As the prairie dogs disappear, they are taking with them at least one of their predators, the black-footed ferret. This member of the weasel family has prairie dogs as its prime food. It has become overspecialized and is caught in an evolutionary trap.
North America's arid areas occur in the southwestern United States and parts of Mexico. Large grazers and browsers include bighorn sheep, mule deer and javelinas, also called peccaries. Hawks, foxes, owls, coyotes, and several species of reptiles are among the carnivores. Among them, the coyote is one of the few which has thrived in the face of human intrusion into its habitat. Not only has it maintained its former range; it has expanded it.
One of the resident birds of the North American southwest is the roadrunner, a member of the cuckoo family. Primarily a ground bird, it can run at speeds of up to 24 kmph (15 mph). Its diet consists of lizards and other reptiles which it kills by repeated blows from its heavy beak. If prey proves too large to swallow, the roadrunner ingests a bit at a time. The birds can be seen dashing along the desert with snakes or lizards hanging from their mouths.
The world's smallest owl, the 14 cm (5 1/2 in) high elf owl, also is a resident of the American desert. This tiny predator uses the hollowed-out nests of woodpeckers, located in cactuses, as its home.
The desert also has its reptiles, including many species of lizards, plus two of the four poisonous snakes of North America — the rattlesnake and coral snake.
Rattlesnakes are pit vipers, a group of reptiles which also includes the fer-de-lance, bushmaster, water moccasin, and the copperhead The pit is an opening below the snake's eyes which contains a heat-sensing organ.
Only two of North America's lizards are poisonous — the gila monster and Mexican beaded lizard. Unlike poisonous snakes which inject their venom through hollow fangs, these lizards bite their victims, hold on, and allow poison to flow into the open wound from fangs which are grooved at the rear.
The coastlands and adjacent lands of the United States are the habitat of a wide variety of reptiles, birds and mammals. Water moccasins and copperheads are found in the warmer portions, and the largest of all North American reptiles, the alligator, lives in the rivers and bayous of the southeast.
Alligators can be distinguished from the closely related crocodiles by their broader heads and the lower teeth which are out of sight when the mouth is closed. A crocodile's teeth are visible at all times.
There are no authenticated cases of wild alligators attacking humans. Crocodiles, on the other hand, can attack people.
Many species of shorebirds live in North America. One of them, the brown pelican, came close to extinction on the continent because of DOT. The pesticide was sprayed and dusted on croplands, then percolated into the ground water and was carried to sea where it entered the ocean's food chain. The pelicans, being ultimate consumers, got heavy doses. Although the chemical didn't kill them, it did weaken the shells of their eggs. The result was few pelican hatchlings. After DDT was banned the pelican population began to grow again. In 1979, 1,200 nests were counted in California, a remarkable comeback.
Marine mammals of the U.S. Pacific coast include four species of pinnipeds — members of the seal group. They are elephant seals, harbor seals, Steller sea lions and California sea lions.
South of the United States and northern Mexico, the character of the land and its wildlife changes. Desert, chaparral, and plains give way to tropical forest. In places rainfall exceeds 500 cm (200 in) annually, and a mild average temperature of 27°C (81°F) prevails.
As in most rain forests, primates dominate. In America they consist of dozens of species of monkeys and marmosets. New World monkeys are only distantly related to those of the Old World. Many species have prehensile tails, a feaure lacking in the Old World monkeys. This "fifth hand" is especially well developed in the spider monkey.
Not all of the rain forest's primates have prehensile tails. Marmosets of the forests of Panama and the Amazon basin lack it. And the uakari has a mere stub of a tail, making it the only short-tailed New World monkey.
Реферат опубликован: 21/12/2008