Food as a limiting factor for small rodent numbers : Tests of two hypotheses

1. Two hypotheses relating small rodent density variations to their food conditions were tested by attempts to refute derived predictions on food conditions and dynamics of populations. One hypotheses assumes an independent rhythm in plant food production and the other gradual overgrazing. The findings were also compared with predictions from hypotheses on self-regulation of rodent populations. 2. Two vole species, representing one herbivore and one partial granivore, were sampled in South, Central and North Sweden in 1971-1975. A two year population peak was found in both species in North Sweden, a one year peak in Central Sweden and no obvious peak at all in South Sweden. 3. Weather conditions, primary production of various plants and chemical composition of food plants were examined in the same regions. Food conditions improved for at least herbivorous rodents during the increases and before peaks in density in Central and North Sweden. Similar annual variations appeared in plant production in South Sweden without any accompanying numerical changes in the rodent populations. 4. In certain respects neither hypothesis could be refuted. However, the observations did not favour the ideas of self-regulation. Thus a new hypothesis is proposed. It assumes that in certain areas small rodents and their predators show a dynamic balance with the rodents below the food resource limitation. However, in disturbed habitats or areas with much snow, predation is difficult. There the herbivorous rodents may overexploit their food but at the same time may be affected by a several-year rhythm in the food plants.