With habitats ranging from tropical to Arctic, U.S. plant life is very diverse. The country has more than 17,000 identified native species of flora, including 5,000 in California (home to the tallest, the most massive, and the oldest trees in the world). More than 400 mammal, 700 bird, 500 reptile and amphibian, and 90,000 insect species have been documented. Wetlands such as the Florida Everglades are the base for much of this diversity. The country's ecosystems include thousands of nonnative exotic species that often adversely affect indigenous plant and animal communities. The Endangered Species Act of 1973 protects threatened and endangered species and their habitats, which are monitored by the U.S. Fish and Wildlife Service.

In 1872, the world's first national park was established at Yellowstone. Another fifty-seven national parks and hundreds of other federally managed parks and forests have since been formed. Wilderness areas have been established around the country to ensure long-term protection of pristine habitats. Altogether, the U.S. government regulates 1,020,779 square miles (2,643,807 kmē), 28.8 percent of the country's total land area. Protected parks and forestland constitute most of this. As of March 2004, approximately 16 percent of public land under Bureau of Land Management administration was being leased for commercial oil and natural gas drilling; public land is also leased for mining and cattle ranching. The energy policy of the United States is widely debated; many people worldwide call on the U.S., as the largest emitter of carbon dioxide from the burning of fossil fuels, to take a leading role in fighting global warming.