Alaska and Bark Beetles
Over the twentieth century, global temperatures increased by an average of about 0.7 degrees Celsius, but some places have warmed a lot more than this, and other places have warmed less. These temperature increases have been enough to trigger changes in ecosystems all over the world, especially in places where the warming has been the greatest. In some places, the changes have been subtle, perhaps a slight shift in vegetation that only a careful observer would notice. In other cases, small changes in climate have sparked a chain of larger effects, leading to massive changes.
The biggest climate-caused ecosystem shifts today are happening at the world’s most northern latitudes, where the temperature over the last century has been rising about two times faster than the global average. In the northernmost state of the United States, Alaska, for example, warming has paved the way for a spike in the numbers of spruce bark beetles. Bark beetles have been a pest to Alaskan white spruce trees for thousands of years, but their numbers were held in check by the cold climate, which forced the insects to hide in the bark of individual trees for most of the year. As the length of the warm season increased over the 1980s and 1990s, however, bark beetles had more time to fly from one tree to the next, burrow, and lay their eggs between the bark and the wood. The beetles had another thing going for them, too: a multi-year drought had weakened many of the spruce trees, leaving them vulnerable to attack. In the mid-1990s, the bark beetle population exploded, and over the next few years the pests wiped out white spruce forests over an area the size of the U.S. state of Connecticut. In the years since, the combined forces of a longer insect-breeding season and forest management practices that left forests overcrowded gave way to similar epidemics farther south. Large swaths of pine and spruce have been destroyed by insects in several other parts of the United States.
In the late 1990s, the effects of the bark beetle epidemic rippled throughout Alaska’s white spruce ecosystem and affected virtually every population of living organism, but not all of the impacts were negative. Fewer spruce trees meant a sunnier area in the forest below the treetops, which allowed grasses to move in and take hold. The grasses, in turn, changed the soil temperature, making the environment more friendly for some other types of vegetation. Animals that feed on grasses, including moose, elk, and some birds, also benefited. But the beetle infestation was bad news for organisms that rely on white spruce for their habitat, like hawks, owls, red squirrels, and voles. Voles a type of small, mouse-like rodent are an especially vital part of the ecosystem because they help spread mycorrhizal fungi, which attach to the roots of plants and help them take in water and nutrients. Voles are also an important food for a number of predators.
Ecosystem changes always hurt some living creatures and help others. It’s hard to say, therefore, whether a change is good or bad overall. Instead, ecologists (people who study ecosystems) often focus on the impacts on a single species: for instance, us. In the short term, the Alaskan spruce beetle epidemic supplied a lot of people with firewood, but only by destroying tons of otherwise valuable timber and threatening the livelihoods of loggers. And no one knows for sure what the long-term impacts on the forest will be. Ecosystems tend to return to their previous states after disturbances like pest outbreaks, fires, or major storm events, but if the Alaskan spruce ecosystem is disturbed too often or too much, it might shift to a different type of forest, a woodland, or a grassland instead.
In extreme cases, major assaults on ecosystems can lead to a total collapse in which the ecosystem doesn’t bounce back to the way it was or transition to a new, healthy state. The result is an area with very little life; in the oceans, biologists refer to these areas as dead zones. One such example is the coral reef die-off that happened in the Indian Ocean in the late 1990s.
【Paragraph 1】Over the twentieth century, global temperatures increased by an average of about 0.7 degrees Celsius, but some places have warmed a lot more than this, and other places have warmed less. These temperature increases have been enough to trigger changes in ecosystems all over the world, especially in places where the warming has been the greatest. In some places, the changes have been subtle, perhaps a slight shift in vegetation that only a careful observer would notice. In other cases, small changes in climate have sparked a chain of larger effects, leading to massive changes.
1. The word “subtle” in the passage is closest in meaning to
A. limited
B. unimportant
C. not obvious
D. gradual
【Paragraph 2】The biggest climate-caused ecosystem shifts today are happening at the world’s most northern latitudes, where the temperature over the last century has been rising about two times faster than the global average. In the northernmost state of the United States, Alaska, for example, warming has paved the way for a spike in the numbers of spruce bark beetles. Bark beetles have been a pest to Alaskan white spruce trees for thousands of years, but their numbers were held in check by the cold climate, which forced the insects to hide in the bark of individual trees for most of the year. As the length of the warm season increased over the 1980s and 1990s, however, bark beetles had more time to fly from one tree to the next, burrow, and lay their eggs between the bark and the wood. The beetles had another thing going for them, too: a multi-year drought had weakened many of the spruce trees, leaving them vulnerable to attack. In the mid-1990s, the bark beetle population exploded, and over the next few years the pests wiped out white spruce forests over an area the size of the U.S. state of Connecticut. In the years since, the combined forces of a longer insect-breeding season and forest management practices that left forests overcrowded gave way to similar epidemics farther south. Large swaths of pine and spruce have been destroyed by insects in several other parts of the United States.
2. Paragraph 2 suggests that the warming of the Alaskan climate affected bark beetles in which of the following ways
A. By making it possible for a beetle to deposit its eggs in a greater number of trees
B. By making it possible for beetles to survive in the bark of trees for longer lengths of time
C. By making it unnecessary for a beetle to protect its eggs by laying them between the bark and the wood
D. By increasing the number of spruce trees, thereby providing the beetles with far more places to live
3. According to paragraph 2, all of the following contributed to the destruction of forests in different parts of the United States EXCEPT
A. a drought that had lasted for several years
B. a lack of forest management practices
C. overcrowding in forests
D. a huge increase in spruce tree pest populations
【Paragraph 3】In the late 1990s, the effects of the bark beetle epidemic rippled throughout Alaska’s white spruce ecosystem and affected virtually every population of living organism, but not all of the impacts were negative. Fewer spruce trees meant a sunnier area in the forest below the treetops, which allowed grasses to move in and take hold. The grasses, in turn, changed the soil temperature, making the environment more friendly for some other types of vegetation. Animals that feed on grasses, including moose, elk, and some birds, also benefited. But the beetle infestation was bad news for organisms that rely on white spruce for their habitat, like hawks, owls, red squirrels, and voles. Voles a type of small, mouselike rodent are an especially vital part of the ecosystem because they help spread mycorrhizal fungi, which attach to the roots of plants and help them take in water and nutrients. Voles are also an important food for a number of predators.
