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Experimental studies of competition among four species of voles Hawes, David Bruce


The competitive interactions among four species of voles, Clethrionomys gapperi, Microtus oregoni, M. longicaudus, and M. townsendii (Cricetidae: Rodentia), were studied in the vicinity of Vancouver, B.C., Canada. Species occurrences in five vegetationally distinct habitats were ascertained by trapline sampling. Microtus oregoni occurred ubiquitously, whereas the other three species were restricted to one or two habitats and did not overlap with each other. Although species occurrences were predictable on the basis of subjectively defined habitats, they were not predictable on the basis of various environmental parameters. Overlapping or contiguous populations of voles were studied by means of mark and recapture methods and experimentation. M. longicaudus could not be induced to utilize atypical, but apparently suitable, habitat by experimental removal of M. oregoni from that habitat. The overlap between C. gapperi and M. oregoni was found to have a seasonal component, being greatest during the non-breeding season. Microtus oregoni and M. townsendii were found to significantly affect each other, and this effect was shown to be a function of substrate topography. Experimental introduction of M. townsendii into an area previously occupied only by M. oregoni caused reductions both in the population size of M. oregoni and in its range of utilized habitat. Subsequent to the introduction of M. townsendii, the M. oregoni population was excluded from flat habitat and restricted to elevated areas. A survey of 14 different sites disclosed that M. oregoni and H. townsendii populations were negatively correlated and that also the height of the plot above the winter water table affected species composition. Predation by hawks and owls did not explain coexistence of these two vole species. Apparent character displacement of M. oregoni populations was observed: individual body weights in M. oregoni populations sympatric with M. townsendii were significantly smaller than those in other populations of M. oregoni. These results indicate that M. townsendii competes with M. oregoni by interference competition, while M. oregoni appears to escape this interference by burrowing. A similar relationship is hypothesized between M. oregoni and M. Iongicaudus. Additionally, specializations of each vole species and limitations imposed by these specializations are discussed. The relevance of these specific findings is discussed in relation to the distribution of microtine rodents in North America.

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