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Implications of Habitat Fragmentation on Vertebrate Species

This section reviews the literature on habitat fragmentation and the resulting influence on the species that inhabit those landscapes. Two additional Chapters of the Assessment examine fragmentation in the South. Chapter SOCIO-1 presents an analysis of southern locations using remotely sensed imagery. In addition, Chapter TERRA-3 examines the influence of roads and powerlines on habitat fragmentation.

The definition of fragmentation

The term “fragmentation” is often used to refer to the insularization of habitat on a landscape. The change in arrangement of existing habitats is often accompanied by a loss of habitat area. A landscape may cover hundreds of square miles or a much smaller area. The definition depends on the context of its use and is shaped by the scale at which ecological processes are discussed (Trani 2002).

Fragmentation may occur when a forested landscape is subdivided into patches. Fragmentation may also occur when numerous openings for such things as fields, roads, and powerlines interrupt a continuous forest canopy. It also can refer to discontinuities of vegetation in the landscape. Wetland habitat can become fragmented when portions are drained for urban development, while prairie habitat can become fragmented by agricultural development. The resulting landscape pattern alters habitat connectivity and edge characteristics, influencing a variety of species.

Factors that contribute to landscape fragmentation

Landscape fragmentation may result from natural processes such as hurricanes, wildfires, and floods. Landscape fragmentation may also occur in association with land-use conversion for urban development, agricultural use, and timber harvesting. The ecological consequences of natural or human-caused fragmentation differ depending on the pattern imposed by these factors.

Landscape modification has occurred for thousands of years. Native inhabitants modified landscapes by burning and clearing forested areas. The first European settlers divided vast forests into farmlands and settlements. This trend continues today. Much of the southern landscape is under intensive management and is becoming an increasingly complex mosaic of forest, urban, and agricultural areas.

Timber harvesting may fragment the landscape, depending on the number, size, and arrangement of harvest units (Trani 1996). Higher levels of fragmentation occur when small, numerous harvest units are dispersed over the landscape than when units are clustered. A dispersed harvest scheme increases spatial heterogeneity, patchiness, and forest edge length. However, the changes in pattern resulting from timber harvest are often temporary because, the harvested area regenerates and reverts to forest. The rate of succession depends on the composition of the residual stand, browsing by herbivores, subsequent management activities, weather, and other disturbances (Wigley and Roberts 1994).

It is important to note that a forested landscape supporting a mosaic of different seral stages is not ecologically the same as a landscape containing isolated forested patches surrounded by agricultural or urban areas. Each seral stage provides habitat that varies in suitability for a particular species as it moves through the forested landscape.

Roads may contribute to forest fragmentation when their placement divides large landscapes into smaller patches and interior forest habitat is converted into edge habitat. As road density increases, the populations of some species may become isolated (Chapter TERRA-3). Roads located along the periphery of a landscape have the least influence on the resulting pattern (Trani 1996). The influence of roads on habitat fragmentation varies with road width and degree of permanence. A six-lane interstate highway has a greater effect on landscape pattern than does a 20-foot forest road. Some roads, such as unimproved dirt roads, may be temporary, while others are paved and quite permanent.

Influence of landscape fragmentation upon terrestrial species

Harris (1988) cited fragmentation as the most serious threat to biological diversity in the nation. Area-sensitive species requiring large tracts of habitat may decline or be extirpated locally. The movement of species between patches may be inhibited. Population persistence may be linked to the number, size, and degree of isolation of forest patches (Robbins and others 1989).

The influence of fragmentation on the landscape can be associated with three related factors: patchiness, edge, and connectivity.

Patchiness. Changes in patch size have been recognized as a major component of fragmentation. Species richness may decline as patch area is reduced (Ambuel and Temple 1983; Lynch and Whigham 1984; Askins and others 1990). Small remnant patches of forest surrounded by open areas constitute unfavorable habitat for many species; these remnants also have increased susceptibility to windthrow disturbance and other processes. Robinson and Wilcove (1994) suggested that fragmented landscapes become population sinks that are only sustained by immigration from nearby forest tracts that are large enough to produce a surplus of individuals.

Matthiae and Stearns (1981) found that the density of red squirrel, gray squirrel, raccoon, and red fox increased with habitat patch size. Fahrig and Merriam (1985) also reported that certain mammals were more common in large forest tracts than in smaller, isolated patches. Populations of white-footed mice and chipmunks in small forest patches declined to a point that local extirpations occurred.

Rosenberg and Raphael (1986) reported that gray foxes, ringtail cats, and northern flying squirrels were sensitive to forest fragmentation. Picton (1979) found that the presence of large mammals was correlated with the size of the mountain ranges where each species occurs. Mammal population can increase when minimum habitat size requirements are met. The insularity of populations increase with continued landscape fragmentation while larger, undeveloped areas protected these species from extinction.

Roads may or may not act as barriers to the movement of species between habitat patches. Extensive networks of roads have negative impacts on black bears, white-tailed deer, and Florida panthers (Chapter TERRA-3). These negative impacts stem from loss of habitat, increased hunter accessibility, and vehicular mortality.

Long-term population declines have been observed for neotropical migrants inhabiting small forest patches. Breeding bird censuses for isolated forest patches indicate general reductions in abundance and diversity of species over the past several years (Lynch and Whitcomb 1977). Critical information for the conservation of bird species includes understanding of the relationship between reproductive success and habitat size and quality. The dependence of many breeding songbirds on large blocks of forest is well established (Whitcomb and others 1981, Robbins and others 1989).

