Habitat destruction, fragmentation, and degradation continue at a rapid pace in the United States, mostly outside parks. Development fragments habitat, creating barriers to wildlife movement and complicating ecosystem management at the regional and landscape levels. Scientific evidence shows that no park ecosystem can remain healthy as an isolated refuge in a sea of development, yet parks—even large ones—are becoming surrounded.

   Fragmentation can expose the interiors of remaining habitat to edge effect, which benefits some species but harms many others. For example, when large forests are fragmented into small pieces, songbirds such as orioles, tanagers, and warblers that nest deep within woodlands are forced to nest near the edge of open lands. On the edges, these birds are more likely to encounter brown-headed cowbirds, which lay their eggs in other birds' nests, leaving their young to be raised by the unsuspecting parents. Upon hatching, the young cowbirds are larger than the host's young, grow more rapidly, and aggressively out-compete them for nourishment and space.

   Human population growth places incredible stress on ecosystems. The global population is likely to climb by about 66 percent in the 21st century to 10 billion people (Cincotta and Engelman 2000). More people living on the planet and near parks means more conversion of habitat to development, a greater infusion of exotic plantings, more demand for energy and water, more pollution, and increased park visitation and recreation which, if not properly managed by the Park Service, can harm fragile parks and their species.

   The re-distribution and concentration of America people and the resulting demands on local resources pose difficult problems for biodiversity and the national parks. States such as Florida, California, and Hawaii, which contain rich biological resources, already have large human populations and are experiencing fast-paced growth. Between April 1990 and April 2000, the population of Florida increased by 23.5 percent, California by 13.8 percent, and Hawaii by 9.3 percent. 

   Such population growth burdens natural ecosystems:

• The Everglades in South Florida is in a race for survival with a human population that demands ever more land and fresh water and ever less of the natural flooding that drives the processes that create and support the Everglades. 
• Across the nation, in California, a commercial water project threatens the Mojave National Preserve, home to threatened desert tortoises and endangered desert bighorn sheep. And overuse, misuse, and pollution threaten the decline of the coral reefs that nurture a large proportion of ocean life.

   The changing economy and its shifting dynamics are altering land-use around national parks. More people are seeking a better quality of life and pleasant environmental and social surroundings. The result is that rural areas—particularly in the West and Southeast, where some of the nation's leading parks are located—are experiencing steadily rising influxes of people and development. Between April 1, 2000, and July 1, 2001, California, Texas, Florida, Georgia, Arizona, North Carolina, and Nevada experienced the largest population growth of all states, according to the U.S. Census Bureau.

   One of the greatest threats stemming from population growth near any park is the increase in roads that population growth brings. Increased motor traffic yields more collisions with wildlife—for example, in Florida, roadkill makes human-caused mortality the leading factor in annual panther deaths. More traffic also means an increased risk of toxic spills and air pollution. An influx of people into a relatively wild area increases contact between people and wildlife, usually to the detriment of the animals. Abandoned logging and mining roads are used increasingly for off-road-vehicle recreation, bringing people into wild areas and into contact with wildlife.

   Agricultural development is another major threat to natural biodiversity. Native grasslands in large Rocky Mountain valley bottoms, for example, have nearly disappeared beneath an onslaught of plowing, grazing, and even housing development. Development of water resources for agriculture and urban use also have fragmented freshwater ecosystems.