托福阅读真题Official 46 Passage 3（五）

托福阅读真题Official 46 Passage 3（五）

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.

Question 9 of 14

What is the main importance of the study discussed in paragraph 3?

A.It examines the response of certain grassland plants to a drought.

B.It contains an index of plants that survived well in times of drought.

C.It provides scientific evidence that diversity helps to make ecosystems stable.

D.It shows that ecosystems contain both resistant species and less tolerant ones.

正确答案：C

题目详解

题型分类：修辞目的题

题干分析：关键词the main importance of the study，考查句子之间的关系。

选项分析：

第三段第1句指出，一些科学证据证实了这些想法，也仍有待探索之处。第2句承上启下，指出，其中一个研究用数据支持了生物多样性与稳定性之间的关系。接下来的内容具体阐述了这个关于草原植物在干旱环境里生长的实验。

C是对第2句中to provide data supporting a relationship between diversity and stability的同义转述，为正确答案。

A选项是对例子的描述，不符合答案。

B选项是对例子的部分描述，不符合答案。

D选项是对例子的部分描述，不符合答案。

Question 10 of 14

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.A diverse ecosystem will have species that respond differently to a variety of conditions.

B.Species within a species-rich ecosystem are more likely to have competitors.

C.An ecosystem is more likely to develop diverse and stable species when it is exposed to extreme conditions.

D.Species with similar ecological functions will perform the function of a lost species.

正确答案：AD

题目详解

题型分类：事实信息题

原文定位：定位词conclusion，by Tilman and Downing。

选项分析：

第4段主要重述了第3段实验的结论，包括两个方面：一是物种丰富的生态系统中可能包含一些对不同自然条件反应不同的物种，这能增加生态系统的稳定性；二是在物种丰富的生态系统中，如果某一个物种消失了，其他的物种可能能够代替其行使相应的职能，从而维持生态系统的稳定。

A选项和D选项分别是对it is more likely to have species with a wide array of responses to variable conditions和a species-rich ecosystem is more likely to have species with similar ecological functions的同义转述，为正确答案。

B选项说在物种丰富的生态系统中，物种更有可能有竞争对手，原文说物种丰富的生态系统更稳定，没有说有更多竞争对手，表述错误。

C选项说发展出的物种不一定稳定，文章中没有提到，无中生有，错误。

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