Biodiversity refers to genetic variability within a species, diversity of populations of a species, diversity of species within a natural community, or the wide array of natural communities and ecosystems throughout the world. Some scientists estimate that there may be between 15 and 100 million species throughout the world. Biodiversity is threatened at the present time more than at any other time in history. In the time since the North American continent was settled, as many as 500 plant and animal species have disappeared in North America. Some recent examples of threats to biodiversity in the United States include: 50 percent of the United States no longer supports its original vegetation; in the Great Plains, 99 percent of the original prairies are gone; and across the United States, we destroy 100,000 acres of wetlands each year.