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.