404 Extinction, Causes of clearing for commercial logging, cattle ranches, cash crop plantations (oil palm, cocoa, rubber, etc.), road building, and mining At the current rate of destruction (approximately 140,000 km2 per year), there will be no large blocks of tropical forest left after the year 2040, except in the relatively small national parks and protected areas and a few remote areas of the Brazilian Amazon, central Africa, and the islands of Borneo and Papua New Guinea (Figure 4) The situation is actually grimmer than these projections indicate because the world’s population is still increasing, and poverty is rising in many developing tropical countries, putting ever-greater demands on the dwindling supply of rain forest Other Threatened Habitats The plight of the tropical rain forests is perhaps the most widely publicized case of habitat destruction, but other habitats are also in grave danger Tropical Deciduous Forests Tropical deciduous forests contain many species and in some places rival the diversity of the tropical rain forests The land occupied by tropical deciduous forests is more suitable for agriculture and cattle ranching than the land occupied by tropical rain forests Moderate seasonal rainfall, in the range of 250–2000 mm per year, allows nutrients to be retained in the soil, where they can be taken up by plants These forests are also easier to clear and burn than rain forests Consequently, human population density is five times greater in dry forest areas of Central America than in adjacent rain forests Today, the Pacific coast of Central America has less than 2% of its original extent of deciduous dry forest remaining, and many species have been lost or are threatened with extinction Grasslands Temperate grasslands have also been almost completely destroyed by human activity, with a consequent loss of species It is relatively easy to convert large areas of grassland to farmland and cattle ranches Illinois and Indiana, for example, originally contained 15 million (37 million acres) of tall-grass prairie, but now only 1400 (3500 acres) of this habitat – one-ten-thousandth of the original area – remain undisturbed The rest have been converted to farmland, and the remaining area of prairie is fragmented and widely scattered across the landscape Though widespread efforts to restore prairies in many areas of the world are underway and should be encouraged, it is impossible to bring back species that have already been lost Wetlands and Aquatic Habitats Wetlands are critical habitats for fish, aquatic invertebrates, aquatic plants, and birds They also provide services to humans such as flood control, drinking water, and power production Wetlands are often filled in or drained for development, or they are altered by channelization of watercourses, dams, chemical pollution, and siltation This habitat destruction increases extinction risk for aquatic species, especially one with limited distributions During the last 200 years, over half of the wetlands in the US have been destroyed The Florida Everglades, a premiere wildlife refuge, is now on the verge of ecological collapse Approximately 40–50% of the freshwater snail species in the southeastern US are either extinct or endangered In California’s San Diego County, more than 97% of the vernal pools, which support a unique endemic biota, have been destroyed Furthermore, massive development projects in other parts of the industrialized world, such as the Three Gorges Dam in China, are destroying aquatic ecosystems on an unprecedented scale Mangroves Mangrove forests are among the most important wetland communities in tropical areas Composed of species that are among the few saltwater-tolerant woody plants, mangrove forests occupy coastal areas with saline or brackish water, typically where there are muddy bottoms Such habitats are similar to salt marshes in the temperate zone Mangroves are extremely important breeding grounds and feeding areas for shrimp and fish They also reduce storm damage Despite their great economic value, mangroves are often harvested for timber and charcoal production and cleared for development In recent years, mangroves have been increasingly cleared for rice cultivation and commercial shrimp hatcheries, particularly in Southeast Asia, where as much as 15% of the mangrove area have been removed for aquaculture Over 35% of the world’s mangrove ecosystems have been destroyed, and 40% of vertebrates endemic to mangroves are threatened with extinction Coral Reefs Tropical coral reefs contain an estimated one-third of the ocean’s fish species in only 1% of its surface area Already 20% of all coral reefs have been destroyed, and an additional 20% have been degraded The most severe destruction is taking place in the Philippines, where a staggering 90% of reefs are dead or dying The main culprits are pollution, which either kills the coral directly or allows excessive growth of algae; sedimentation following the removal of forests; overharvesting of fish, clams, and other animals; climate change; invasive species; and, finally, fishermen blasting with dynamite and releasing cyanide and other poisons to collect the few remaining living creatures Extensive loss of coral reefs is expected within the next 40 years in tropical East Asia, around Madagascar and East Africa, and throughout the Caribbean (Figure 4) In the Caribbean, a combination of overfishing, hurricane damage, water pollution, and disease is responsible for a dramatic decline of a large proportion of the coral reefs and their replacement by fleshy macroalgae Elkhorn and staghorn corals, which were formerly common and gave structure to the community, have already become rare in many locations Desertification Many biological communities in seasonally dry climates are degraded into man-made deserts by human activities, a process known as desertification These communities include tropical grasslands, scrub, and deciduous forests, as well as temperate shrublands, such as those found in the Mediterranean region, southwestern Australia, South Africa, central