EARTH Phần 9

3 REEF FISH The total weight of fish, or biomass, on a coral reef is not large compared with some other marine habitats, but there is an amazing diversity of species. They have evolved because there are so many different ways to make a living on the reef. Some, such as parrotfish, eat the coral itself. Others nibble at algae, catch plankton, or prey on each other. Gray reef sharks 4 INVERTEBRATES All kinds of colorful invertebrates live on coral reefs, including delicate prawns, flamboyant sea slugs, and deadly venomous cone shells. The biggest is the giant clam which, like corals, has masses of photosynthetic organisms called zooxanthellae living within its tissues. These provide the clam with sugar in exchange for nutrients that the clam obtains by filtering plankton from the water. 5 Giant clam 4 Potato cod Brightly colored sea slugs nibble at encrusting animals on the reef Crown-of-thorns starfish 6 103 CYPRESS SWAMP Most trees cannot survive in waterlogged conditions, but some species like the American bald cypress have special “knee roots” that gather vital oxygen from above the water. They grow in flooded cypress swamps in the subtropical southern United States, famous for the rare, beautiful orchids that take root on the tree branches. WETLANDS Most wetlands are freshwater habitats where most of the water is hidden by dense vegetation. Many are transition zones between open water and dry forest or grassland. They range from overgrown lake and river margins to waterlogged forests with tall trees. Many support a wide diversity of wildlife. Others, such as acid peat bogs, are colonized by only a few specialized plants and animals. Yet The margins of many African lakes and rivers are choked with a type of giant sedge called papyrus, as seen here in the Okavango Delta in Botswana. The matted plants can also form floating islands. Virtually nothing else grows in these papyrus swamps, but they provide safe refuges for a great variety of animal life including waterbirds, crocodiles, and herds of hippos that spend their days in the water and emerge at night to feed on the surrounding grasslands. MARSH AND FEN Low-lying waterlogged land supports grasses, sedges, and reeds that root in the mud, forming a marsh. As the plants die they do not decay fully in the waterlogged soil, but build up as peat. Over time, water-tolerant trees such as willow and alder take root, dry out the peat, and turn Cattle egret the marsh into fen woodland. 104 MANGROVES Sheltered tropical coasts and river estuaries are colonized by mangroves—evergreen trees that can grow in salty, waterlogged soil thanks to root modifications like those of swamp cypresses. The mangrove forests are flooded at high tide, providing safe havens for many fish. Low tide reveals muddy swamps, alive with fiddler crabs and air-breathing fish called mudskippers. ACID PEAT BOG In cool, wet regions, spongy sphagnum moss grows on top of waterlogged plant remains to create acid peat bogs. Few other plants can grow in the acid, infertile conditions, but those that can include specialists such Mosquitoes as carnivorous fly-traps, which feed on the mosquitoes that breed in the bog pools. TUNDRA SWAMP In the far north, evergreen forest gives way to the open tundra that surrounds the poles. Here, the ground is permanently frozen at depth, forming a layer of permafrost. The surface thaws in summer, but the waterproof permafrost layer prevents the meltwater from draining away, so the defrosted tundra becomes a waterlogged swamp. It resembles an acid peat bog, but colder, and only a few tough plants can survive the combination of waterlogged soils, icy winds, and winter freezing. TROPICAL SEASONALWETLAND During the tropical rainy season, the great rivers that drain the forests and savannas burst their banks to flood the landscape. In southern Amazonia this creates the Pantanal, which at peak flood covers 75,000sqmiles (195,000sq km), making it the largest wetland in the world. The whole area becomes a habitat for aquatic animals, such as these spectacled caymans, the anaconda—the world’s biggest snake—and the giant river otter. SALT MARSH Muddy estuaries in temperate regions are colonized by low-growing salt-tolerant plants, forming tidal salt marshes. The regions nearest to the coast are dominated by fleshy plants and grasses, but other areas are more shrubby. They provide homes for a variety of small animals, including the endangered salt marsh harvest mouse of California. 105 TROPICAL RAIN FOREST Near the equator, heavy rain and high temperatures throughout the year create ideal conditions for tree growth, and these are the most luxuriant forests in the world. The trees are broad-leaved evergreens that grow to immense heights, creating a multilayered habitat that teems with life—most of it living high above the forest floor. 106 107 ... - tailieumienphi.vn