Biodiversity implications of northern rodent fluctuations

Heikki Henttonen

Finnish Forest Research Institute, POB 18, FIN-01031 Vantaa, Finland

Many northern areas are characterized by strong and regular population fluctuations of voles and lemmings. Small rodents are a keystone group in the North, and they have a great impact on trophic levels below, above and inside them. The numbers and the breeding intensity of most predators and birds of prey depend on the fluctuating rodents.

In low rodent years, predator numbers are low, birds of prey may not stay in their normal breeding grounds, and breeding may not happen at all.  During rodent peaks, nomadic birds of prey, like the snowy owl, may suddenly populate specific restricted areas. High rodent numbers affect the vegetation, i.e. the species composition, dynamics, seed and berry crops of many herbs, grasses and dwarf shrubs. Most vole and lemming species are quite selective in the feeding, and the exact impact depends on the rodent species. The most drastic example is the Norwegian lemming Lemmus lemmus, which during the peak years has a major impact on the field layer of the alpine zone, and also in forest zone during the migrations.

The plant species composition and moss biomass in the alpine is strongly modified by lemmings. Grey sided vole Clethrionomys rufocanus feeds mainly on shoots of the bilberry Vaccimium myrtillys in winter. The cyclic consumption affects the age structure, and consequently, the flower and berry production, of this highly valued berry producer.  The fluctuating grazing impact and production of faeces certainly affects the soil animal fauna, microbial flora and nutrient cycling though these patterns are not well-known yet.

The complicated life cycles of many parasites cause inherent lags in the capacity to respond immediately to changes in rodent host densities. However, more long-term host changes are reflected in the dynamics of rodent parasites. The ratio of monoecious/dioecious parasites may be related to the degree of rodent fluctuations in northern areas, the latter ones being more prone to extinctions during extreme low phases of the host.

During recent 10-15 years, there has been a change in rodent dynamics in northern Fennoscandia: some species have become more stable, some have almost disappeared. These changes have profound impacts on trophic levels both below and above.