Species sensitive to patch size tend to be highly migratory, are forest-interior specialists, build open nests, and/or nest on the ground (Whitcomb and others 1981). The worm-eating warbler, the hooded warbler, and the black-and-white warbler are generally absent in patches <50 acres (Hamel 1992). Other species that are sensitive to patch size include the swallow-tailed kite, broad-winged hawk, barred owl, pileated woodpecker, and black-billed cuckoo (Hamel 1992). While many species avoid small patches, widespread permanent residents and short-distance migrants tend to predominate in small patches (Askins and others 1990).

Habitat isolation has been associated with population declines in large snakes due to increasing networks of roads (Gibbons and Buhlmann 2001). These networks divide forested habitat into smaller and smaller parcels. Likewise, amphibian mortality is intensified when a heavily traveled road separates individuals from the forest they live in and the wetland they require for breeding.

Edge. An edge is the place where two different plant communities, successional stages, or land uses come together. Fragmentation can increase the amount of edge habitat in a landscape. Inherent edges are caused by changes in soil type or topography, whereas induced edges are those created by disturbance. Induced edges can be created by land uses, including cultivation, fertilization, and harvest, and by environmental disturbances such as fires, blowdowns, and floods.

The creation of forest edge influences seedling establishment and vegetative composition. For some species, these effects persist hundreds of yards into the forest interior (Chen and others 1992). For example the edge habitat may serve as an access point, attracting cowbirds into the interior of a forested landscape (Askins 1994).

Many species occur in edge habitat, particularly those that use one habitat for food and another for cover. Game birds, such as the American woodcock and northern bobwhite, occur in edge habitats. Many species in urban and agricultural landscapes are edge-adapted. Many woodland passerines favor edge habitat (Yahner and Scott 1988), which may provide enhanced forage and/or improved habitat conditions.

In contrast, excessive edge may lead to reduced populations of species dependent on large blocks of forest interior (Robbins and others 1989). Species that use continuous mature forest may be replaced by generalist species. Southern breeding birds that nest only in the interior of forests include the sharp-shinned hawk, Cooper's hawk, hairy woodpecker, winter wren, and veery (Hamel 1992). Edge can negatively affect these species, particularly in patches with large perimeter-to-area ratios (Noss 1983).

An increase in density of forest-edge and farmland species along edges may exclude certain interior and long-distance migrant species. Competition by the edge-adapted starling exerts a direct negative impact on many forest species (Harris 1988). This competition may influence bird community composition more than area-dependent changes in habitat (Ambuel and Temple 1983).

Species that occur in edge habitats are subject to high rates of mortality from predators attracted to these habitats. The raccoon, least weasel, and striped skunk often hunt for small mammals along edges. Ground nests receive predation pressure where mammals and reptiles are the dominant predators (Chasko and Gates 1982). Predation reduces the recruitment of the Kentucky warbler, scarlet tanager, wood thrush, yellow-throated vireo, and ovenbird (Temple and Cary 1988). Increases in edge density contribute to the escalation of nest predation and parasitism to levels that can bring reproductive success below replacement rates.

Nest parasitism by cowbird species may be an important factor in the decline of some breeding birds. Brood parasites lay their eggs in the nests of other species, reducing the reproductive success of their hosts. The brown-headed cowbird may have contributed to the population declines of the Acadian flycatcher, veery, American redstart, and Louisiana waterthrush (Brittingham and Temple 1983).

Connectivity. Connectivity, the degree of continuity of a landscape, is also affected by fragmentation. Connectivity may facilitate dispersal and improve habitat quality by connecting patches of habitat. It has been suggested that the population dynamics of species are affected by the spatial pattern of fragmentation (Hanski 1991, Haddad and others 2000). There is disagreement, however, on the value of corridors for the conservation of biological diversity. One view is that populations linked by corridors are vulnerable to the spread of disease and several environmental stressors (Gilpin 1987, Quinn and Hastings 1987). If corridors spread the risk of environmental stress among isolated populations, persistence time may actually be longer in fragmented landscapes (Fahrig and Paloheimo 1988).

Another view suggests that species persistence is lower in fragmented habitats than in contiguous habitats (Tilman and others 1994). These studies suggest that corridors are valuable as a conservation tool. This point of view is discussed further below.

Heany and Patterson (1986) presented an extensive review of the regional patterns of mammal distribution as affected by habitat connectivity. Pelton (1986) described how the loss of connectivity restricts the distribution of black bears. When disturbance causes local extirpation, populations may be reestablished through the dispersal of individuals from source populations. Jackson (1987) reported corridors aided red-cockaded woodpeckers in colonizing existing habitat Forest birds can often use small tracts of forest connected to large tracts by wooded corridors (Robbins 1979). Forest interior birds and small mammals (Merriam 1990) persist in forest fragments connected by woodland corridors that ease colonization.

Species that are able to move between connected habitat patches operate demographically as a metapopulation. Corridors may permit the survival of extinction-prone populations through the immigration of individuals. Corridors also may facilitate movement of an individual within its home range. Such movement may be particularly important for species whose home range area requirements exceed the average patch size. For example, Rosenburg and others (1997) reported that migratory amphibians, such as red-spotted newts, may require corridors among seasonally used habitats. The loss of connectivity may cause local extirpation. Many amphibian and reptile species cannot move through relatively large, deforested areas to reach other suitable forest habitat. Where declines of herpetofaunal populations occur, population sizes will not be rebuilt quickly in a fragmented landscape (Gibbons and Buhlmann 2001).