托福閱讀真題Official 46 Passage 3（三）

托福閱讀真題Official 46 Passage 3（三）

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 5 of 14

What is the function of paragraph 2 in the passage?

A.To present a hypothesis about ecosystem diversity and some reasons why it might be true

B.To give examples of types of ecosystems that have the greatest diversity

C.To contradict a previous belief about the stability of species-rich ecosystems

D.To contrast species-rich and species-poor ecosystems

正確答案：A

題目詳解

題型分類：修辭目的題

題干分析：題干問的是第二段整段在文章中的作用，那么要找到第二段的段落大意，屬于考察一個段落的主旨。主旨句是本段的第一、二句話。

選項分析：

第2段主要闡述了一個理論：和物種貧瘠的生態(tài)系統(tǒng)相比，物種豐富的生態(tài)系統(tǒng)可能更穩(wěn)定些，以及三個支持該理論的觀點。

A選項符合要求。

B選項中關(guān)于greatest diversity的描述并沒有在文中出現(xiàn)，為無中生有。

C選項和D選項的contradict“反駁”和contrast“對比”，這兩種關(guān)系均未在該段中出現(xiàn)，均屬于無中生有。

Question 6 of 14

According to paragraph 2,which of the following increases the stability of an ecosystem?

A.Species in which producers outnumber predators

B.New or exotic species that increase ecosystem diversity

C.Heavily populated species that are free of disease

D.Species that are diverse but have similar nutritional roles

正確答案：D

題目詳解

題型分類：事實信息題

原文定位：關(guān)鍵詞increase the stability。

選項分析：

第二段主要介紹了三類生物多樣性對穩(wěn)定性所造成的原因，第一種：物種繁多意味著食物網(wǎng)復(fù)雜，營養(yǎng)角色具有替代的多樣性；第二種，多樣性的生態(tài)系統(tǒng)不易受到外來物種的侵略；第三，物種多的生態(tài)系統(tǒng)使得疾病的擴散很慢。

D選項是對第2段中給出的第一個原因With greater redundancy among species in terms of their nutritional roles.的同義轉(zhuǎn)述，正確答案。

A選項無中生有，并沒有提及物種種類和捕食者之間的關(guān)系。

B選項無中生有，文中提到物種多樣性使得生態(tài)系統(tǒng)不易被外來物種入侵，并沒有說外來物種會增加物種多樣性。

C選項太極端，多樣物種的大量存在并不會使其免疫于疾病。

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