Large ecosystems, such as the Amazon rainforest, will collapse and disappear alarmingly quickly, once a crucial tipping point is reached, according to calculations based on real-world data.
Writing in Nature Communications, researchers from Bangor University, Southampton University and the University of London, reveal the speed at which ecosystems of different sizes will disappear, once they have reached a point beyond which they collapse — transforming into an alternative ecosystem. For example, once the “point of no return” is reached, the Amazon rainforest could shift to a savannah-type (稀树草原型) ecosystem with a mix of trees and grass within 50 years, according to the research. Some scientists argue that many ecosystems are currently teetering (摇晃) on the edge of a cliff, with the fires and destruction of both the Amazon and Australian rainforests.
“Unfortunately, what our paper reveals is that humanity needs to prepare for changes  far sooner than expected,” says joint lead author Dr Simon Willcock of Bangor University.  “These rapid changes to the world’s largest and most typical ecosystems would impact the benefits which they provide us with, including everything from food and materials, to the oxygen and water we need for life.”
Ecosystems made up of a number of interacting species, rather than those dominated by one single species, may be more stable and take longer to shift to alternative ecosystem states. “These ecosystems provide opportunities to mitigate or manage the worst effects”, say the authors. For example, elephants are termed a “key stone” species as they have a disproportionately（不成比例地） large impact on the landscape — pushing over trees, but also dispersing seeds over large distances. The authors state that the loss of key stone species, such as this, would lead to a rapid and dramatic change in the landscape within our lifetime. “This is yet another strong argument to avoid degrading our planet’s ecosystems; we need to do more to conserve biodiversity,” says Dr Gregory Cooper, University of London.

32.What is the phenomenon described in paragraph 2?
A.Many ecosystems are in danger currently.
B.Large ecosystems have reached a tipping point.
C.A new ecosystem will appear in the Amazon forest.
D.Ecosystems are changed mainly due to natural disasters.
33.What is paragraph 3 mainly about?
A.The benefits of large ecosystems.
B.The ways to prepare for the changes.
C.The introduction to typical ecosystems.
D.The influences of ecosystems’ changes on humanity.
34.Which of the following best explains “mitigate” in the last paragraph?
A. cause.    B. ease.    C. increase.    D. judge.
35.Why does the author mention elephants in the passage?
A.To prove the destructive power of elephants.
B.To present the great influences of large animals.
C.To explain the reason for the reducing grassland.
D.To stress the importance of conserving biodiversity.
 32-35 ADBD