4. Which of the following statements most accurately describes the relationship of paragraph 3 to paragraph2
A. Paragraph 2 explains the causes of the spruce bark beetle epidemic in Alaska, and paragraph 3 discusses the chain of
events that occurred as a result of that epidemic.
B. Paragraph 2 shows that warming air temperatures can affect a large number of species, and paragraph 3 shows that
warming soil temperatures can have even greater effects.
C. Paragraph 2 discusses one explanation for the disappearance of spruce trees from a part of Alaska, but paragraph 3
shows that an alternative explanation is more likely to be correct.
D. Paragraph 2 describes the negative consequences of climate warming for some species, but paragraph 3 shows that there
are also some positive consequences for these same species.
5. According to paragraph 3, which of the following effects did the bark beetle epidemic have on moose, elk, and some birds
A. The epidemic increased the availability of water for these animals.
B. The epidemic increased the availability of food for these animals.
C. The epidemic destroyed the habitat of these animals.
D. The epidemic meant that these animals experienced more competition from hawks, owls, red squirrels, and voles.
6. According to paragraph 3, a decline in the vole population in Alaska may have which TWO of the following consequences？ To receive credit, you must select TWO answer choices.
A. Some predators may have less to eat.
B. Hawk and red squirrel populations may be more successful.
C. Plants may find it more difficult to absorb water and nutrients.
D. Mycorrhizal fungi numbers may increase.
【Paragraph 4】Ecosystem changes always hurt some living creatures and help others. It’s hard to say, therefore, whether a change is good or bad overall. Instead, ecologists (people who study ecosystems) often focus on the impacts on a single species: for instance, us. In the short term, the Alaskan spruce beetle epidemic supplied a lot of people with firewood, but only by destroying tons of otherwise valuable timber and threatening the livelihoods of loggers. And no one knows for sure what the long-term impacts on the forest will be. Ecosystems tend to return to their previous states after disturbances like pest outbreaks, fires, or major storm events, but if the Alaskan spruce ecosystem is disturbed too often or too much, it might shift to a different type of forest, a woodland, or a grassland instead.
7. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage Incorrect choices change the meaning in important ways or leave out essential information.
A. Ecosystems like the spruce ecosystem in Alaska tend to return to their previous states after disturbances such as pest
outbreaks, fires, or major storm events.
B. While ecosystems tend to return to their previous states after disturbances, the Alaskan spruce ecosystem might not if
it is disturbed too often or too much.
C. Ecosystems tend to return to their previous states after disturbances, so Alaska might again become covered with
woodlands or grasslands.
D. After certain types of disturbances such as pest outbreaks, ecosystems do not always return to their previous states but
shift to being woodlands or grasslands instead.
【Paragraph 5】In extreme cases, major assaults on ecosystems can lead to a total collapse in which the ecosystem doesn’t bounce back to the way it was or transition to a new, healthy state. The result is an area with very little life; in the oceans, biologists refer to these areas as dead zones. One such example is the coral reef die-off that happened in the Indian Ocean in the late 1990s.
8. In paragraph 5, coral reefs in the Indian Ocean are presented as an example of which of the following
A. Ecosystems that totally collapsed
B. Ecosystems that transitioned to a new, healthy state
C. Ecosystems that bounced back to the way they were
D. Ecosystems that were affected by a nearby dead zone
Paragraph 4：■Ecosystem changes always hurt some living creatures and help others. It’s hard to say, therefore, whether a change is good or bad overall. ■Instead, ecologists (people who study ecosystems) often focus on the impacts on a single species: for instance, us. ■In the short term, the Alaskan spruce beetle epidemic supplied a lot of people with firewood, but only by destroying tons of otherwise valuable timber and threatening the livelihoods of loggers. ■And no one knows for sure what the long-term impacts on the forest will be. Ecosystems tend to return to their previous states after disturbances like pest outbreaks, fires, or major storm events, but if the Alaskan spruce ecosystem is disturbed too often or too much, it might shift to a different type of forest, a woodland, or a grassland instead.
9. Look at the four squares【■】that indicate where the following sentence could be added to the passage. But even from this limited perspective, the answer is not completely straightforward.
Where would the sentence best fit？
10. 【Directions】 An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Over the twentieth century, global temperatures increased by an average of about 0.7 degrees Celsius.
●
●
●
Answer Choices
A. Global warming has led to changes in ecosystems all over the world, with ecosystems at northern latitudes being affected the most.
B. A longer warm season in Alaska caused a sharp increase in the number of bark beetles, leading to the destruction of spruce forests, which in turn seriously affected many other species.
C. Sometimes ecosystems are able to recover from disturbances or to develop into different, but healthy, systems, but in extreme cases, they may collapse completely.
D. The loss of spruce forests caused an epidemic in mycorrhizal fungi, and these fungi damaged the roots of many plants, making them unable to take in water and nutrients.
E. Whereas some types of changes are good for the majority of species in an ecosystem, ecologists believe that most disturbances to ecosystems are bad overall.
F. Coral reefs may die off as a result of the global increase in temperatures, but after a transition period as a dead zone, they are able to return to their original state.
答案请付费后查阅：
