托福阅读official69 The author mentions "giant tortoises" and "giant birds" in paragraph 3 in order 答案解析【小站备考】

The Pacific Ocean accounts for one-third of Earth’s surface and half of the world’s ocean area. It has about 25,000 islands, of which about 7,500 are oceanic, being relatively far from a continental shore. The great majority of all Pacific islands were born barren of life: hard, dense, volcanic rock pimples on the sea's surface. New Zealand is the chief exception; it is among the continental islands of the western Pacific, together with Fiji, the Solomons, and others to the west.

Life arrived on most other islands by accident or by drift. Some plants arrived by air transport; seeds carried in the digestive tracts of birds account for nearly 40 percent of Hawaii's early plants. The first invaders were either creatures that could float well enough, in air or water, to cross stretches of ocean, or those whose seeds could survive a voyage in some bird's gut. At times of lower sea level, land bridges linked, or nearly linked, many islands in the far western Pacific, so some species colonized these islands without being notably good floaters or stowaways. In the eastern Pacific (Easter Island, for example) only the best floaters and travelers arrived and survived. Consequently, the western islands have far more species and far greater biodiversity than do the eastern islands of Polynesia. Mammals found it hard to get anywhere in the island Pacific; only bats and rats successfully colonized east of New Guinea. Almost all species derive from Asia; the early Pacific was an Asian lake, with only a tiny proportion of species from the Americas. As a rule of thumb, the further from Indonesia, the more impoverished the plant and animal life and, in consequence, the less stable and resilient in the face of disturbance. This attenuation is strong for land species, less strong for marine species, and nonexistent for oceanic birds, although fairly strong for land birds.

Pacific ecosystems evolved in relative (but differential) isolation from the continental crucibles of biological evolution. This meant opportunities for speciation: the development of new species occupying ecological niches that elsewhere were already filled. The finches described by the naturalist Charles Darwin on the Galapagos Islands—birds that divided into different species, each specialized for a narrow niche—are the classic example. On islands that had no mammals, reptiles and birds took their place. Thus the Galapagos have giant tortoises , and New Zealand once had giant birds that functioned more or less like browsing or grazing mammals. Throughout most of the Pacific, the paucity of grazing animals meant that plants developed no defenses, such as spines, poisonous chemicals, or bitterness. The remoter islands had a very high proportion of endemism—that is，of species that existed only there. In the case of Hawaii, as many as 99 percent of the species were endemic. All this led to a certain biological vulnerability among the terrestrial island species, should they ever be obliged to compete for niche space with the winners of the more intense continental competitions for survival. This vulnerability increased toward the east and toward the remoter corners of the Pacific along a gradient defined chiefly by the degree of isolation.

A second source of vulnerability, perhaps more decisive, arose from the arrival of humankind in the Pacific. Island animals evolved with no experience of the ways of humankind, or indeed of any large terrestrial predators. As a result, they had no immunities to predators or the effects of human action. Pacific animals were often unwary and easy prey. At the extreme, again the Galapagos, Darwin found many birds almost tame, so naively trusting that they would allow him to get within arm’s reach. Pacific plants had little experience with fire, because natural fires were very rare, except in a few places. Thus few plants were well adapted to fire, and most proved vulnerable to it. In contrast, continental species that had evolved in the presence of humankind, or in places where natural fire is much more common, could recover easily after burns, and some could flourish as a result of fire. In short, Pacific ecosystems were different from continental ones on account of their isolation. They were well adapted to their circumstances but very vulnerable to alien invasion and human impact.

OFFICIAL69 The author mentions "giant tortoises" and "giant birds" in paragraph 3 in order to

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