托福阅读真题Official 46 Passage 3（七）

托福阅读真题Official 46 Passage 3（七）

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 13 of 14

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.

正确答案：A

题目详解

题型分类：插入句子题

题干分析：关键词more research，some work。

选项分析：

插入句两部分，第一部分It seems clear that there is room for a great deal more research和前半句有联系，说明前文中已经提及到和research相关的文段。第二部分although some work has been done衔接后文，说明后文中一定有例证说明已经做过的work。原文中从第一个方框往后的句子都在阐述D.Tilman和J A.Downing的实验，而待插入句又是个小结性的句子。

因此，它只可能填入第一个方框选项处，起承上启下的过渡作用。

第二、三、四个方框选项均为后文中对research的进一步说明和解释，逻辑关系与待插入句子不符。

Question 14 of 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.

Biologists have long been worried about the possible effect of the extinction of species on whole ecosystems.

Answer Choices:

A.

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.

B.

The absence of sea otter predation in a Pacific kelp bed ecosystem dramatically changed the entire ecosystem by stabilizing the total kelp population.

C.

Ecosystems having species with similar functions but different responses to adverse conditions can survive environmental disturbances.

D.

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.

E.

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.

F.

More research is needed on the relationship between species diversity and ecosystem stability,though a simple explanation is unlikely.

正确答案：CEF

题目详解

题型分类：文章总结题

题干分析：选择概括性的正确选项。

选项分析：

C选项对应第4段的内容Both of these,variability in responses and functional redundancy,could be thought of as insurance against disturbances，为正确选项。

E选项的表述对应了文章最后一段的第一句话，即The Minnesota grassland research has been widely accepted as strong evidence for the diversity-stability theory，同时在第三段关于T和D的研究中阐明了diversity和stability的关系。

F选项对应最后一段的内容：Clearly,this is a complex issue that requires further field research with a broad spectrum of ecosystems and species，故该选项为正确选项。

A选项第1段中it is possible that the loss of a single important species could start a cascade of extinctions that might dramatically change an entire ecosystem不符。

B选项中关于sea otter移除后kelp的数量的变化关系与原文中第一段最后一句的表述相矛盾，kelp最终消失了，而不是稳定了kelp的数量。

D选项在原文中并没有推测和合理的依据。