Herbivores prefer diseased plants.
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. They get their name from the smut-like spore masses that develop on infected plants. The smut fungus Urocystis trientalis specializes on chickweed wintergreen, infecting up to 50 per cent of plants in some areas in Sweden Unhealthy diet. 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.