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保护生物学家长期以来一直担心，物种灭绝可能对整个生态系统（相互作用的生物群体及其栖息的物理环境）的稳定性产生重大影响。

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.

一个生态系统可以在某些物种消失后继续存在，但是如果足够多的物种消失，生态系统将会严重退化。

An ecosystem could survive the loss of some species, but if enough species were lost, the ecosystem would be severely degraded.

事实上，一个重要物种的消失可能会引发一连串的物种灭绝，这可能会极大地改变整个生态系统。

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.

一个很好的例子是，海獭从一些太平洋海带（海藻）床生态系统中被消灭后：海带床实际上也被毁掉了，因为在没有海獭捕食的情况下，海胆数量激增，吃掉了大部分的海带和其他大型藻类。

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.

人们通常认为物种丰富的生态系统比物种贫乏的生态系统更稳定。

It is usually claimed that species-rich ecosystems tend to be more stable than species-poor ecosystems.

提出了三种关于更高多样性提升生态稳定性的机制。

Three mechanisms by which higher diversity increases ecosystem stability have been proposed.

首先，如果一个生态系统中有更多的物种，那么它的食物网就会更加复杂，物种之间的营养作用也会更加冗余。

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.

换句话说，在一个丰富的系统中，如果一个物种消失了，其他物种很有可能会取代它作为猎物、捕食者、生产者、分解者或它所扮演的任何角色的功能。

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.

其次，多样化的生态系统可能不太会受到新物种的入侵，尤其是外来物种（生活在其原生范围之外的外来物种），这将破坏生态系统的结构和功能。

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.

第三，在一个物种丰富的生态系统中，疾病传播的速度可能会更慢，因为大多数物种的数量相对较少，从而增加了同一物种个体之间的平均距离，阻碍了疾病在个体之间的传播。

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.

阐明这些想法的科学证据进展缓慢，许多问题依然存在。

Scientific evidence to illuminate these ideas has been slow in coming, and many shadows remain.

第一批为多样性和稳定性之间的关系提供数据支持的研究之一，考察了草原植物对干旱的反应。

One of the first studies to provide data supporting a relationship between diversity and stability examined how grassland plants responded to a drought.

研究人员D·蒂尔曼（D·Tilman）和J·A·唐宁（J·A·Downing）在明尼苏达州雪松溪自然历史区一块草地的207个地块上，利用1988年（经过两年干旱）和1986年（干旱前）的地上生物量比例，作为生态系统对干旱破坏响应的指标。

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.

在1982年开始的一项实验中，他们将这些数值与每个地块的植物物种数量进行了比较，发现植物物种数量较多的地块，生物量减少的幅度较小。

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.

1988年，超过10个物种生物量的地块大约是1986年的一半，而在两年的干旱之后，少于5个物种生物量的地块仅为1986年的八分之一左右。

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.

显然，物种丰富的地块可能含有一些抗旱植物物种，它们在干旱的年份生长得更好，弥补了那些不那么耐旱的物种生长不良的情况。

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.

从更一般的角度来说，一个物种丰富的生态系统可能更稳定，因为它更有可能拥有对干旱等多变条件做出广泛反应的物种。

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.

此外，一个物种丰富的生态系统更有可能拥有具有类似生态功能的物种，因此，如果一个物种从生态系统中消失，另一个物种，可能是一个竞争对手，会蓬勃发展，并占据其功能角色。

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.

反应的可变性和功能的多样性这两者，可以被认为是防止干扰的保障。

Both of these, variability in responses and functional redundancy, could be thought of as insurance against disturbances.

明尼苏达草地研究被广泛认为是多样性-稳定性理论的有力证据；然而，它的发现受到了质疑，对其他生态系统的类似研究并不总是得出多样性和稳定性之间的正相关关系。

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.

显然，这是一个复杂的问题，需要对广泛的生态系统和物种进行进一步的实地研究：草原植物和计算机模型只能带我们走这么远。

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.

最后，尽管我们试图发现一些普遍模式，但我们可能会发现很难将这个话题归结为一个简单的、普遍的真理。

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.

According to paragraph 1, why has the extinction of species been a concern for conservation biologists?

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

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

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.

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

What is the main importance of the study discussed in paragraph 3?

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

According to paragraph 5, which of the following is true about Tilman and Downing’s findings?

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.

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.

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.