托福閱讀真題Official 46 Passage 3（四）

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 7 of 14

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.In any ecosystem,as the number of individuals in the same species increases,the rate of disease transmission slows down.

B.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.

C.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.

D.The average distance between individuals in a species-rich ecosystem increases,so diseases are prevented from being communicated between species.

正確答案：C

題目詳解

題(ti)型分類：簡化句子題(ti)

題干(gan)分析：高亮句的(de)意思是，在(zai)一個(ge)物(wu)種豐富(fu)的(de)生態系統里，疾病可能(neng)會傳(chuan)播得(de)更慢(man)些，因(yin)為絕大(da)(da)多數物(wu)種的(de)數量(liang)不那么龐大(da)(da)most species will be relatively less abundant，所以這使得(de)同一個(ge)物(wu)種的(de)個(ge)體能(neng)有更大(da)(da)的(de)空間(jian)，這能(neng)阻礙疾病在(zai)個(ge)體間(jian)的(de)傳(chuan)播diseases may spread more slowly；increasing the average distance between individuals of the same species；hampering disease transmission among individuals。

選項分析：

C選項是原文的同(tong)義改寫，且(qie)邏(luo)輯正確。

A選項并不是(shi)(shi)在any ecosystem，而是(shi)(shi)in a species-rich ecosystem，因此表述不正確，后(hou)半句由(you)于原文的邏(luo)輯因果關系(xi)顛倒。

B選項表(biao)述和原文(wen)的因果邏(luo)輯(ji)顛倒(dao)。

D選(xuan)項只說明(ming)了結果，并沒有說出(chu)原因(yin)。

Question 8 of 14

The phrase“compensating for”in the passage is closest in meaning to

A.working against

B.leaving out

C.making up for

D.spreading over

正確答案：C

題目詳解

題型分類：詞匯題

選項分析：

詞匯所在(zai)句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.原(yuan)句意為(wei)：在(zai)物(wu)種豐富的地里可能會包(bao)含一些抗(kang)旱的植(zhi)物(wu)，這(zhe)些植(zhi)物(wu)能在(zai)干旱的年份里生(sheng)長得(de)更(geng)好，從而彌補(bu)那(nei)些不怎么(me)抗(kang)旱植(zhi)物(wu)的收成(cheng)。這(zhe)里compensating for意為(wei)“彌補(bu)”。

C選項：making up for意為(wei)償還，賠(pei)償，符(fu)合文意。

A選項：working against意為對……不利(li)，反對。

B選項：leaving out意為遺漏，忽略。

D選項：spreading over意為遍布(bu)。

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