How does an ecosystem （生態(tài)系統(tǒng)） 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 （掠食動(dòng)物） always eats huge numbers of a single prey （獵物） ，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 （滅絕） 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 （臨界點(diǎn)） ，it is remarkably difficult for them to return.
（1）A strong link is found between two species when a predator CC.
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？ DD
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？ AA
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？ DD
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.
【考點(diǎn)】說(shuō)明文．
【答案】C；D；A；D
【解答】
【點(diǎn)評(píng)】
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發(fā)布：2024/4/20 14:35:0組卷：10引用：3難度：0.5
相似題
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1．It is widely believed that biodiversity（生物多樣性）is promoted to save species.But in reality species exist only as part of ecosystems and cannot survive unless their ecosystems are protected along with as much as possible of the diversity they contain.
Protecting habitats（棲息地）is important for keeping biodiversity.In 2003，over 102，000 habitats covering nearly 19 million square kilometres，or 11.5% of the world's land surface，were under some form of protection.Though this is a great improvement since 1962，when just 1，000 protected habitats were listed，it is still not considered enough to stop the ecocide that is endangering the world's biodiversity.
For a number of environmentalist，protecting the world's 25 biodiversity hotspots（熱點(diǎn)地區(qū)）is critical to saving habitats and species.But many hotspots are endangered already，having lost up to three-quarters of their original plants.
The poor state of most biodiversity hotspots results directly from population growth and migration（遷移） into these areas.A study found that by the mid-1990s around 1.1 billion people lived in these hotspots.Moreover，the annual population growth rate in these areas was 1.8%，higher than the global average of 1.4%.The PAI report concluded that human-related environmental changes will continue to put pressure on hotspots.Therefore，keeping biodiversity requires paying close attention to population size.
Protecting hotspots is not simply a matter of putting up fences and employing guards.The best results are achieved when local people are educated about the value of wildlife，and actually gain a share of the benefits from eco-tourism.Only then do they have a chance to see the benefits of protecting hotpots.
（1）What can we infer from paragraph 1？
A．The loss of any species can affect humans.
B．Endangered species are paid more attention to by humans.
C．Species can still survive when their ecosystems are destroyed.
D．Ecosystem protection is as important as diversity protection.
（2）What does the underlined word "ecocide" in paragraph 2 mean？
A．Ecosystem protection.
B．Reduction of the number of species.
C．Serious damage to habitats.
D．Habitats protection.
（3）What's the direct cause of the terrible state of the hotspots？
A．Population growth in hotpots.
B．Migration out of these areas.
C．Global warming.
D．The global average population growth rate.
（4）What's the author's opinion on protecting the hotspots？
A．It's simple to carry out.
B．It leads to the increase of tourists.
C．It's closely connected with local people.
D．It suggests that the local people are well educated.發(fā)布：2025/1/6 16:30:6組卷：0引用：1難度：0.5 -
2．International students who stayed in the US as their classes moved online during the COVID may be forced to leave，following guidelines of the US Immigration and Customs Enforcement published due to the pandemic（流行?。?
"You know I don't want to put my family at risk，I don't want to put myself at risk to travel during a pandemic and go back to Pakistan，and even if I could delay the program for a year，it is not certain that I will get the visa again." said a Pakistani student.
Many students say they need to be in the US for more than attending cljye.ais.As a PhD student，the library is an extremely important resource for them，although it is because of COVED that those services have been closed，which means even this advantage is gone.
"You know more than 90% of the international students who were studying the US when COVID-19 hit and college campuses were closed have to remain in the US and this creates such trouble in their lives and such uncertainty that is unreasonable." said a lady from a university.
International students staving in the United States say the new ruling has disturbed their plans to return. "I know it is a fact that when it comes to rules，guidelines，laws and stuff like that，international students are sort of treated as low-level citizens.Even I can't even use the word "citizen" to begin with because that's what they make clear we are not."
The government said that the policy speaks for itself and that online coursework is not worth a visa.So far，not any sign of change has been within sight.As students are waiting for the policy to improve，their fall programming and their future remain in limbo.
（1）Why would international students prefer to stay in the US during the pandemic？
A．They couldn't delay the program for a year.
B．They can make full use of the library resource.
C．They wouldn't bring any potential danger to their family.
D．They can learn better with their classes moved online.
（2）What do international students staying in the United States think of the new rule？
A．The new rule treats them as American citizens.
B．The new rule shows great unreasonable opinion against them.
C．The new rule guides them to plan to return to the US.
D．The new rule treats them like low-level students in school.
（3）What is the government's attitude towards the new rule？
A．Favorable.
B．Neutral（中立）.
C．Unfavorable.
D．Unclear.
（4）What does the underlined phrase "in limbo" in the last paragraph mean？
A．Horrible.
B．Flexible.
C．Uncertain.
D．Unbelievable.發(fā)布：2025/1/6 15:30:8組卷：0引用：1難度：0.5 -
3．Most of us spend our lives seeking the natural world.We go fishing，sit in the garden，have a picnic，live in the suburbs or go to the seaside.The most popular leisure activity in Britain is going for a walk.When joggers jog，they don't run on the streets.Every one of them tends to go to the park or the river.
But despite this，our children are growing up naturedeprived（剝奪）.I spent my boyhood climbing trees.These days，children are robbed of the ancient freedom，due to problems like crime，traffic，the loss of the open space and strange new ideas about what is best for children，that is to say，things that can be bought，rather than things that can be found.
The truth is to be found elsewhere.A study in the US：families had moved to better housing and the children were assessed for ADHD—Attention Deficit Hyperactivity Disorder（注意力缺陷多動(dòng)癥）.Those whose accommodation had more natural views showed an improvement of 19%；those who had the same improvement in material surroundings but no nice view improved just 4%.
ADHD is one of the great problems of modern childhood.One study after another indicates that contact with nature gives huge benefits to ADHD children.However，we spend money on drugs rather than on green places.
The life of old people is measurably better when they have access to nature.The increasing concern for the growing population of old people is in quality rather than quantity of years.And study after study finds that a garden is the single most important thing in finding that quality.Even problems with crime and aggressive behaviour are reduced when there is contact with the natural world.
We need the wild world.It is essential to our wellbeing，our health and our happiness.
（1）According to the author，people enjoyto seek nature.
A．jogging on the street
B．running in the gym
C．shopping in the supermarket
D．sitting in the garden
（2）From the second paragraph，we can see that.
A．Children don't want to approach nature
B．children probably spend less time in nature nowadays
C．climbing trees will certainly do good to the children
D．children tend to be happier as a result of their material satisfaction
（3）In what way do people benefit from their contact with nature？
A．Children with ADHD can be cured.
B．Children's performance at school is greatly improved.
C．Problems with crime and violent behavior will easily be solved.
D．A garden nearby improves the quality of old people's life.
（4）what is the main idea of this passage？
A．Access to nature improves our life.
B．Nature treats children for ADHD.
C．Getting close to nature reduces crime.
D．Man can't live without natural areas.發(fā)布：2025/1/6 16:30:6組卷：0引用：1難度：0.5
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