Sprawl Threatens Wildlife and Habitat

More than one-third of the known species in the United States are considered in danger of extinction (Stein et al. 2000). The main threat to these species, and biodiversity in general, is habitat loss and fragmentation. While habitat can be consumed and altered in numerous ways, poorly planned development and unmanaged growth, or sprawl, is one of the major contributors. In a recent California study, sprawl was found to be the leading cause of species imperilment (National Wildlife Federation 2001).

Sprawl, especially through the building of impervious surfaces and roads, destroys and fragments habitat and disrupts ecological processes. Invasive species thrive and pollution increases in these disturbed environments, causing numerous additional problems for species and their habitat.

Sprawl has been devouring land and habitat at an alarming pace. The rate of sprawl in the United States almost quadrupled between 1954 and 1997 and doubled between 1992 and 1997. About 3 million acres (roughly the size of Connecticut) of mainly forestland, pastureland, rangeland, and crop land are converted to urbanized landscapes annually (U.S. Department of Agriculture 1997).

The rate of development in the U.S. doubled in the last decade with some 3 million acres of farmland, forestland, and wildlife habitat now converted annually-- an area the size of Connecticut (Environmental Protection Agency 2001). Currently, the rate of land consumption outpaces population growth. For example, Chicago suburbs experienced a 46% increase in land consumption during the last decade while only increasing population by 4% (Benfield et al. 1999). And once converted, these urbanized landscapes no longer provide the same options for wildlife, thus displacing or eliminating many species, especially those that depend upon large blocks of forest, shrubland or grassland.

Habitat Loss

Studies show that habitat loss is the leading threat to endangered and extremely rare species. In a study of 1,207 rare U.S. species that suffer from habitat loss, 35% are threatened by commercial development and 38% by agricultural development (Wilcove et al. 2000). Since most agricultural development occurred years ago, commercial development is now the most immediate threat.

Habitat loss concerns:

Lowland habitat. In the U.S., the most biologically diverse areas occur in low elevation bottomlands, the same places where most major cities are located. Unfortunately, biodiversity has been much better protected at the high elevations than the lower elevations (Scott et al. 2001). Habitat loss in the lowlands has severed connections between populations, creating isolated habitat islands for species that would otherwise move through valleys (Shafer 1990, Saunders et al. 1991). In addition, road mortality and human-species conflicts occur when developed areas block the paths of animals looking to disperse, migrate, and locate suitable habitat.

Wetlands. Many species depend upon wetland habitat to support some stage in their life cycle. At the same time, human demand to develop these areas is high. More than 50% of the wetlands in the U.S. have been drained and filled (Environmental Protection Agency 2001).

Agricultural lands. Farms, if managed properly, can provide wildlife habitat for some species, but many populations of species historically associated with native prairie or forests that are now converted to agriculture have declined dramatically. In fact, grassland birds are considered the most threatened group of birds in the U.S. because of habitat loss (U.S. Geological Survey 1998).

At the beginning of the 20th century the most significant form of habitat loss was logging and farmland conversion. Although more of the U.S. landscape has been converted to agriculture than to development, today habitat lost to development is the more pressing problem. Currently, not much new farmland is being created. For more information about agricultural issues, go to American Farmland Trust

Habitat Fragmentation

Sprawling development, such as roads and subdivisions, scatters and disperses throughout large blocks of habitat. This uncontrolled development pattern chops the landscape into smaller, disconnected pieces that cannot sustain healthy wildlife populations. Species dependent on habitat interiors or with large habitat area requirements are especially vulnerable to fragmentation. In general, fragmentation reduces the number of species (species diversity) found in the habitat patch (Bessinger 1982). Only certain species, such as those that are adapted to habitat edges or dependent upon human activity, are able to persist in these fragmented habitats.

Interior habitat decreases. When a habitat is fragmented, the amount of edge habitat -- the zone along the boundary of a habitat -- increases while the amount of interior core habitat decreases. Species dependent on interior habitat suffer, while edge dependent species, including invasive species and predators, thrive. In particular, forests that are fragmented are less likely to successfully provide the food, cover, or reproduction needs of interior forest species. Predators, such as crows and raccoons, and nest parasites, like the brown-headed cowbird (which lays eggs in other birds' nests for the host bird to nurture), find target nests more easily in edge habitats (Askins 2000).

Plant communities are altered. The increase of edge habitat impacts the habitat's microclimate, including light, soil, temperature, moisture, and wind conditions, which, in turn, alters the composition of plant communities. Because different plant communities support different collections of wildlife and rare plant species, changes in habitat also shift and displace wildlife. These changes disrupt the pollination mechanisms plants depend upon, and without animal pollinators or seed dispersers, plant communities slowly lose species (Buchmann and Nabhan 1996). While small populations of plants may persist for some time, the community in its entirety may not function as it did without its former wildlife and area.

Large predators suffer. Fragmentation also prevents species with large habitat requirements, such as top-level carnivores, from remaining in the ecosystem. Without large predators, the number of smaller or mid-level predators (e.g. coyotes, raccoons, opossums, skunks, squirrels) increases with a damaging effect on prey populations. For example, the recent explosion of coyote populations in the eastern U.S. is, in part, due to increased fragmentation of the once largely intact forests. This habitat alteration contributed to the disappearance of the gray wolf, a top-level predator and historically one of the coyote's main competitors, and allowed the coyote to spread beyond its historic range.