The health of park ecosystems depends on some of nature's most powerful recurring forces: free-flowing rivers, fire, erosion, and even major storms such as hurricanes. The ongoing human drive to dominate and harness nature, as well as the effects of historic patterns of human action that were based on incorrect information or lack of understanding, has meant the disruption of these forces in many places.

   The need to restore more-natural hydrologic cycles, flooding regimes, and water tables to protect the quality and quantity of park waters is urgent. Prominent examples are Everglades National Park, Big Cypress National Preserve, and Biscayne National Park.

   These parks, located at the end of Florida's river of grass, are affected directly by the impoundment, diversion, and controlled release of waters coming from Lake Okeechobee. Built for flood control and ground water recharge, these alterations allowed the draining and clearing of land for homes, businesses, and agriculture. They tend to release water to the protected lands during times of water abundance, such as the rainy season, when flow is not needed, and to hold it back during dry seasons.

   Alteration in natural water flow has brought fundamental ecological changes to the Everglades and other South Florida marshes, including disruptions in bird nesting habitat and feeding areas. Some bird species, including long-legged wading birds such as wood storks and egrets, have declined in the area by as much as 90 percent since water diversions began in the early 1900s.

   Fire suppression is another cause of biodiversity decline. Fire is a key factor in numerous ecosystems, including boreal forest, pine woods, grasslands, sequoia forests, chaparral, and even some wetlands. 

Fires recycle nutrients, rejuvenate soils, physically change the landscape, and break down organic matter such as dead leaves and grasses, which in turn helps prevent the unnaturally catastrophic fires that occur in areas with a heavy build up such material. Fires create a mosaic of different habitats that support diverse species.

   During the last century, human fire suppression has dramatically altered parks, often reducing diversity by removing fire as a force for maintaining a natural variety of plant species. In some areas, fire suppression can lead to habitats dominated by a few tree species. As the well-known Yellowstone fires of the late 1980s showed, the blazes that sweep the area roughly every 250 years cause a tremendous rejuvenation of sun-loving wildflowers and grasses, released from the constraints of the mature forest to start a new cycle of life. 

   The slash pine forests of the Everglades, the sequoia groves of Kings Canyon National Park in California, as well as fireweed at Alaska's Wrangell-St. Elias National Park are among many species benefited by fire. Some tree species, such as jack pines, will not even germinate unless their seeds are burned over by a light fire.

   To restore the proper fire regimen to national parks, the National Park Service prefers to let naturally occurring fires burn or to initiate fires through a process called prescribed burns, in which an area is marked out for burning and then controlled. The ecological need for prescribed burns presently outstrip the Park Service's available staff and funding resources. Lack of funds is itself a threat to park biodiversity.