Nutritional Ecology of Microtine Rodents: Resource Utilization near Atkasook, Alaska

Abstract

Three species of microtine rodents occur near Atkasook, Alaska: the brown lemming (Lemmus sibiricus), the collared lemming (Dicrostonyx torquatus), and the tundra vole (Microtus oeconomus). Population densities do not fluctuate as widely as do those of the brown lemming near Barrow, where densities may reach 200 ha^–1. Total microtine densities at Atkasook seldom reach more than 10 ha^–1, except in the most favorable habitats near streams. Collared lemmings, unlike those at Barrow, occur in a wide variety of habitats and are almost as numerous as brown lemmings. Tundra voles, which do not occur at Barrow, are restricted to habitats where lemmings are uncommon — on stream banks, on floodplains, and near human disturbance. The summer distribution of lemmings appears to be controlled by the availability of food (low growing willow for Dicrostonyx and lush growth of sedges for Lemmus), but the winter nests of Dicrostonyx also appear to be associated with deep snow.

Stomachs of brown lemmings mostly contained monocotyledons, particularly sedges, and small amounts of mosses; collared lemmings mostly eat dicotyledon leaves, particularly willow, supplemented by small amounts of monocotyledons; and tundra voles eat large amounts of sedges, willows, and a wide variety of herbaceous dicotyledons. None of the microtines eats the abundant, aromatic evergreen shrubs. Comparison of stomach contents with forage availability indicated strong food preferences. Palatability trials gave results consistent with these preferences, and many plants were not consumed. Microtines will consume rabbit pellets dosed with alcohol extracts of unpalatable plants. Results of feeding trials indicated that the most common sedge (Carex aquatilis) contains a compound(s) deleterious to collared lemmings, that the most common deciduous shrub (Salix pulchra) contains a compound(s) deleterious to brown lemmings, and that the most common aromatic shrub (Ledum palustre) contains a compound(s) deleterious to all three species of microtines. Apparently Lemmus and Dicrostonyx specialize on particular foods and cannot tolerate the secondary compounds in one another's major food items, whereas Microtus acts as a generalist whose food habits overlap with both lemmings. This suggests that competitive interactions between the vole and the lemmings may restrict the distribution of voles.

Our results confirm and extend previous hypotheses about these microtines. They also indicate that the grazing pressure on graminoids exerted by microtine rodents at Atkasook is much less severe than at Barrow but that chronic damage to dicotyledons, particularly willow, is more widespread.