托福閱讀真題Official 46 Passage 3（五）

托福閱讀真題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.

正確答案：C

題目詳解

題型分類：修辭目的題

題干分析：關(guān)鍵詞the main importance of the study，考查句子之間的關(guān)系。

選項(xiàng)分析：

第三段第1句指出，一些科學(xué)證據(jù)證實(shí)了這些想法，也仍有待探索之處。第2句承上啟下，指出，其中一個(gè)研究用數(shù)據(jù)支持了生物多樣性與穩(wěn)定性之間的關(guān)系。接下來的內(nèi)容具體闡述了這個(gè)關(guān)于草原植物在干旱環(huán)境里生長的實(shí)驗(yàn)。

C是對第2句中to provide data supporting a relationship between diversity and stability的同義轉(zhuǎn)述，為正確答案。

A選項(xiàng)是對例子的描述，不符合答案。

B選項(xiàng)是對例子的部分描述，不符合答案。

D選項(xiàng)是對例子的部分描述，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.

正確答案：AD

題目詳解

題型分類：事實(shí)信息題

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

選項(xiàng)分析：

第4段主要重述了第3段實(shí)驗(yàn)的結(jié)論，包括兩個(gè)方面：一是物種豐富的生態(tài)系統(tǒng)中可能包含一些對不同自然條件反應(yīng)不同的物種，這能增加生態(tài)系統(tǒng)的穩(wěn)定性；二是在物種豐富的生態(tài)系統(tǒng)中，如果某一個(gè)物種消失了，其他的物種可能能夠代替其行使相應(yīng)的職能，從而維持生態(tài)系統(tǒng)的穩(wěn)定。

A選項(xiàng)和D選項(xiàng)分別是對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的同義轉(zhuǎn)述，為正確答案。

B選項(xiàng)說在物種豐富的生態(tài)系統(tǒng)中，物種更有可能有競爭對手，原文說物種豐富的生態(tài)系統(tǒng)更穩(wěn)定，沒有說有更多競爭對手，表述錯(cuò)誤。

C選項(xiàng)說發(fā)展出的物種不一定穩(wěn)定，文章中沒有提到，無中生有，錯(cuò)誤。

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