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The African elephant-- mythic symbol of a continent, keystone of its ecology and the largest land animal remaining on earth- has

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问题     The African elephant-- mythic symbol of a continent, keystone of its ecology and the largest land animal remaining on earth- has become the object of one of the biggest, broadest international efforts yet mounted to turn a threatened species off the road to extinction. But it is not only the elephant’s survival that is at stake, conservationists say. Unlike the endangered tiger, unlike even the great whales, the African elephant is in great measure the architect of its environment. As a voracious eater of vegetation, it largely shapes the forest-and-savanna surroundings in which it lives, thereby setting the terms of existence for millions of other storied animals- from zebras to giraffes and wildebeests- that share its habitat. And as the elephant disappears, scientists and conservationists say, many other species will also disappear from vast stretches of forest and savanna, drastically altering and impoverishing whole ecosystems.
   It is the elephant’s metabolism and appetite that make it a disturber of the environment and therefore an important creator of habitat. In a constant search for the 300 pounds of vegetation it must have every day, it kills small trees and underbrush and pulls branches off big trees as high as its trunk will reach. This creates innumerable open spaces in both deep tropical forests and in the woodlands that cover part of the African savannas. The resulting patchwork, a mosaic of vegetation in various stages of regeneration, in turn creates a greater variety of forage that attracts a greater variety of other vegetation-eaters than would otherwise be the case.
   In studies over the last 20 years in southern Kenya near Mount Kilimanjaro, Dr. Western has found that when elephants are allowed to roam the savannas naturally and normally, they spread out at "intermediate densities". Their foraging creates a mixture of savanna woodlands (what the Africans call bush) and grassland. The result is a highly diverse array of other plant-eating species: those like the zebra, wildebeest and gazelle, that graze; those like the giraffe, bushbuck and lesser kudu, that browse on tender shoots, buds, twigs and leaves; and plant-eating primates like the baboon and vervet monkey. These herbivores attract carnivores like the lion and cheetah.
   When the elephant population thins out, Dr. Western said, the woodlands become denser and the grazers are squeezed out. When pressure from poachers forces elephants to crowd more densely onto reservations, the woodlands there are knocked out and the browsers and primates disappear.
   Something similar appears to happen in dense tropical rain forests. In their natural state, because the overhead forest canopy shuts out sunlight and prevents growth on the forest floor, rain forests provide slim picking for large, hoofed plant-eaters. By pulling down trees and eating new growth, elephants enlarge natural openings in the canopy, allowing plants to regenerate on the forest floor and bringing down vegetation from the canopy so that smaller species can get at it.
   In such situations, the rain forest becomes hospitable to large plant-eating mammals such as bongos, bush pigs, duikers, forest hogs, swamp antelopes, forest buffaloes, okapis, sometimes gorillas and always a host of smaller animals that thrive on secondary growth. When elephants disappear and the forest reverts, the larger mammals give way to smaller, nimbler animals like monkeys, squirrels and rodents.
In the opening paragraph, the author mentions tigers and whales in order to emphasize which point about the elephant?
选项 A、Like them, it faces the threat of extinction.
B、It is herbivorous rather than carnivorous.
C、It is the largest extant land mammal.
D、Unlike them, it physically alters its environment.
答案D
解析 根据文章第一段第三句“Unlike the endangered tiger,unlike even the great whales． the African elephant is in great measure the architect of its environment．”可知，老虎和鲸鱼不能 主宰环境，而大象是环境的主宰者，它们可以改变环境。据此判断，答案是D。
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