Demographic and genetic response of woodland amphibians to landscape change in southern Connecticut.

摘要

A critical question in conservation biology concerns how changes in land-use patterns affect the persistence of plant and animal populations and the distribution of genetic variation within them. This dissertation focused on woodland amphibians occupying forested habitats arrayed along an urban-rural gradient in southern Connecticut for examining these issues. My objectives were: (1) to identify determinants of the structure of the forest mosaic, the arena within which demographic and genetic processes in woodland amphibians operated; (2) to examine whether amphibian dispersal occurred randomly with respect to several prominent, natural and anthropogenic features of the landscape; (3) to determine thresholds of sensitivity to habitat fragmentation for different species of woodland amphibians; and (4) to test several hypotheses about microevolution in amphibian populations subjected to induced subdivision.; Distinct trends in the structure and dynamics of forested lands, and hence the habitats available to woodland amphibians, occurred along a spatially continuous urban-rural gradient. Primary determinants of forest occurrence in 1990 included slope and drainage attributes of underlying soils, proximity to transportation corridors and urban centers, and historical 'common land' designations. Amphibian movements, measured from captures in strategically placed pit-fall traps, were generally facilitated by stream beds, hindered by roads, and variously affected by ecotones between forested lands and open lands. Notably, the most dispersive species with bi-phasic life cycles (Rana sylvatica and Notophthalmus viridescens) were the least tolerant of habitat fragmentation, whereas the least dispersive species with direct development (Plethodon cinereus) was the most tolerant. Tolerances to habitat fragmentation could be largely predicted with a spatially explicit population model. Genetic differentiation, measured with molecular genetic markers and morph frequencies in the redback salamander (Plethodon cinereus), was greater among fragmented subpopulations than among contiguous subpopulations. Among fragmented populations, differentiation was greater among those lacking historical forest connections than among those retaining such connections. Furthermore, genetic divergence was correlated with geographic distance in a population occupying continuous forest, whereas no such pattern was evident in a population occupying fragmented forests. Notably, overall levels of genetic diversity appeared to be affected little by population fragmentation.