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Invasive Species
Invasive exotic species are non-native species that disrupt naturally occurring
plant and animal communities. Within their natural range, population numbers of
most plants and animals are limited by competition, food resources, predation,
and disease. Introduced to new areas outside their range, species may be freed
from those constraints. With no natural competitors or predators some introduced
species out-compete native plants and animals for space, food, or water. Others
can fundamentally alter natural fire patterns and other ecological processes,
making it difficult or impossible for native species to survive (Defenders of
Wildlife 1998).
Invasive exotic species thrive in disturbed sites and are a particular problem in rapidly developing areas. When human activity destroys, fragments, or degrades a habitat site, the physical and chemical characteristics of the area is altered. This change may allow invasive species to establish themselves and crowd out native species. In the canyons of San Diego, invasive plants and animals, such as hottentot fig, pampas grass, and house mice, from the surrounding urban areas have taken over most of the native chaparral and coastal sage scrub habitats (Alberts et al. 1993, Cox 1999).
Invasive birds commonly thrive in disturbed urban settings. In the 1890s, one man released 100 European starlings in New York City because of his interest in birds mentioned in Shakespeare's works. Over 200 million European starlings now live in North America, thriving in urban and suburban areas where they out-compete other cavity-nesters, such as woodpeckers and tree swallows. Other birds introduced to the urban areas of North America, such as the domestic pigeon, house sparrow, and mute swan, have had similar devastating effects on populations of native bird species (Cox 1999).
Most highways around the United States are lush with non-native vegetation. Automobiles contribute to the spread of invasive species by carrying seeds attached to their undercarriage and tires and depositing them miles away. Along the West Coast, French, Scotch and Spanish broom are prevalent along highways and disturbed areas and are invading chapparal and woodland habitat.
Want to read more about invasive species? Go to Defenders' main invasive species section.
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.
Sprawl encourages the growth and expansion of invasive exotic species.
Sprawl increases the amount of impervious surfaces such as roads, parking lots, sidewalks, and rooftops.
The paving over of wetlands, grasslands, forests and other sensitive areas have resulted in many poorly functioning ecosystems.
Sprawl extends pollution into areas where environmental quality may not yet have been severely compromised.
