托福阅读真题Official 46 Passage 3（三）

托福阅读真题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

正确答案：A

题目详解

题型分类：修辞目的题

题干分析：题干问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，属于考察一个段落的主旨。主旨句是本段的第一、二句话。

选项分析：

第2段主要阐述了一个理论：和物种贫瘠的生态系统相比，物种丰富的生态系统可能更稳定些，以及三个支持该理论的观点。

A选项符合要求。

B选项中关于greatest diversity的描述并没有在文中出现，为无中生有。

C选项和D选项的contradict“反驳”和contrast“对比”，这两种关系均未在该段中出现，均属于无中生有。

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

正确答案：D

题目详解

题型分类：事实信息题

原文定位：关键词increase the stability。

选项分析：

第二段主要介绍了三类生物多样性对稳定性所造成的原因，第一种：物种繁多意味着食物网复杂，营养角色具有替代的多样性；第二种，多样性的生态系统不易受到外来物种的侵略；第三，物种多的生态系统使得疾病的扩散很慢。

D选项是对第2段中给出的第一个原因With greater redundancy among species in terms of their nutritional roles.的同义转述，正确答案。

A选项无中生有，并没有提及物种种类和捕食者之间的关系。

B选项无中生有，文中提到物种多样性使得生态系统不易被外来物种入侵，并没有说外来物种会增加物种多样性。

C选项太极端，多样物种的大量存在并不会使其免疫于疾病。