C
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.
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.
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.
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.
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.
46. 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.
47. 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
48. 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.
49. 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.
50. 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.
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