Most free-living nematodes should have a global distribution if they would follow general tendencies of microbial organisms. Information on free-living nematodes presented in this review demonstrates that this cosmopolitanism is less common than assumed by theory. While very large distribution ranges are observed in a number of nematode species, various examples of endemism are described for isolated units like islands, extreme environments and ancient pre-Quaternary lakes. Endemism is

more »

... rare among microorganisms, but a typical observation for larger organisms. The biogeography of nematodes thus reflects their intermediate position between macro-and microorganisms and future studies on this interesting group may help identifying why the positive relationship between body size and range size observed in large animals shifts to a negative relationship in microbial organisms.