POPULATION CYCLES IN SMALL MAMMALS
32 1
lemmings were thought to bring about their own demise by eating the
vegetation which provided cover and protection while the predators
acted as the agents of mortality. Pitelka (1958) reformulated the food
hypothesis and suggested that the lack of food brought about by high
density lemming populations led to malnutrition and reduced reproduction, and thus a population decline.
To analyze the relationship of microtines to their habitat we will
consider three questions: (1) Are microtines selective in their food
choices? (2) What is the effect of microtine grazing on the habitat?
and (3) Does the quantity or quality of the food supply become limiting
to increasing microtine populations?
1. Selectivity of microtine food habits and habitats
Microtines live in a variety of habitats from woodland (Clethrionomys)
to meadows, grasslands and old fields (Microtus, Symptomys, Pitymys)
and to alpine meadows and tundra (Microtus, Lemmus, Dicrostonyx).
For the grassland and tundra dwellers food and cover are provided by
the same plants.
Various workers have raised the question of the selectivity of
microtines in choosing food plants from their habitat. The preferred
food plants of Microtus pennsylvanicus in Minnesota were found to be
clover and dandelion, neither of which are common in the natural
habitat of this animal (Thompson, 1965). Introduced grasses are
readily eaten by M . ochrogaster, M . pennsylvanicus (Thompson, 1965;
Zimmerman, 1965) and M . californicus (Batzli and Pitelka, 1971).
Therefore, microtines appear to be very catholic in their food habits
and they generally take what is most rea’dily available (Martin, 1956),
often introduced grass species. However, Godfrey (1953) reported that
Helicotrichon pubescens occurred more often in fecal pellets of Microtus
agrestis than would be predicted from its abundance in the habitat,
and Batzli and Pitelka (1971) also had some evidence of food preferences
of M . californicus. Fleharty and Olson (1969) found that, although
availability of food types was an important factor, M . ochrogmter
showed some selectivity based on the growth stage and the palatability
of the food plants. Kalela and Koponen (1971) state that lemmings
show preference in the types of mosses they eat, particularly favoring
those of the genus Dicranum.
Thompson’s studies of food preference of Microtus pennsylvanicus
showed that plants from old field habitats were more acceptable than
those from marshes, tall grass prairie or boreal forest, and the developmental environment of the individual Microtus did not influence its
food preference. Also Thompson found that the quality of the soil
on which a plant was grown did not influence its acceptability to M.
32 1
lemmings were thought to bring about their own demise by eating the
vegetation which provided cover and protection while the predators
acted as the agents of mortality. Pitelka (1958) reformulated the food
hypothesis and suggested that the lack of food brought about by high
density lemming populations led to malnutrition and reduced reproduction, and thus a population decline.
To analyze the relationship of microtines to their habitat we will
consider three questions: (1) Are microtines selective in their food
choices? (2) What is the effect of microtine grazing on the habitat?
and (3) Does the quantity or quality of the food supply become limiting
to increasing microtine populations?
1. Selectivity of microtine food habits and habitats
Microtines live in a variety of habitats from woodland (Clethrionomys)
to meadows, grasslands and old fields (Microtus, Symptomys, Pitymys)
and to alpine meadows and tundra (Microtus, Lemmus, Dicrostonyx).
For the grassland and tundra dwellers food and cover are provided by
the same plants.
Various workers have raised the question of the selectivity of
microtines in choosing food plants from their habitat. The preferred
food plants of Microtus pennsylvanicus in Minnesota were found to be
clover and dandelion, neither of which are common in the natural
habitat of this animal (Thompson, 1965). Introduced grasses are
readily eaten by M . ochrogaster, M . pennsylvanicus (Thompson, 1965;
Zimmerman, 1965) and M . californicus (Batzli and Pitelka, 1971).
Therefore, microtines appear to be very catholic in their food habits
and they generally take what is most rea’dily available (Martin, 1956),
often introduced grass species. However, Godfrey (1953) reported that
Helicotrichon pubescens occurred more often in fecal pellets of Microtus
agrestis than would be predicted from its abundance in the habitat,
and Batzli and Pitelka (1971) also had some evidence of food preferences
of M . californicus. Fleharty and Olson (1969) found that, although
availability of food types was an important factor, M . ochrogmter
showed some selectivity based on the growth stage and the palatability
of the food plants. Kalela and Koponen (1971) state that lemmings
show preference in the types of mosses they eat, particularly favoring
those of the genus Dicranum.
Thompson’s studies of food preference of Microtus pennsylvanicus
showed that plants from old field habitats were more acceptable than
those from marshes, tall grass prairie or boreal forest, and the developmental environment of the individual Microtus did not influence its
food preference. Also Thompson found that the quality of the soil
on which a plant was grown did not influence its acceptability to M.