Diet quality and food limitation in herbivores: the case of the snowshoe hare

Abstract

Arguments concerning the regulation of animal populations through food shortage are hampered by the difficulty of measuring available food. This is avoided by considering how animals respond to declining food supplies. Three alternatives exist: (a) maintain a constant rate of food intake by including more poor food, (b) increase the quantity eaten to compensate for poor quality, or (c) eat only high-quality foods and thus decrease the quantity eaten. Experiments with snowshoe hares (Lepus americanus) fed ad libitum food of different crude protein values, show hares maintain a relatively constant intake rate as quality falls. This result allows us to use mean diet quality to predict whether animals can maintain body weight under natural conditions. The threshold diet quality below which weight loss occurred was 11% crude protein in the laboratory. There is a strong correlation between diet crude protein and faecal crude protein. By collecting faecal pellets in the field, one can monitor the diet of the population. If faecal crude protein falls below 7.5%, animals lose weight as a result of insufficient good food. Field data for 1977–1980 show faecal protein for some animals dropping below this level in late winter of 1979 when the hare population was near its peak density.