We studied the influence of preferred food, forbs, and vegetative cover on survival and reproduction at different population densities of Microtus ochrogaster and M. pennsylvanicus in alfalfa, bluegrass, and tallgrass prairie habitats in east-central Illinois for 25 years. Population densities of M. ochrogaster were greatest in alfalfa, least in tallgrass, and intermediate in bluegrass, whereas those of M. pennsylvanicus were greatest in tallgrass, least in alfalfa, and intermediate in bluegrass. For both species, preferred food availability was greatest in alfalfa, intermediate in bluegrass, and least in tallgrass. Vegetative cover was relatively sparse in alfalfa, especially in winter, and dense throughout the year in bluegrass and tallgrass. Variation in survival emerged as the most important factor explaining population differences between the 2 species. Reproduction had little differential impact on abundance of either species in any of the 3 habitats. Survival of M. ochrogaster was higher in alfalfa than in bluegrass or tallgrass; survival of M. pennsylvanicus was higher in tallgrass than in alfalfa or bluegrass. Differential survival among habitats and between species was influenced primarily by amount of vegetative cover. We suggest that M. ochrogaster is less susceptible than M. pennsylvanicus to predation by raptors and large carnivores (predators that hunt from above vegetative cover), whereas M. pennsylvanicus is less susceptible than M. ochrogaster to snakes and small carnivores (predators that hunt under vegetative cover).
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