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The animals of Americas - Реферат

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North and South America comprise the only continuous land mass that reaches from the north to south polar regions, a distance of more than 14,500 km (9,000 mi). The combined area of the two continents is 41.4 million sq km (16 million sq mi), in which are found all terrestrial biomes.
The two continents have been joined for the past two or three million years. Earlier South America was an island, set apart from the northern land mass for at least 60 million years. This gave time for animal species unique to the continent to evolve. After the Isthmus of Panama emerged, there was an interchange of animals between North and South America, much as that experienced by Eurasia and America during the Ice Ages. One of the animals found in both Eurasia and America is the polar bear. Its habitat is along the entire Arctic coast. It has even been sighted hunting seals on ice floes hundreds of miles at sea. The polar bear's heavy coat insulates it from the icy water and air. Thick hair growing between its toes keep it from slipping on the ice. The thick, white pelt made the animal a prized trophy and reduced its population. The bear is now protected throughout its range.
The musk ox, resident of the far north, also has had to be protected from excessive hunting. At one time it came very close to extinction. A member of the cow family, the musk ox has adapted to the bitter cold by developing a heavy, shaggy coat consisting of two parts - a coarse outer covering of long guard hairs and a soft inner coat so dense that neither cold nor moisture can penetrate.
Musk oxen form a defensive ring when threatened. Adults stand along the perimeter, heads and horns pointing out, and the calves cluster together inside. This defensive posture works well against the ox's chief enemy, wolves, but is of little avail when high-powered rifles are the enemy.
Wolves prey on many species in the north - musk ox, caribou, moose, deer, hares, and even rodents. These carnivores are among the most maligned of all animals, victims of false myths and legends and systematic programs of extermination. They are accused of attacking humans and destroying entire herds of domestic animals. But their depredations of livestock are less severe than often claimed. And unprovoked attacks by healthy wolves in North America on humans are unknown. Those recorded from Europe's Middle Ages are thought to have been made by rabid animals or hybrids.
The world will be a far lonelier place if the last wolf dies. As biologist Ernest P. Walker wrote in his book, Mammals of the World, "The howl of the wolf and coyote, which to some people is of more enduring significance than superhighways and skyscrapers, should always remain a part of our heritage."
Some Arctic wolves remain snow white year round, an adoption to their environment. Three other predators of the far north- the snowy owl, Arctic fox, and weasel- are white at least part of the year.
The life cycle of the snowy owl demonstrates the close relationship which can exist between predator and prey. This owl hunts hares and lemmings. When these mammals are plentiful, female owls lay clutches of seven to ten eggs. When the food supply drops, only one to three eggs are laid.
Lemmings are among the most plentiful animals of the far north. These tiny rodents, found throughout the Arctic, are characterized by wide fluctuations in population. When vegetation is plentiful, the lemmings' numbers skyrocket. This population density seems to trigger a drive to migrate. Hordes of lemmings move out. Nothing deters them - swamps, forests, lakes, rivers. Eventually some reach the sea, which seems just one more obstacle. They plunge in, swim out, and drown.
Each summer the far north comes alive with the millions of birds which have migrated from the south to mate, build nests and raise their young. Waterfowl make up the majority of these migrants. Shore birds, pelagic birds, geese and ducks abound in the short Arctic summer. Some have come thousands of miles. The champion migrant is the Arctic tern, which flies o 16,000 km (10,000 mi) from the Antarctic, and in autumn flies back again.
When the birds leave the Arctic at the end of summer, they follow ancient flyways south. One of the flyways follows the Pacific coastline from Alaska to California. Small ponds and estuaries along the coast resound to the gabbling of hundreds of ducks.
The southern edge of North America's tundra borders on the taiga. Here wildlife tends to stay on the forest's edge, in meadows, along streams, on lakes and in old burns. Grass, sedges, and willows grow most profusely in these openings.
The lakes of Wood Buffalo Park in Canada's taiga are the summer nesting sites of the whooping crane, the rarest of all cranes and the object of a decades-long conservation effort. In 1949 there were only 21 left out of a population which once ranged from the East Coast to the Rocky Mountains. With complete protection, the population rose to 109 birds by 1979. Eighty-three lived in the wilderness; the others were captives.
Twice a year the wild birds migrate a hazardous 4,000 km (2,500 mi) from their nesting grounds in Wood Buffalo Park to the Aransas Wildlife Refuge on the Texas coast. The possibility of a major storm or devastating disease striking this flock is a threat which makes biologists shudder. One of the basic rules in the management of an endangered species is to spread the risk. A daring experiment was undertaken with the whooping cranes. Eggs were removed from nests in Wood Buffalo Park for artificial incubation and placement under setting sandhill cranes, a related, more plentiful species. The artificially incubated eggs are hatching and producing birds that are raised in captivity. Several whooping cranes have been hatched and are being raised by their foster parent sandhills in Idaho. If the experiment succeeds, a new flock of whooping cranes will have been produced, one which migrates a much smaller distance, over a different route, than the original group. A fringe benefit of taking eggs is that it stimulates the female bird to continue laying, thus generating more than the usual number of clutches per year. The most common grazing animal of the American coniferous and deciduous forests is the white-tailed deer. In the far West, it is replaced by the mule deer. There are actually more deer now in North America than when Europeans first arrived, because of the clearing of forest land, plus game management.
Bears once occurred throughout the forests of America north of Mexico. The world's largest is a brown bear, the Alaskan or Kodiak. The grizzly, also a brown bear, has been known to launch unprovoked attacks against humans.
American black bears are quite common in much of their range - practically all the wooded areas of North America north of central Mexico. They usually occurin their familiar black color phase, but also have been known to be a cinnamon color, brown, and even blue. The rare blue or glacier bear occurs only in southeastern Alaska, where there are about 500 left.
South of North America's taiga is the immense grassland known as the Great Plains. This covers most of the continent's interior and stretches 3,900 km (2,400 mi) from southern Canada deep into Mexico. It is prairie country, a seemingly flat land, devoid of trees excepting along the river courses. Almost all of the original grasses were plowed under for the raising of crops, and of the tremendous number of wild animals which once lived there, practically nothing remains. As the naturalist Peter Farb wrote, "Not even the eastern forests have suffered the almost complete destruction that European man has brought to the grassland."
The story of the American pronghorn, the only "antelope" native to the New World, illustrates his point. When Europeans first settled in the Western Hemisphere, there were an estimated 50 to 100 million pronghorn on the plains. Four centuries later by the turn of the 20th century, only 20,000 were left. Today, through strenuous conservation efforts, the prong-horn is safe, although consigned to a small fraction of its former range.
Another example of what happened to the plains' wildlife concerns a "dog." Before the Europeans came, hundreds of millions of rodents, called prairie dogs because of their dog-like call, lived in underground "towns" from southern Canada to Mexico. One such system of burrows in Texas covered more than 65,000 sq