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.
生态系统是如何运作的？是什么决定了不同物种的种群数量？为什么有这么多苍蝇而狼却这么少？为了找到答案，科学家们构建了食物网的数学模型，记录了谁吃谁以及每个物种吃了多少。

With such models，scientists have found out some key principles operating in food webs. Most food webs，for instance，consist of many weak links rather than a few strong ones. 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. Food webs may be dominated by many weak links because that arrangement is more stable over the long term. If a predator can eat several species，it can survive the extinction（灭绝）of one of them. 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. The weak links may thus keep species from driving one another to extinction.
通过这样的模型，科学家们发现了食物网中运作的一些关键原则。例如，大多数食物网由许多弱联系而非少数强联系组成。当一个捕食者总是大量捕食单一猎物时，这两个物种之间就形成了强联系；当一个捕食者依赖于多种物种时，它们之间就是弱联系。食物网可能由许多弱联系主导，因为这种安排在长期内更稳定。如果一个捕食者可以吃几种物种，它就能在其中一种物种灭绝时生存下来。如果一个捕食者能在一种猎物变得稀少时转向另一种更容易找到的物种，这种转换允许原来的猎物恢复。因此，弱联系可能使物种免于相互推向灭绝。

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. 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.
数学模型还揭示了食物网可能是不稳定的，顶级捕食者的微小变化可能导致整个生态系统产生巨大的影响。在20世纪60年代，科学家们提出，食物网顶端的捕食者对其他物种的种群规模有着惊人的控制力——包括那些它们并未直接攻击的物种。

And unplanned human activities have proved the idea of top-down control by top predators to be true. 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. These actions have greatly affected the ecological balance.
人类未计划的活动证明了顶级捕食者的自上而下的控制是真实的。在海洋中，我们以工业规模捕捞顶级捕食者，如鳕鱼，而在陆地上，我们消灭了大型捕食者，如狼。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. 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. Prevention is key，scientists says because once ecosystems pass their tipping point（临界点），it is remarkably difficult for them to return.
科学家们基于数学模型建立了一个早期预警系统。理想情况下，这个系统会告诉我们何时调整那些正在推动生态系统走向崩溃的人类活动，甚至可能允许我们将生态系统从边缘拉回。科学家说，预防是关键，因为一旦生态系统超过了它们的临界点，要让它们恢复就非常困难了。

1. What have scientists discovered with the help of mathematical models of food webs？
   A. The living habits of species in food webs.
   B. The rules governing food webs of the ecosystems.
   C. The approaches to studying the species in the ecosystems.
   D. The differences between weak and strong links in food webs.
   答案：B

2. A strong link is found between two species when a predator__.
   A. has a wide food choice
   B. can easily find new prey
   C. sticks to one prey species
   D. can quickly move to another place
   答案：C

3. What will happen if the populations of top predators in a food web greatly decline？
   A. The prey species they directly attack will die out.
   B. The species they indirectly attack will turn into top predators.
   C. The living environment of other species will remain unchanged.
   D. The populations of other species will experience unexpected changes.
   答案：D

4. What conclusion can be drawn from the examples in Paragraph 4？
   A. Uncontrolled human activities greatly upset ecosystems.
   B. Rapid economic development threatens animal habitats.
   C. Species of commercial value dominate other species.
   D. Industrial activities help keep food webs stable.
   答案：A

5. How does an early-warning system help us maintain the ecological balance？
   A. By getting illegal practices under control.
   B. By stopping us from killing large predators.
   C. By bringing the broken-down ecosystems back to normal.
   D. By signaling the urgent need for taking preventive action.
   答案：D