托福阅读TPO46真题Part3阅读原文：Ecosystem Diversity and Stability
Conservation biologists have long been concerned that species extinction could have significant consequences for the stability of entire ecosystems—groups of interacting organisms and the physical environment that they inhabit. An ecosystem could survive the loss of some species, but if enough species were lost, the ecosystem would be severely degraded. In fact, it is possible that the loss of a single important species could start a cascade of extinctions that might dramatically change an entire ecosystem. A good illustration of this occurred after sea otters were eliminated from some Pacific kelp (seaweed) bed ecosystems: the kelp beds were practically obliterated too because in the absence of sea otter predation, sea urchin populations exploded and consumed most of the kelp and other macroalgae.
It is usually claimed that species-rich ecosystems tend to be more stable than species-poor ecosystems. Three mechanisms by which higher diversity increases ecosystem stability have been proposed. First, if there are more species in an ecosystem, then its food web will be more complex, with greater redundancy among species in terms of their nutritional roles. In other words, in a rich system if a species is lost, there is a good chance that other species will take over its function as prey, predator, producer, decomposer, or whatever role it played. Second, diverse ecosystems may be less likely to be invaded by new species, notably exotics (foreign species living outside their native range), that would disrupt the ecosystem’s structure and function. Third, in a species-rich ecosystem, diseases may spread more slowly because most species will be relatively less abundant, thus increasing the average distance between individuals of the same species and hampering disease transmission among individuals.
Scientific evidence to illuminate these ideas has been slow in coming, and many shadows remain. ■ One of the first studies to provide data supporting a relationship between diversity and stability examined how grassland plants responded to a drought. ■ Researchers D. Tilman and J A. Downing used the ratio of above-ground biomass in 1988 (after two years of drought) to that in 1986 (predrought) in 207 plots in a grassland field in the Cedar Creek Natural History Area in Minnesota as an index of ecosystem response to disruption by drought. ■ In an experiment that began in 1982, they compared these values with the number of plant species in each plot and discovered that the plots with a greater number of plant species experienced a less dramatic reduction in biomass. ■ Plots with more than ten species had about half as much biomass in 1988 as in 1986, whereas those with fewer than five species only produced roughly one-eighth as much biomass after the two-year drought. Apparently, species-rich plots were likely to contain some drought-resistant plant species that grew better in drought years, compensating for the poor growth of less-tolerant species.
To put this result in more general terms, a species-rich ecosystem may be more stable because it is more likely to have species with a wide array of responses to variable conditions such as droughts. Furthermore, a species-rich ecosystem is more likely to have species with similar ecological functions, so that if a species is lost from an ecosystem, another species, probably a competitor, is likely to flourish and occupy its functional role. Both of these, variability in responses and functional redundancy, could be thought of as insurance against disturbances.
The Minnesota grassland research has been widely accepted as strong evidence for the diversity- stability theory; however, its findings have been questioned, and similar studies on other ecosystems have not always found a positive relationship between diversity and stability. Clearly, this is a complex issue that requires further field research with a broad spectrum of ecosystems and species: grassland plants and computer models will only take us so far. In the end, despite insightful attempts to detect some general patterns, we may find it very difficult to reduce this topic to a simple, universal truth.
1. The word "significant" in the passage is closest in meaning to
2. According to paragraph 1, why has the extinction of species been a concern for conservation biologists?
O When ecosystems lose just one species, they undergo permanent change.
O The extinction of a particular predator species could cause an overpopulation of certain prey species.
O The loss of one or more species could cause the decline of a whole ecosystem.
O The extinction of a single species is evidence that plant-food sources are in danger of disappearing.
3. According to paragraph 1, what was the result of the removal of Pacific sea otters?
O The kelp and sea urchins were destroyed by new predators.
O The uncontrolled population of sea urchins ate most of the kelp plants.
O Without sea otters, the kelp beds soon became overgrown.
O Macroalgae remained as the primary population in the ecosystem.
4. The word “redundancy” in the passage is closest in meaning to
5. What is the function of paragraph 2 in the passage?
O To present a hypothesis about ecosystem diversity and some reasons why it might be true
O To give examples of types of ecosystems that have the greatest diversity
O To contradict a previous belief about the stability of species-rich ecosystems
O To contrast species-rich and species-poor ecosystems
6. According to paragraph 2, which of the following increases the stability of an
O Species in which producers outnumber predators
O New or exotic species that increase ecosystem diversity
O Heavily populated species that are free of disease
O Species that are diverse but have similar nutritional roles
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.
O In any ecosystem, as the number of individuals in the same species increases, the rate of disease transmission slows down.
O Ecosystems that have a small number of different species tend to be disease-free, because the species’ habitats are at a safe distance from each other.
O In ecosystems with many species, diseases spread more slowly because there are fewer individuals in a species and, as a result, the individuals are more widely scattered.
O The average distance between individuals in a species-rich ecosystem increases, so diseases are prevented from being communicated between species.
8. The phrase ”compensating for" in the passage Is closest in meaning to
O working against
O leaving out
O making up for
O spreading over
9. What Is the main importance of the study discussed in paragraph 3?
O It examines the response of certain grassland plants to a drought.
O It contains an index of plants that survived well in times of drought.
O It provides scientific evidence that diversity helps to make ecosystems stable.
O It shows that ecosystems contain both resistant species and less tolerant ones.
10. Select the TWO answer choices that, according to paragraph 4, are conclusions that can be drawn from the study by Tilman and Downing. To receive credit you must select TWO answer choices.
□ A diverse ecosystem will have species that respond differently to a variety of conditions.
□ Species within a species-rich ecosystem are more likely to have competitors.
□ An ecosystem is more likely to develop diverse and stable species when it is exposed to extreme conditions.
□ Species with similar ecological functions will perform the function of a lost species.
11. The word “detect” in the passage is closest in meaning to
12. According to paragraph 5, which of the following is true about Tilman and Downing’s findings?
O General patterns of diversity and stability have been established as a result of the findings.
O Questions about the findings have been refuted by computer models.
O The findings have been tested in a broad spectrum of ecosystems with similar results.
O The findings are not sufficient to prove a definite link between diversity and stability in ecosystems.
13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
It seems clear that there is room for a great deal more research, although some work has been done.
Where would the sentence best fit? Click on a square [■] to add the sentence to the passage.
14. 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.
Drag your answer choices to the spaces where they belong To remove an answer choice, click on it
To review the passage, click VIEW TEXT
Biologists have long been worried about the possible effect of the extinction of species on whole ecosystems.
Conservation biology studies Indicate that the loss of a single important species may bring temporary change to an ecosystem but it seldom results in lasting damage.
Ecosystems having species with similar functions but different responses to adverse conditions can survive environmental disturbances.
The Minnesota grassland study by Tilman and Downing presented evidence that the
greater the diversity of species in an ecosystem, the more stable the ecosystem.
The absence of sea otter predation in a Pacific kelp bed ecosystem dramatically changed the entire ecosystem by stabilizing the total kelp population.
The findings of the Minnesota grassland study by Tilman and Downing indicated an equal number of drought-resistant and drought-tolerant plant species in species-rich plots.
More research is needed on the relationship between species diversity and ecosystem stability, though a simple explanation is unlikely.
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