Abstract
The history of the "principle of competitive exclusion" is briefly reviewed. Next, it is shown that taxonomically closely related (carabid) species, i.e. species in the same genus, can indeed be considered to be also ecologically closely related. This opened the possibility to test whether or not exclusion plays a demonstrable role in the distribution of species belonging to the same or to different genera over different habitats. The method used was proposed by SIMPSON (1949) and applied for the first time by WILLIAMS (1951). With the help of the pitfall-catches of 149 carabid species during nine years and in 73 different habitats, and by using three, different "values" it could thus be shown that congeneric species coexist more frequently in the same habitats than could be expected from a random distribution of the available species over habitats. This "coexistence principle" was further illustrated by two examples from habitats studied during a number of years. It could also be shown that interaction groups of coexisting congeneric (carabid) species do not die out more frequently than those of species belonging to different genera, not even in the course of a century. These findings are thus in agreement with the conclusion of BIRCH (1979) from a review of the field evidence, that "competitive exclusion" must be considered an only exceptional outcome of the possible interactions between species. The most parsimonious hypothesis for understanding the findings discussed in this paper therefore is: Taxonomically closely related (carabid) species are also ecologically closely related, and will thus more often than not be found coexisting in the same habitats.

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