Evolutionary diversity and ecology of endemic small mammals of southeastern Alaska with implications for land management planning.

摘要

The dynamic geological history and naturally fragmented landscapes of southeastern Alaska create an environment with a high potential for endemism. The temperate rainforest of the region regenerates and develops slowly, and old-forest characteristics do not appear until >=300 years following disturbance. The challenges of managing forest resources are intensified in island archipelagos because of the increased sensitivity of indigenous biota to disturbance and higher rates of extinction, especially among endemic organisms. Early expeditions of the large islands of the Alexander Archipelago (~1% of all named islands) documented 27 endemic mammalian taxa. More recent studies with modern techniques found that some reputed endemics showed nominal levels of genetic divergence from other conspecific populations, but more divergence existed among several taxa than was reflected in the current taxonomy. Furthermore, the mammal fauna of southeastern Alaska has a nested structure with complex phylogeographic patterns suggesting multiple colonization events. Of eight taxa examined through phylogeographic analyses, five species showed acute genetic variation and divergence in mitochondrial sequences. Four species were comprised of coastal and continental clades (i.e. populations of recent common descent), whereas the fifth species showed a third clade. Conversely, the northern flying squirrel (Glaucomys sabrinus) and southern red-backed vole (Clethrionomys gapperi) were represented by relatively shallow divergent lineages. Still, G. sabrinus showed a distinct mitochondrial lineage on 11 islands (Prince of Wales Island complex), which exhibited severely reduced genetic variation. Moreover, flying squirrels in southeastern Alaska are genetically distinct from populations in the Pacific Northwest; as different as each is from the southern flying squirrel (G. volans). Recent ecological studies of endemic populations of the northern flying squirrel and the southern red-backed vole (reputed old-growth associates) suggest that risk of extirpation in managed landscapes is likely less than was presumed during recent land management planning in the region because: (1) abundant noncommercial forests apparently contribute to breeding populations of northern flying squirrels, which appear to have a more general lifestyle than populations in the Pacific Northwest; and (2) red-backed vole populations may be able to exist in managed young-growth stands that originated from clearcut logging. Still, there are essential questions for both species regarding the influence of annual population fluctuations on habitat distribution and population demography, stand and landscape features that restrict dispersal, and vegetative and structural characteristics of second-growth stands that will sustain breeding populations of both species.