patterns of coexistence in synaptomys cooperi and microtus pennsylvanicus
The microtine rodents Synaptomys cooperi and Microtus pennsylvanicus are sympatric, have a similar life styles, and occur in the same general habitats. The object of this study was to determine the importance of competition in a Microtus-Synaptomys system in southwestern Virginia as a test of the general hypothesis that herbivorous rodents compete directly for space and indirectly for food. The following patterns were observed: (1) Undisturbed populations of Microtus and Synaptomys exhibited microhabitat partitioning when coexisting in heterogeneous habitats that were marginal for Microtus. (2) In the presence of Microtus, Synaptomys microhabitats were characterized by significantly higher densities of deciduous trees and shrubs. (3) Disappearance of Microtus from some habitats during a population decline was accompanied by a shift of Synaptomys into areas formerly occupied by Microtus. Recolonization by Microtus resulted in a return to the previous pattern of microhabitat segregation. (4) Removal of Microtus from an experimental grid was followed by immediate colonization by Synaptomys. (5) One year following removal of Synaptomys from an experimental grid, most of the former Synaptomys habitat remained empty even though Microtus numbers were increasing. (6) Fecal analysis indicated that Synaptomys diet consisted largely of vegetation that is low digestible nutrients (Andropogon in summer, moss in winter). Although Microtus ate Andropogon, this species included more dicotyledons and other monocotyledons in its diet. (7) When Synaptomys was living on a grid from which Microtus had been removed, similarity to food habits of Microtus previously living on the removal grid was much greater than on a grid where the two species coexisted in separate microhabitats (72 vs. 37%). These results 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. Although these species do compete, this competition is relaxed when Microtus populations decline. When given access to preferred habitats the food habits of Synaptomys change and resemble those of Microtus, suggesting that the ultimate limiting resource is food. The combination of wider habitat tolerances by Synaptomys and temporal variation in intensity of competition allows coexistence of these species on a regional basis.