TPO46阅读文本及答案解析

TPO46阅读文本3. Ecosystem Diversity and Stability

Conservation biologists have long been concerned that speciesextinction could have significant consequences for the stability of entireecosystems—groups of interacting organisms and the physical environment thatthey inhabit. An ecosystem could survive the loss of some species, but ifenough species were lost, the ecosystem would be severely degraded. In fact, itis possible that the loss of a single important species could start a cascadeof extinctions that might dramatically change an entire ecosystem. A goodillustration of this occurred after sea otters were eliminated from somePacific kelp (seaweed) bed ecosystems: the kelp beds were practicallyobliterated too because in the absence of sea otter predation, sea urchinpopulations exploded and consumed most of the kelp and other macroalgae.

It is usually claimed that species-rich ecosystems tend to be morestable than species-poor ecosystems. Three mechanisms by which higher diversityincreases ecosystem stability have been proposed. First, if there are morespecies in an ecosystem, then its food web will be more complex, with greaterredundancy 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 otherspecies will take over its function as prey, predator, producer, decomposer, orwhatever role it played. Second, diverse ecosystems may be less likely to beinvaded by new species, notably exotics (foreign species living outside theirnative range), that would disrupt the ecosystem’s structure and function.Third, in a species-rich ecosystem, diseases may spread more slowly becausemost species will be relatively less abundant, thus increasing the averagedistance between individuals of the same species and hampering diseasetransmission among individuals.

Scientific evidence to illuminate these ideas has been slow incoming, and many shadows remain. ■ One of the first studies to provide datasupporting a relationship between diversity and stability examined howgrassland plants responded to a drought. ■ Researchers D. Tilman and JA. Downing used the ratio of above-ground biomass in 1988 (after two years ofdrought) to that in 1986 (predrought) in 207 plots in a grassland field in theCedar Creek Natural History Area in Minnesota as an index of ecosystem responseto disruption by drought. ■ In an experiment that began in 1982, theycompared these values with the number of plant species in each plot anddiscovered that the plots with a greater number of plant species experienced aless dramatic reduction in biomass. ■  Plots with more than ten species had about halfas much biomass in 1988 as in 1986, whereas those with fewer than five speciesonly produced roughly one-eighth as much biomass after the two-year drought.Apparently, species-rich plots were likely to contain some drought-resistantplant species that grew better in drought years, compensating for the poorgrowth of less-tolerant species.

To put this result in more general terms, a species-rich ecosystemmay be more stable because it is more likely to have species with a wide arrayof responses to variable conditions such as droughts. Furthermore, aspecies-rich ecosystem is more likely to have species with similar ecologicalfunctions, 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 bethought of as insurance against disturbances.

The Minnesota grassland research has been widely accepted as strongevidence for the diversity- stability theory; however, its findings have beenquestioned, and similar studies on other ecosystems have not always found apositive relationship between diversity and stability. Clearly, this is acomplex issue that requires further field research with a broad spectrum ofecosystems and species: grassland plants and computer models will only take usso far. In the end, despite insightful attempts to detect some generalpatterns, we may find it very difficult to reduce this topic to a simple,universal truth.

1. The word "significant" in the passage is closest inmeaning to

O      direct

O      important

O      long-term

O      surprising

2. According to paragraph 1, why has the extinction of species beena concern for conservation biologists?

O When ecosystems lose just one species, they undergo permanentchange.

O The extinction of a particular predator species could cause anoverpopulation of certain prey species.

O The loss of one or more species could cause the decline of a wholeecosystem.

O The extinction of a single species is evidence that plant-foodsources are in danger of disappearing.

3. According to paragraph 1, what was the result of the removal ofPacific sea otters?

O The kelp and sea urchins were destroyed by new predators.

O The uncontrolled population of sea urchins ate most of the kelpplants.

O Without sea otters, the kelp beds soon became overgrown.

O Macroalgae remained as the primary population in the ecosystem.

4. The word “redundancy” in the passage is closest in meaning to

O duplication

O variety

O requirements

O flexibility

5. What is the function of paragraph 2 in the passage?

O To present a hypothesis about ecosystem diversity and some reasonswhy it might be true

O To give examples of types of ecosystems that have the greatestdiversity

O To contradict a previous belief about the stability ofspecies-rich ecosystems

O To contrast species-rich and species-poor ecosystems

6. According to paragraph 2, which of the following increases thestability of an ecosystem?

O Species in which producers outnumber predators

O New or exotic species that increase ecosystem diversity

O Heavily populated species that are free of disease

O Species that are diverse but have similar nutritional roles

7. Which of the sentences below best expresses the essentialinformation in the highlighted sentence in the passage? Incorrect choiceschange the meaning in important ways or leave out essential information.

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

O Ecosystems that have a small number of different species tend tobe disease-free, because the species’ habitats are at a safe distance from eachother.

O In ecosystems with many species, diseases spread more slowlybecause there are fewer individuals in a species and, as a result, theindividuals are more widely scattered.

O The average distance between individuals in a species-richecosystem increases, so diseases are prevented from being communicated betweenspecies.

8. The phrase ”compensating for" in the passage Is closest inmeaning to

O working against

O leaving out

O making up for

O spreading over

9. What Is the main importance of the study discussed in paragraph3?

O It examines the response of certain grassland plants to a drought.

O It contains an index of plants that survived well in times ofdrought.

O It provides scientific evidence that diversity helps to makeecosystems stable.

O It shows that ecosystems contain both resistant species and lesstolerant ones.

10. Select the TWO answer choices that, according to paragraph 4,are conclusions that can be drawn from the study by Tilman and Downing. Toreceive credit you must select TWO answer choices.

□ A diverse ecosystem will have species that respond differently toa variety of conditions.

□ Species within a species-rich ecosystem are more likely to havecompetitors.

□ An ecosystem is more likely to develop diverse and stable specieswhen it is exposed to extreme conditions.

□ Species with similar ecological functions will perform thefunction of a lost species.

11. The word “detect” in the passage is closest in meaning to

O repeat

O alter

O find

O emphasize

12. According to paragraph 5, which of the following is true aboutTilman and Downing’s findings?

O General patterns of diversity and stability have been establishedas a result of the findings.

O Questions about the findings have been refuted by computer models.

O The findings have been tested in a broad spectrum of ecosystemswith similar results.

O The findings are not sufficient to prove a definite link betweendiversity and stability in ecosystems.

13. Look at the four squares [■] that indicate where thefollowing 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 [■] toadd the sentence to the passage.

14. Directions: An introductory sentence for a brief summary of thepassage is provided below. Complete the summary by selecting the THREE answerchoices that express the most important ideas in the passage. Some sentences donot belong in the summary because they express ideas that are not presented inthe passage or are minor ideas in the passage. This question is worth 2 points.

Drag your answer choices to the spaces where they belong To removean answer choice, click on it

To review the passage, click VIEW TEXT

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

Answer Choices

Conservation biology studies Indicate that the loss of a singleimportant species may bring temporary change to an ecosystem but it seldom resultsin lasting damage.

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

The Minnesota grassland study by Tilman and Downing presentedevidence that the greater the diversity of species in an ecosystem, the morestable the ecosystem.

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

The findings of the Minnesota grassland study by Tilman and Downingindicated an equal number of drought-resistant and drought-tolerant plantspecies in species-rich plots.

Moreresearch is needed on the relationship between species diversity and ecosystemstability, though a simple explanation is unlikely.

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