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  3. The Rainforest City A) A patch of tropical rainforest has twice the number of mammal species, five times the bats and birds

The Rainforest City A) A patch of tropical rainforest has twice the number of mammal species, five times the bats and birds

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问题                                 The Rainforest City
    A) A patch of tropical rainforest has twice the number of mammal species, five times the bats and birds and ten times the types of trees than an identical sized patch of temperate forest. Explaining this diversity is extremely difficult, but much of the answer lies in the unique complexity, productivity and dynamism of the place. These three features have simultaneously fed upon each other to erect and populate the equivalent of vast, buzzing metropolises in the living world.
    B) In fact, the more we look at the rainforest, the more we see parallels with a city. Just like a city, the rainforest has "guilds"—groups that share a common livelihood. Where the city may have guilds of locksmiths and fishmongers (鱼贩), the rainforest has guilds of understorey nectar-eaters and emergent epiphytes (附生植物).
    C) And, just as a large city offers more employment opportunities than a small town, the rainforest has significantly more guilds than other habitats. This is partly due to its more complex structure—the fact that there is an understorey means species can find a livelihood in the understorey—but the rainforest is also effectively open all year and so it offers employment that is simply not available in other habitats.
    D) A deckchair attendant in Britain has to do odd jobs in the winter, but in Thailand it’s a year-round occupation. Similarly, no animal can be just a seed-eater in an oak forest, because acorns only fall in autumn. In the rainforest, seeds are always falling from the canopy (树冠), and so seed-eating is a legitimate profession—it has its own guild. Similarly, due to the year-round demand in cities, specialists such as carpet-cleaners, copywriters and couriers can thrive, while in a small town, they are absent.
    E) The rainforest "job market" is also enormous as a result of its permanently booming "economy". In nature, energy is the currency, and the incredible productivity of the rainforest ensures that there’s always enough of it around to enable millions of species to live side by side. And, to avoid competition, natural selection has made sure that, even within a guild there are tiny differences in the diets, habitats or behaviours of each member.
    F) The rainforest could therefore be regarded as a vast association of specialists, a community of animals and plants that ply their own very particular trade. In insects, the specialisation is extreme. Most live on only one or two species of plant. One tree in Panama was found to have 163 species of beetles that were exclusive to that type of tree.
    G) Most rainforest plants protect their leaves with poisons. In order to eat a plant’s leaves, the insects have to evolve to become tolerant to its particular cocktail of toxins (毒素). After thousands of years, most herbivorous insects are committed to living on their host plant alone.
    H) So, every poison-laced rainforest tree has a whole community of species living on, under and around it that are not found anywhere else. This situation is not unique to the rainforest. The same happens in Britain. In oak or Scots pine forests, a host of species live on just the oak trees or Scots pines. But the fact is that in these forests, virtually every tree is an oak or a Scots pine.
    I) What makes the rainforest so special, and so diverse, is that in one hectare there can be 300 different types of trees, each with its own exclusive community. In one tract of forest there are thousands, and worldwide there could be up to 50000 canopy-tree species. To an insect, the rainforest isn’t just one job market, but thousands of different job markets, all located in the same city.
    J) This "mosaic (组合) of trees" is probably the single most important cause of diversity in the rainforest, and yet we don’t really understand how it happens—that is, why you don’t normally find groves of trees in the rainforest.
    K) It could be that the 50000 different trees suit 50000 different types of plot and that the best tree for the spot excludes all the others. Or, it could be that all the trees are as "good" as each other and that the forest is trapped in an endless game of tick-tack-toe (三连游戏) with no ultimate winner. Or, it could be that some species are better than the others and are in the process of taking over, but because this process would take centuries, they never quite manage it before something such as a storm or landslide puts them back to square one.
    L) But none of these explanations answers a simple question: if this is true for the rainforest, why isn’t it true for an oak forest in England? The only theory that solves this puzzle is one that looks back to animals for an answer. Remember the guild of seed-eaters? In the rainforest a long list of species belongs to this guild.
    M) There are beetles and weevils (象鼻虫), squirrels and mice, rats, birds and larger mammals such as forest pigs, deer and tapirs. When this gang finds a tree in fruit, they feast until virtually no seed survives. The only seeds that are spared are those scattered far and wide, lying alone on the forest floor. It is these seeds that will go on to create the next generation of canopy trees—one that, like the previous generation, is also scattered far and wide.
    N) This is how the seed-eaters might create a mosaic of trees—by stopping any one tree from becoming too common. It wouldn’t happen in an English oak forest because there is no guild of seed-eaters—it’s not a year-round occupation.
    O) No one doubts that the rainforest is extremely valuable, but not everyone sees this value in the same way. Timber merchants, for example, see one kind of value, and environmentalists see another. To many scientists, a rainforest is most valuable when left alone to prosper without human interference, but with a growing human population, a global market for extracted goods and the extent of poverty around the equator, an evaluation of the rainforest has to be more practical than this.
    P) A new breed of rainforest valuation attempts to fit into the accounting books of nations and international organizations. It speaks the language of accountants, costs both the benefits of an intact rainforest and the losses of a vanished one, works out a forest’s "natural capital" and assesses its contribution to "environmental services". Its grand conclusion; each hectare of intact rainforest is worth about £4 500. It may not sound much, but that puts more than £7. 5 trillion in the pockets of some of the most troubled countries on Earth.
    Q) So where does this figure come from? Three quarters of it represents the uncollected harvest of a living rainforest. Managed sensibly, a wild forest can yield all sorts of sustainable crops—timber, fruit, nuts, fibre, pulpwood, gums, resins, oils, veneer—and there are many more treasures in there that we haven’t discovered yet. At least 3000 fruits are known from rainforest plant collections, but only 200 of these are now in the marketplace. There are thousands of timbers, resins and oils that we’ve never analysed.
    R) And, perhaps most important of all, the rainforest is a natural pharmacy (药房). All those poisonous plants are a goldmine of possible drugs. To date, less than one percent of rainforest plants have been examined for medicinal uses, but even this tiny percentage yields a quarter of all prescription drugs. Estimates suggest that the market value of those still secreted in the forest would run to hundreds of billions of pounds.
    S) The remaining quarter stems from the value of the chores that the rainforest carries out on our behalf— purifying air and water, preventing floods and drought, pollinating (授粉) our crops, controlling our pests, fertilising our soil and reducing the effects of global warming by storing carbon. If the rainforest disappears, we’ll have to pick up the tab for all of these services, and this means that each time a hectare of forest is felled it actually costs us money. T) When you’ve done the sums, the rainforest is actually worth more whole than in pieces. Each time another hectare is removed, humanity effectively takes a loss. Many of us already knew this, but now it has been written in a language that everyone can understand.
The insects feed themselves on a particular plant because they can only tolerate one kind of poison from the leaves.
选项
答案G
解析 由题干中的feed themselves on a particular plant定位到G段第二、三句。细节推理题。G段提到，雨林中的树木往往在树叶部分进化出毒素以保护自己。而昆虫为了吃到树叶就必须进化为能够适应某种特定的毒素。几千年后，长期的进化就把它们固定在某种特定的树木上了。题干中的feed themselves on a particular plant对应原文中的are committed to living on their host plant alone，故选G。
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