Pollution makes marine species vulnerable to disease by weakening their immune systems. By increasing demands for food, fuel, shelter, and other amenities, population growth and urbanization contribute to the loss of forests around the world. Nearly one-half of the forests that covered the Earth 8,000 years ago are gone, primarily because of human activities, according to a recent study by the World Resources Institute and the World Conservation Monitoring Centre. 53 Logging threatens 70 percent, and agriculture threatens 20 percent, of the world's remaining frontier forests. The world's forests are essential for a number of reasons.
Forests provide wood pulp and timber and offer areas of natural beauty and recreation for people.
They provide a habitat for millions of species of plants and animals, most of which have not yet been catalogued. Forests are integral to global weather systems and the world's oxygen supply, and they serve as watersheds that absorb water and prevent soil erosion. Forests act as storehouses of the global carbon supply, and thus are crucial to the international negotiations on global warming, as explained below. This represents a decline of about 450 million hectares since 1960. Between 1960 and 1990, about one-fifth of all natural tropical forest cover in less developed countries was lost.