Microtus pennsylvanicus and M. ochrogaster are sympatric in southern Indiana grasslands. From June 1965 to August 1967 four populations were lived trapped, three of them in 0.8—hectare (2—acre) outdoor pens. Both species increased during 1965 and reached peak densities in summer 1966. Microtus ochrogaster declined abruptly that fall and remained low; M. pennsylvanicus declined the following spring. One of the fenced populations increased to a density about three times that of its unfenced control. By early fall 1966 it had nearly destroyed its food resources and then suffered a severe decline associated with obvious overgrazing and starvation. No such overgrazing has been seen on any unfenced grasslands in this area. Dispersal is probably necessary for normal population regulation in these voles, since fenced populations seem unable to regulate their density below the limit set by starvation. Both species bred extensively in the winter of 1965—66 during the phase of population increase. There was little or no breeding during the winter after the peak. Survival of females in the trappable population of both species was high and relatively constant until the end of the cycle. In males, periods of low survival punctuated the increase and peak phases, and these periods of low male survival did not occur at the same time in the two Microtus species. Some mortality processes are thus highly specific for sex and species. In the fenced populations survival rates were very high and no sporadic male losses occurred. Increasing and peak populations of M. pennsylvanicus and M. ochrogaster are characterized by adults of large body size. During the increase and peak phases some voles stopped growing at low weights (30—40 g) while others reached high asymptotic weights (45—55 g). The demography of these Microtus species in southern Indiana is similar to that of other cycle voles and lemmings in temperate and arctic areas.

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We present five case studies highlighting the effects of habitat fragmentation on the genetic structure of small mammal populations. The studies reflect different spatial scales and components of genetic variation. In marginal and central populations of Sigmodon hispidus we found less allozymic variation within the marginal population, whereas patterns of morphological variability were the converse. In the rice rat (Oryzomys spp.), nucleotide diversity in mtDNA was similar in an island population in the Florida Keys to mainland populations in the Everglades. This observation contrasts with insular vole populations (Microtus spp.), where isolation on islands results in genetic structuring. Temporal changes in abundance in mainland populations had no effects on genetic differentiation (FST values) because subpopulations did not experience bottlenecks. In an experimentally fragmented landscape, fragmentation influenced demographic processes but not genetic structure. We conclude that (1) with extreme fragmentation, small mammal populations become depauperate of genetic variation and differentiate genetically; (2) different components of genetic variation lead to different genetic structuring; (3) spatial and temporal scales should both be considered when examining genetic structure of populations; (4) demographic and ecological processes are more likely influenced by fragmentation than genetic structure; and (5) there is an interaction between demographic processes and genetic structure.

The beach vole, Microtus breweri, was trapped monthly on Muskeget Island, Massachusetts, from May 1972 until June 1975. Populations of the meadow vole, Microtus pennsylvanicus, were trapped in Barnstable and Middleborough counties, Massachusetts, during the same time. Three of the four meadow vole populations showed typical cycles, whereas the beach vole showed only annual changes in density. The beach vole had a sex ratio weighted towards males (1.2 : 1), whereas the meadow vole showed a 1: 1 sex ratio. Microtus pennsylvanicus, as compared to M. breweri, had a larger litter size (4.5 versus 3.4), a higher pregnancy rate (0.60 versus 0.46), winter pregnancy, a 30 percent lower weight at sexual maturity, and lower pre-implantation (0.3 ova versus 0.5) and post-implantation (0.1 embryos versus 0.3) mortality. These parameters, plus the sex ratio, indicate that M. breweri is K-selected. This is a result of their remaining at high densities virtually all the time.

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