Population Dynamics of Common and Rare Helminths in Cyclic Vole Populations

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Abstract
(1) Population dynamics of vole helminths in Finnish Lapland were studied for a 10-year period covering a peak phase of a vole cycle (1977-78), an entire cycle (1980-85), and an increase phase of a third cycle (1986-87). The host material consists of 1032 bank voles Clethrionomys glareolus and 175 red voles Clethrionomys rutilus. (2) Age-prevalence relationships were analysed, as well as seasonal and long-term population dynamics of vole heminths with special reference to the various regulatory factors. (3) The common helminths (Heligmosomum mixtum, Nematoda, and Catenotaenia sp., Cestoda) were characterized by regular seasonal changes in prevalence and intensity, with the peak of infection occurring in winter. Generally, the density of C. glareolus did not determine the infection levels of common helminths. However, during the low and increase phase of a typical vole cycle the prevalence of common helminths followed those of the host (C. glareolus). (4) Compared to the common heminths, the rare species (Mastophorus muris, Capillaria sp., Syphacia petrusewiczi, Nematoda, and Paranoplocephala kalelai, Cestoda) showed quite different seasonal dynamics with a spring, summer or autumn peak in overwintered voles. Multi-annually they were less predictable than the common species. In addition, the dynamics of the rare helminths were not affected by the phases of low host density, probably because of their strictly seasonal occurrence and, consequently, long response times. (5) The regulation of common helminths seemed to be accomplished primarily at the level of host individuals, the role of the hosts'' immunological mechanisms being important. The transmission was impaired only when extremely low host densities prevailed for long periods of time. No evidence of host mortaily caused by common helminths was found. (6) The drastic seasonal variation in transmission efficiency was the dominant feature in the dynamics of rare helminths. The long-term results suggest that stochastic factors affecting the survival of infective stages, and hence transmission, may be more important for rare than common species. (7) The life histories (modes of feeding and reproduction, life cycles) of vole helminths did not determine the nature of their population dynamics and regulation. However, the common species were characterized by a low degree of aggregation, whereas all the rare nematodes were highly aggregated within the host population.