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.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.
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
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.
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.
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.
By the end of the century，if not sooner，the world’s oceans will be bluer and greener thanks to a warming climate，according to a new study.
At the heart of the phenomenon lie tiny marine microorganisms(海洋微生物) called phytoplankton. Because of the way light reflects off the organisms，these phytoplankton create colourful patterns at the ocean surface. Ocean colour varies from green to blue，depending on the type and concentration of phytoplankton. Climate change will fuel the growth of phytoplankton in some areas，while reducing it in other spots，leading to changes in the ocean's appearance.
Phytoplankton live at the ocean surface，where they pull carbon dioxide(二氧化碳) into the ocean while giving off oxygen. When these organisms die，they bury carbon in the deep ocean，an important process that helps to regulate the global climate. But phytoplankton are vulnerable to the ocean's warming trend. Warming changes key characteristics of the ocean and can affect phytoplankton growth，since they need not only sunlight and carbon dioxide to grow，but also nutrients.
Stephanie Dutkiewicz，a scientist in MIT's Center for Global Change Science，built a climate model that projects changes to the oceans throughout the century. In a world that warms up by 3℃，it found that multiple changes to the colour of the oceans would occur. The model projects that currently blue areas with little phytoplankton could become even bluer. But in some waters，such as those of the Arctic，a warming will make conditions riper for phytoplankton，and these areas will turn greener. “Not only are the quantities of phytoplankton in the ocean changing. ”she said，“but the type of phytoplankton is changing. ”
1.What are the first two paragraphs mainly about?
A. The various patterns at the ocean surface.
B. The cause of the changes in ocean colour.
C. The way light reflects off marine organisms.
D. The efforts to fuel the growth of phytoplankton.
2.What does the underlined word “vulnerable” in Paragraph 3 probably mean?
A. Sensitive. B. Beneficial
C. Significant D. Unnoticeable
3.What can we learn from the passage?
A. Phytoplankton play a declining role in the marine ecosystem.
B. Dutkiewicz's model aims to project phytoplankton changes
C. Phytoplankton have been used to control global climate
D. Oceans with more phytoplankton may appear greener.
4.What is the main purpose of the passage？
A. To assess the consequences of ocean colour changes
B. To analyse the composition of the ocean food chain
C. To explain the effects of climate change on oceans
D. To introduce a new method to study phytoplankton
California has lost half its big trees since the 1930s, according to a study to be published Tuesday and climate change seems to be a major factor(因素).
The number of trees larger than two feet across has declined by 50 percent on more than 46, 000 square miles of California forests, the new study finds. No area was spared or unaffected, from the foggy northern coast to the Sierra Nevada Mountains to the San Gabriels above Los Angeles. In the Sierra high country, the number of big trees has fallen by more than 55 percent; in parts of southern California the decline was nearly 75 percent.
Many factors contributed to the decline, said Patrick McIntyre, an ecologist who was the lead author of the study. Woodcutters targeted big trees. Housing development pushed into the woods. Aggressive wildfire control has left California forests crowded with small trees that compete with big trees for resources(资源).
But in comparing a study of California forests done in the 1920s and 1930s with another one between 2001 and 2010, McIntyre and his colleagues documented a widespread death of big trees that was evident even in wildlands protected from woodcutting or development.
The loss of big trees was greatest in areas where trees had suffered the greatest water shortage. The researchers figured out water stress with a computer model that calculated how much water trees were getting in comparison with how much they needed, taking into account such things as rainfall, air temperature, dampness of soil, and the timing of snowmelt(融雪).
Since the 1930s, McIntyre said, the biggest factors driving up water stress in the state have been rising temperatures, which cause trees to lose more water to the air, and earlier snowmelt, which reduces the water supply available to trees during the dry season.
1.What is the second paragraph mainly about?
A.The seriousness of big-tree loss in California.
B.The increasing variety of California big trees.
C.The distribution of big trees in California forests.
D.The influence of farming on big trees in California.
2.Which of the following is well-intentioned but may be bad for big trees?
A.Ecological studies of forests.
B.Banning woodcutting.
C.Limiting housing development.
D.Fire control measures.
3.What is a major cause of the water shortage according to McIntyre?
A.Inadequate snowmelt. B.A longer dry season.
C.A warmer climate. D.Dampness of the air.
4.What can be a suitable title for the text?
A.California’s Forests: Where Have All the Big Trees Gone?
B.Cutting of Big Trees to Be Prohibited in California Soon
C.Why Are the Big Trees Important to California Forests?
D.Patrick McIntyre: Grow More Big Trees in California
假如你是《当代中学生》的主编Li Hua，你收到了一封读者来信，请根据信件内容，给Confused daughter写封建议信。
Dear Sir,
I have a mum who is just like a camera in the home. She wants to know everything about me and decides everything for me. I know that is her special love to me, yet I still hope she can respect and trust me. Could you please give me some advice?
Best regards,
Confused daughter
注意:1. 词数100左右;
2. 信的开头已为你写出，不计入总词数。
参考词句:
私人空间: private space
代 沟:generation gap
Dear Confused daughter,
I’m deeply concerned about your situation.
__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Yours,
Li Hua
根据必修一、必修二背诵段落，准确补全以下句子。
1.I can well remember that _____ a deep blue sky, the song of the birds, moonlight and flowers could never have kept me spellbound.
2.Although she didn’t know the best way of getting to places, she insisted that _____ properly.
3.“The last thirty years have seen the greatest number of laws stopping our rights and progress, until today we have reached a stage _____.”
4.Only athletes who have reached the agreed standard for their event _____.
5.I have even been put into space rockets and sent to explore the Moon and Mars. Anyhow, my goal is _____ a life of high quality.
根据首字母或所给汉语意思，写出空缺单词的正确形式(每空只写一词；请使用必修一、二单词表词汇)。
1.The anti-smoking movement has only partly _______(成功).
2.Anytime we give a speech, we should speak loud and _______(自信).
3.It’s _______(恐惧) to think how easily children can be hurt.
4.We are discussing a theme _______(探索) by Mrs. Gaskell in her later novels.
5.I notice that the _______(广告) misses out the price of the product.
6.We finished the project three weeks ahead of s_______.
7.He may look young, but he is a________45 years old.
8.My boss is r________ from a heart attack.
9.Jenifer cut the cake into six e________ parts, so everyone can have the same piece.
10.The Titanic s________after hitting an iceberg.
