introduction - Pennsylvania Natural Heritage Program

amount of habitat fragmentation, edge characteristics, connectivity between habitat patches anddiversity of habitats found with the landscape.Habitat FragmentationFragmentation of formerly continuous forested landscapes into smaller, more isolated tracts hasan effect on plant and animal composition and structure. The size of a landscape and the way it isperceived varies among the individual species (Pearson et al. 1995). For small or relativelyimmobile species, such as plants or insects, a few hectares of habitat may be sufficient. Whereasother species such as the black bear, Northern Goshawk, bobcat, fisher, and Barred Owl— havelarge individual home ranges and require large expanses of forest to support a viable population.Dramatic declines have been documented across the region in some of these species and in othersthat depend on core forest and large tracts of forest (Yahner 1988, Hansen & Urban 1992,Robinson et al. 1995). Because few large, unfragmented areas of forest remain, those that doremain are especially important as refuges for these species. Pennsylvania has a high proportionof the forest land remaining in the mid-Atlantic states, and thus our forests are critically importantto the regional survival of populations of birds and other forest wildlife (Goodrich et al. 2003,Rosenberg and Wells 1995).Edge EffectAs a forested landscape is fragmented, the amount of forest edge relative to core areas increases.Traditionally, good wildlife management often was synonymous with created edge habitats sincemany “game” species are more abundant near edges. Today, it is recognized that many “nongame”species evolved within extensive areas of unfragmented forest. Consequently, edges maybe detrimental due to the increased presence of predators and non-native species. Forest edgesdiffer in vegetative structure, generally making them less suitable for native species andincreasing the likelihood of success by invasives. The influence of an edge may extend up to300’; therefore, those areas greater than 300’ from an edge are considered “core” forest areas thatoffer better-quality habitat conditions for “forest interior” species such as the Allegheny woodrat,woodland salamanders, Scarlet Tanager, Ovenbird, and Black-throated Blue Warbler. Thepattern of human development has created a landscape in which the majority of forest inPennsylvania is influenced by edge effects, and does not offer “core” conditions, because itborders roads, utility rights-of-way, and other non-forest uses (Goodrich et al. 2003).ConnectivityThe features that fragment natural cover into small tracts are often impassable to wildlife andinterrupt the mechanisms by which plant propagules disperse. When a patch of natural landscapebecomes isolated from other natural habitat, the short- and long- term survival of species withinthat patch are threatened. Many species depend on several habitat types in the course of theirlives, and will immediately decline if isolated from one of the necessary habitats. For example,some species of salamander breed in wetlands but live in upland forest outside of breedingseason. Even where a species can meet its habitat requirements within a patch, isolation threatensthe long-term survival of a population by curtailing opportunities for immigration or emigrationwith neighboring areas. The presence of corridors may facilitate the movement of species acrossboundaries or through inhospitable habitats. These movements across the landscape not only helpto sustain the numbers of a population, but also sustain its genetic viability by exchanging geneticmaterial between populations. Over time, isolated small populations lose genetic diversity, andthus the capacity to respond to change in the environment (Ridley 2003).4