How does an ecosystem（生态系统）work？What makes the populations of different species the way they are？Why are there so many flies and so few wolves？To find an answer，scientists have built mathematical models of food webs，noting who eats whom and how much each one eats.
一个生态系统是如何工作?是什么促使不同物种的种群成为现在的样子?为什么有那么多苍蝇和如此少的狼?为了找到答案,科学家建立了食物网的数学模型,并指出谁吃谁,吃多少。
With such models，scientists have found out some key principles operating in food webs.
通过这样的模型，科学家们发现了在食物网中起作用的一些关键原理。
Most food webs，for instance，consist of many weak links rather than a few strong ones.
例如，大多数食物网是由许多薄弱环节组成的，而不是由一些强环节组成的。
When a predator（掠食动物）always eats huge numbers of a single prey（猎物），the two species are strongly linked；when a predator lives on various species，they are weakly linked.
当捕食者总是吃大量的猎物,这两个物种息息相关,当捕食者以不同的种类为生,他们是弱相关。
Food webs may be dominated by many weak links because that arrangement is more stable over the long term.
食物网可能由许多薄弱环节主导，因为从长远来看，这种安排更稳定。
If a predator can eat several species，it can survive the extinction（灭绝）of one of them.
如果捕食者可以吃几个种类,如果他们其中一种灭绝，它还可以存活。
And if a predator can move on to another species that is easier to find when a prey species becomes rare，the switch allows the original prey to recover.
如果捕食者可以转移到另一个更容易找到的物种上，当一个猎物变得稀有时，这种转换使原来的猎物得以恢复。
The weak links may thus keep species from driving one another to extinction.
因此，这些薄弱的环节可能会阻止物种间的相互灭绝。
Mathematical models have also revealed that food webs may be unstable，where small changes of top predators can lead to big effects throughout entire ecosystems.
数学模型还显示，食物网可能是不稳定的，食物链顶端的捕食者的微小变化可能会导致整个生态系统的巨大影响。
In the 1960s，scientists proposed that predators at the top of a food web had a surprising amount of control over the size of populations of other species---including species they did not directly attack.
在20世纪60年代，科学家们提出，处于食物链顶端的捕食者对其他物种的种群大小有着惊人的控制力——包括那些它们没有直接攻击的物种。
And unplanned human activities have proved the idea of top-down control by top predators to be true.
无计划的人类活动证明了顶级掠食者自上而下控制的观点是正确的。
In the ocean，we fished for top predators such as cod on an industrial scale，while on land，we killed off large predators such as wolves.
在海洋中，我们以工业规模捕捞鳕鱼等顶级掠食者，而在陆地上，我们捕杀狼等大型掠食者。
These actions have greatly affected the ecological balance.
这些行为极大地影响了生态平衡。
Scientists have built an early-warning system based on mathematical models.
科学家已经建立了一个基于数学模型的预警系统。
Ideally，the system would tell us when to adapt human activities that are pushing an ecosystem toward a breakdown or would even allow us to pull an ecosystem back from the borderline.
理想情况下，这个系统会告诉我们，当人类活动把一个生态系统推向崩溃，或者甚至允许我们把一个生态系统从边缘拉回来时，该系统会告诉我们什么时候去改变人类活动。
Prevention is key，scientists says because once ecosystems pass their tipping point（临界点），it is remarkably difficult for them to return.
预防是关键,科学家们说,因为一旦生态系统通过临界点,这是非常难以恢复的。
46.What have scientists discovered with the help of mathematical models of food webs？（ ）
借助食物网的数学模型，科学家们发现了什么?
A. The living habits of species in food webs.
物种在食物网中的生活习性。
B. The rules governing food webs of the ecosystems.
控制生态系统食物网的规则。
C. The approaches to studying the species in the ecosystems.
研究生态系统中物种的方法。
D. The differences between weak and strong links in food webs.
食物网中弱链和强链的区别。
答案分析：考查推理判断，第一段说为了弄清生态系统的运转，科学家建立了教学模型，科学家找到了生态系统食物网中运行的关键原则，所以选择B。
47.A strong link is found between two species when a predator______
一个强大的链接被发现在两个物种之间，当捕食者——
A. has a wide food choice
有广泛的食物选择
B. can easily find new prey
很容易找到新猎物
C. sticks to one prey species
只吃一种猎物
D. can quickly move to another place
可以很快搬到另一个地方
答案分析：考查细节理解，根据题干的关键词A strong link，我们把答案定位在第二段，根据该短的关键信息 when a predator……strongly linked。可知，如果掠食动物只吃一种动物，那么它们的联系就非常紧密。所以选C。A、B、D都是a weak link.
48.What will happen if the populations of top predators in a food web greatly decline？（ ）
如果食物链中的顶级捕食者的数量大幅减少，将会发生什么?
A. The prey species they directly attack will die out.
它们直接攻击的猎物种类将会灭绝。
B. The species they indirectly attack will turn into top predators.
它们间接攻击的物种会变成顶级掠食者。
C. The living environment of other species will remain unchanged.
其他物种的生存环境将保持不变。
D. The populations of other species will experience unexpected changes.
其他物种的数量将经历意想不到的变化。
答案分析：根据题干的关键词，挺短掠食动物大大的减少，我们把答案定在第三段，该段说顶端掠食动物数量的细小变化都会给整个生态系统带来大变化，所以选D。
49.What conclusion can be drawn from the examples in Paragraph 4？（ ）
从第4段的例子可以得出什么结论?
A. Uncontrolled human activities greatly upset ecosystems.
不受控制的人类活动极大地破坏了生态系统。
B. Rapid economic development threatens animal habitats.
经济的快速发展威胁着动物的栖息地。
C. Species of commercial value dominate other species.
具有商业价值的物种支配其他物种。
D. Industrial activities help keep food webs stable.
工业活动有助于保持食物网稳定。
答案分析：考查细节理解，第四段开头说，无计划的人类活动已经证明顶端掠食动物能自上而下地控制其他动物。然而说到人类对海洋和陆地顶端掠食动物的捕杀，并且说，这些行为极大地影响了生态平衡。所以选A。
50.How does an early-warning system help us maintain the ecological balance？（ ）
预警系统是如何帮助我们维持生态平衡的?
A. By getting illegal practices under control.
通过控制非法行为。
B. By stopping us from killing large predators.
阻止我们捕杀大型食肉动物。
C. By bringing the broken-down ecosystems back to normal.
通过使破损的生态系统恢复正常。
D. By signaling the urgent need for taking preventive action.
通过表明采取预防措施的迫切需要。
答案分析：考查推理判断，根据题干的关键信息，早期预警系统。我们把答案定在最后一段。该段说，这个系统是以数学模型为基础建立的，能告诉我们什么时候应该改变人类把生态系统逼向崩溃的活动，然后说，预防是关键。所以选D。
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