COEXISTENCE IN HELMINTHS OF THE BANK VOLE CLETHRIONOMYS-GLAREOLUS .2. INTESTINAL DISTRIBUTION AND INTERSPECIFIC INTERACTIONS

1. We studied the intestinal distribution of helminths (cestodes and nematodes) in the bank vole Clethrionomys glareolus, and changes in distribution due to the presence and varying abundance of other species. The material was obtained from two localities in Finland: Pallasjarvi in western Lapland and Lammi in southern Finland. 2. The intestinal distribution of the common species Heligmosomum mixtum (Nematoda) was affected by the rare species Capillaria sp. (Nematoda) and Paranolocephala gracilis (Cestoda). The common species showed inconsistent negative interactions with other common species. 3. The data indicate that interactions between the intestinal helminths of the bank vole are most likely to occur between species with large intestinal overlap, but not necessarily feeding overlap. 4. Competitive ability is favoured in the rare species, which occur frequently with the common species. On the other hand, the rare species' strict microhabitat specificity could be connected with competitive ability. 5. The coexistence of helminths is promoted by the niche segregation among the common species, by the trade-offs between competitive and colonization ability, and by the independently aggregated distributions of helminths within the host population.