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Vole Power:
Herbivores prefer diseased plants.

Maxine-Fay Miller

Ecologists from Sweden have discovered that grazing by bank voles can affect relations between a plant and a disease-causing fungus. Lars Ericson and Anders Wennström of Umeå University studied links between four species living on an island off Sweden's east coast: the perennial herb chickweed wintergreen Trientalis europaea, a smut fungus Urocystis trientalis, which infects the plant and causes a disease, and two herbivores (plant-eaters) that feed on the plant -- the bank vole and a species of scale insect.

The researchers set up a number of study plots, surrounding some with cages to exclude voles and leaving others open to act as controls. They counted the number of shoots of chickweed wintergreen in each
plot, noting whether the shoots were healthy or infected with smut fungus.
bullet.GIF (43 bytes)  Smut fungi parasitize flowering plants, causing severe damage and often death.
bullet.GIF (43 bytes)  They get their name from the smut-like spore masses that develop on infected plants.
bullet.jpg (1952 bytes)  The smut fungus Urocystis trientalis specializes on chickweed wintergreen, infecting up to 50 per cent of plants in some areas in Sweden

Unhealthy diet. Herbivores eating chickweed wintergreen prefer smut-fungus-infected shoots to healthy ones. (Photo: Bob Gibbons/Ardea)

After two summers, they discovered that the level of fungal disease was lower in the open plots -- the ones with voles -- than it was in the caged plots (Oikos, vol. 80, pp107-11). Further investigation revealed that voles prefer to feed on diseased shoots of the plant -- as do scale insects -- though the reason for this preference isn't yet clear.

Whatever the reason, the voles have a beneficial effect on the plant, as by removing diseased shoots from the population, they reduce the spread of the smut fungus.