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Home > Programs and Policy > Habitat Conservation > Habitat Conservation Basics > Habitat and Sprawl > Threats to Wildlife

Impervious Surfaces and Roads

Sprawling development invariably increases the amount of impervious surfaces, such as roads, parking lots, sidewalks, and rooftops, on the landscape. In a watershed without paved surfaces, water that falls on the landscape during a rain event can infiltrate the soil and is either absorbed by vegetation or slowly makes it way into the stream and river system. However, when a watershed has a significant amount of impervious surface, water cannot penetrate these surfaces, and large volumes of water flow quickly into streams and rivers rather than permeating the soil and recharging groundwater levels (Riley 1998).

This rerouting of water substantially changes the natural hydrologic regime of a watershed. With less groundwater and more surface runoff, streams and rivers in developed areas often experience extreme fluctuations in water levels. During rain events, water levels are higher than normal as water reaches rivers and lakes faster and in greater volumes than normal. During dry seasons, water levels in developed watersheds are often lower than normal since the impervious surfaces prevents water infiltration and groundwater recharge. In essence, urbanized watersheds often have more frequent occurrences of both floods and droughts in part due to the amount of impervious surface in these systems (Riley 1998).

Such hydrologic changes can significantly impact wildlife habitat such that many species have difficulty surviving or can no longer survive there. Many species are adapted to a very specific hydrologic regime; they require specific levels of water at certain times. Too much or too little water at the wrong time can threaten the survival of many species by washing out habitat, including prime nesting areas, increasing water velocity and erosion, altering water clarity and temperature, and diminishing food supply. For example, increased flooding and drought due to human modification of the hydrologic regime have led to massive population declines of many species in the Everglades and brought the ecosystem almost to the brink of collapse (Douglas 1988).

Increased runoff due to impervious surface can also degrade water quality. As water flows over paved surfaces, it may pick up pollutants, such as oils, fertilizers, and other toxic chemicals, high levels of nutrients, debris, and pathogens and carry them unfiltrated into creeks, rivers, and valuable estuary systems, such as the Chesapeake and San Francisco bays. Polluted runoff can sometimes cause eutrophication, through which the increased nutrients alter the chemical composition of the water and deprive aquatic species of valuable oxygen supplies. Impervious surface runoff may also cause warmer water temperatures and a reduction in water clarity. In watersheds with less impervious surface, vegetation serves to filter and slow down the water before it enters the stream and river system. Populations of fish and other aquatic species can be severely damaged by nearby urban centers, as well as those miles upstream. In fact, a higher percentage of freshwater species are threatened with extinction than any other group of species in the United States (Stein et al. 2000). For a more detailed description of the environmental effects of impervious surfaces, click here.

Roads


Roads are an inevitable part of development. They are estimated to affect at least 20% of the U.S. landscape (Forman 2000). Sprawling communities are dependent on automobiles and roads for commuting and access to services. This on-going demand for roads compounds the pattern of sprawl and leads to more vehicle travel and even more roads built, which cause many problems for wildlife.

Roads impact wildlife directly through roadkill and behavior modification and indirectly by destroying, fragmenting, and degrading habitat. For many species, it is difficult to carry out their life cycle without venturing across a road. Mortality from collisions with vehicles is the greatest directly human-caused source of wildlife mortality in the U.S. Even the presence of a road may alter an animal's behavior, such as its movement, dispersal, and migration patterns and reproductive and feeding behavior. Also, the construction of a new road destroys the habitat it has replaced and slices the surrounding habitat into fragments.

After the road is built, habitat is destroyed and degraded in the following major ways:

    By altering the physical environment, such as increasing temperature and light and redirecting water flow;

    By altering the chemical environment, such as adding dioxins, hydrocarbons, and gasoline additives to the environment;

    By spreading exotic species; and

    By allowing increased human use of surrounding areas. (Trombulak and Frissell 2000).

For more information, go to Defenders of Wildlife's Habitats & Highways campaign.

Threats to Wildlife
More than one-third of wildlife species in the United States are considered in danger of extinction. The main threat to wildlife is habitat loss and fragmentation.
Invasive Species
Sprawl encourages the growth and expansion of invasive exotic species.
Impervious Surfaces
Sprawl increases the amount of impervious surfaces such as roads, parking lots, sidewalks, and rooftops.
Natural Cycle Disruption
The paving over of wetlands, grasslands, forests and other sensitive areas have resulted in many poorly functioning ecosystems.
Pollution
Sprawl extends pollution into areas where environmental quality may not yet have been severely compromised.