【推荐3】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 say, because once ecosystems pass their tipping point (临界点) , it is remarkably difficult for them to return.

1. 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

2. 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.

3. 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.

4. 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.

【推荐1】Zoe Chambers was a successful PR (Public Relations) consultant (顾问) and life was going well—she had a great job, a beautiful flat and a busy social life in London. Then one evening in June last year, she received a text message telling her she was out of work. The first two weeks were the most difficult to live through.” she said. “After everything I’d done for the company, they dismissed me by text!I was So angry and I just didn’t feel like looking for another job. I hated everything about the city and my life.”

Then, Zoe received an invitation from an old school friend, Kathy, to come and stay. Kathy and her husband, Huw, had just bought a farm in north-west Wales. Zoe jumped at the chance to spend a weekend away from London, and now, ten months later, she is still on the farm.

“The moment I arrived at Kathy’s farm, I loved it and I knew I wanted to stay.” said Zoe. “ Everything about

my past life suddenly seemed meaningless.”

Zoe has been working on the farm since October of last year and says she has no regrets. “It’s a hard life, physically very tiring. ” she says. “In London I was stressed and often mentally exhausted. But this is a good, healthy tiredness. Here, all I need to put me in a good mood is a hot bath and one of Kathy’s wonderful dinners. ”

Zoe says she has never felt bored on the farm. Every day brings a new experience. Kathy has been teaching her how to ride a horse and she has learnt to drive a tractor. Since Christmas, she has been helping with the lambing-—watching a lamb being born is unbelievable, she says, “It’s one of the most moving experiences I’ve ever had. I could never go back to city life now. ”

1. When working as a PR consultant in London, Zoe thought she lived a        life.

A．satisfying

B．tough

C．meaningless

D．boring

2. The most important reason why Zoe went to visit Kathy’s farm is that          .

A．Zoe lost her job as a PR consultant

B．Kathy persuaded her to do so

C．Zoe got tired of the city life

D．Zoe loved Wales mare than London

3. How does Zoe feel about the country life according to the passage?

A．Tiresome and troublesome.

B．Romantic and peaceful

C．Mentally exhausting but healthy

D．Physically tiring but rewarding.

4. Which of the following is closest to the main idea of the passage?

A．A friend in need is a friend indeed.

B．Where there is a will, there is a way.

C．A misfortune may turn out a blessing.

D．Kill two birds with one stone.

【推荐2】Pete Sparks was a sensitive boy. So sensitive that even his hair was ticklish (怕痒的). You only had to touch his hair a little bit and he would start laughing. When this ticklish laughter started, no one could make him stop.

Pete tried everything to control his ticklishness, but nothing worked. So he began to feel sad for being different.

One day in the street, he met a special clown. The clown was very old and could hardly walk, but when he saw Pete in tears, he went to cheer him up. It didn’t take long to make Pete laugh. Later Pete told him about his ticklish problem, and he asked the clown how such an old man could carry on being a clown. “I have no one to take my place, ”said the clown. “I have a very serious job to do. ”Pete looked at him, surprised. “Come, I’ll show you, ”said the clown. So the clown took Pete to many hospitals full of children who were sick. As soon as they saw the clown, their faces changed completely and lit up with a smile. The short time spent with the clown changed everything for them. That day was even more special, because in every show Pete’s laughter would end up with the kids lying on the floor, dying with laughter. When the visit came to an end, the old clown said to Pete, “Now do you see why I can’t retire, even at my age?”

That was how Pete became a clown, and took the special old man’s place. From that day on, the fact that Pete was different actually made him happy, thanks to his special gift .

1. Pete wanted to change because he wanted to_______.

A．make a difference	B．just be a normal boy
C．make everyone happy	D．just be a special clown

2. The clown took Pete to the hospitals in order to _________.

A．treat his ticklishness	B．encourage him to be a clown
C．show him the meaning of his job	D．know whether Pete had told a lie

3. Which of the following is TRUE about the old clown?

A．He was a sensitive man.	B．He liked his job as a clown.
C．He was unable to make Pete laugh.	D．He knew how to control his ticklishness.

4. What does the story teach us?

A．Practice makes perfect.	B．Everyone is born useful.
C．Well begun is half done.	D．Where there is life, there is hope.