THE EFFECTS OF HABITAT FRAGMENTATION ON THE GENETIC-STRUCTURE OF SMALL MAMMAL POPULATIONS

We present five case studies highlighting the effects of habitat fragm entation on the genetic structure of small mammal populations. The stu dies reflect different spatial scales and components of genetic variat ion, In marginal and central populations of Sigmodon hispidus we found less allozymic variation within the marginal population, whereas patt erns 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 Evergla des, This observation contrasts with insular vole populations (Microtu s spp,), where isolation on islands results in genetic structuring, Te mporal changes in abundance in mainland populations had no effects on genetic differentiation (F(ST)values) because subpopulations did not e xperience bottlenecks, In an experimentally fragmented landscape, frag mentation influenced demographic processes but not genetic structure, We conclude that (1) with extreme fragmentation, small mammal populati ons become depauperate of genetic variation and differentiate genetica lly; (2) different components of genetic variation lead to different g enetic structuring; (3) spatial and temporal scales should both be con sidered when examining genetic structure of populations; (4) demograph ic and ecological processes are more likely influenced by fragmentatio n than genetic structure; and (5) there is an interaction between demo graphic processes and genetic structure.