The microtine rodents Microtus pennsylvanicus and Synaptomys cooperi are sympatric and occur in the same general habitats. Interspecific competition that varies in intensity would be predicted because of similarities in life history characteristics and temporal variation in Microtus density. Microdistribution of Microtus and Synaptomys was monitored during peak, trough, and early increase phases of a Microtus population cycle. Food habits, activity patterns, and behavioral characteristics were also compared.

Field studies at seven sites extended from June 1978 to February 1981. These sites represented a range in habitat suitability for Microtus, as indicated by relative population densities. Undisturbed populations of Microtus and Synaptomys were studied to determine the effects of natural changes in Microtus density on Synaptomys microdistribution. Removal of one species or the other from study grids was used as an experimental tool. Vegetation analysis of study sites included estimates of percentage ground cover and tree/shrub density. Discriminant function analysis was used to determine which of 22 vegetation variables contributed most to distinguishing habitats and microhabitats of the two species.

The dominant plant species in habitats where Microtus and Synaptomys coexisted were Juniperus virginiana and Andropogon scoparius. Even at peak population levels, Microtus occurred in relatively low densities (17-50/ha). At the low point in a natural population cycle, the decline or disappearance of Microtus was accompanied by a microdistributional shift by Synaptomys into areas formerly occupied by Microtus. This shift occurred when differences in daily activity rhythms were maximal. Recolonization by Microtus during the early increase phase of population growth resulted in a return to the initial distributional pattern. Displacement of Synaptomys occurred when both species were primarily nocturnal. Removal of Microtus from an experimental grid was followed by colonization by Synaptomys. When Synaptomys was removed from an experimental grid, the distribution of Microtus did not change. Behavioral interaction studies revealed high levels of avoidance and aggression, suggesting that spatial separation is behaviorally maintained.

Vegetation studies indicated that habitats with heavy ground cover supported permanent Microtus populations and were inaccessible to Synaptomys. Microhabitat partitioning occurred in heterogeneous habitats with sparse ground cover and denser tree canopy. In the presence of Microtus, Synaptomys occupied microhabitats with higher densities of deciduous trees and shrubs, but shifted to more open microhabitats when Microtus was absent.

Food habits analysis indicated that Synaptomys diet consists of vegetation that is low in nutrient content (Andropogon in summer, moss in winter). While Microtus will eat Andropogon (especially in winter), this species also included dicots and other monocots in its diet. Synaptomys diet was more diverse when living on a grid from which Microtus had been removed.

These findings suggest that in the southern Appalachians, Synaptomys is excluded from preferred habitats by Microtus and, as a result, lives where cover is sparse and food is low in nutritional value. Competition is relaxed when Microtus declines to trough population density. The combination of wider habitat tolerances by Synaptomys and temporal variation in intensity of competition allows coexistence on a regional basis.