from each other at least during the last glacial period, are now sympatric. Occurrences and biotope preferences of the various species were investigated in several regions, with attention to the position of each region within the ranges of the species: central or peripheral. For some species the previously known distribution boundary is partially precised. The population densities of related species in their zones of overlap are discussed. Three pairs of species are considered in particular. Triturus helveticus and T. vulgaris have a large sympatric area in which they appear to be gradually replacing one another. Competition is considered to be unimportant, the distribution boundary of T. vulgaris in central France is determined primarily by climatological factors, causing typically a vague border. Rana temporaria and R. dalmatina occur largely sympatric. Very little can be found suggestive of mutual influences. The intensity of occurrence of R. temporaria and the preference of habitats seem to be dependent upon local climatological circumstances. The closely related Triturus cristatus and T. marmoratus both appear to be common in a relatively narrow zone. Within this zone the species occupy rather distinct biotopes, presumably due to ecological displacement. In this case the distribution boundaries are sharp. Phenomena connected to relationship are considered for pairs of species of Europe.