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植物群落可以灵活地组合起来，其特定结构取决于该地区的特定历史。

Plant communities assemble themselves flexibly, and their particular structure depends on the specific history of the area

生态学家使用“更迭”一词来指代植物群落和生态系统随时间发生的变化。

Ecologists use the term “succession” to refer to the changes that happen in plant communities and ecosystems over time

更迭中的第一个群落称为先锋群落，而更迭最后阶段出现的长期性群落被称为顶级群落。

The first community in a succession is called a pioneer community, while the long-lived community at the end of succession is called a climax community

据说先锋群落和中间群落变化区间1-500年时间之内。

Pioneer and successional plant communities are said to change over periods from 1 to 500 years

这些变化（植物数量和物种混合）是累积的。

These changes—in plant numbers and the mix of species—are cumulative

顶级群落本身会发生变化，但时间超过500年。

Climax communities themselves change but over periods of time greater than about 500 years.

如今研究池塘的生态学家很可能会在一年的时间内发现它都相对不变。

An ecologist who studies a pond today may well find it relatively unchanged in a year’s time

某条鱼可能会被换掉，但是鱼的数量倾向于年复一年保持相同。

Individual fish may be replaced, but the number of fish will tend to be the same from one year to the next

可以说，生态系统的特性比组成生态系统的单个生物更稳定。

We can say that the properties of an ecosystem are more stable than the individual organisms that compose the ecosystem.

生态学家一度相信物种多样性使生态系统稳定。

At one time, ecologists believed that species diversity made ecosystems stable

他们认为生物多样性越高，生态系统就越稳定。

They believed that the greater the diversity the more stable the ecosystem

这一观点来自以下观察：与先锋群落相比，长久存在的顶级群落通常具有更复杂的食物网和更高的物种多样性。

Support for this idea came from the observation that long-lasting climax communities usually have more complex food webs and more species diversity than pioneer communities

生态学家得出结论，顶级生态系统的稳定性取决于其复杂性。

Ecologists concluded that the apparent stability of climax ecosystems depended on their complexity

举一个极端的例子，以单一作物为主的农田非常不稳定，一年的恶劣天气或某种害虫的入侵就会破坏整个作物。

To take an extreme example, farmlands dominated by a single crop are so unstable that one year of bad weather or the invasion of a single pest can destroy the entire crop

相反，复杂的顶级群落（如温带森林）则能够承受天气或害虫造成的巨大破坏。

In contrast, a complex climax community, such as a temperate forest, will tolerate considerable damage from weather or pests.

但是，生态系统稳定性的问题很复杂。

The question of ecosystem stability is complicated, however

第一个问题是生态学家们对“稳定性”含义的看法不同。

The first problem is that ecologists do not all agree what “stability” means

稳定性可以简单定义为缺乏变化。

Stability can be defined as simply lack of change

在那种情况下，顶级群落将被认为是最稳定的，因为根据定义，它随着时间的推移变化最小。

In that case, the climax community would be considered the most stable, since, by definition, it changes the least over time

或者，可以将稳定性定义为生态系统在发生重大干扰（例如火灾）后恢复为特定形态的速度。

Alternatively, stability can be defined as the speed with which an ecosystem returns to a particular form following a major disturbance, such as a fire

This kind of stability is also called resilience

在这种情况下，顶级群落将是最脆弱、最不稳定的群落，因为它们可能需要数百年才能恢复到顶级（群落）状态。

In that case, climax communities would be the most fragile and the least stable, since they can require hundreds of years to return to the climax state.

即使稳定性被简单定义为缺乏变化，它也不一定总是与最大化生物多样性相关。

Even the kind of stability defined as simple lack of change is not always associated with maximum diversity

至少在温带地区，最大化的生物多样性通常出现在中间更迭群落，而不是在顶级群落中。

At least in temperate zones, maximum diversity is often found in mid-successional stages, not in the climax community

例如，红杉森林成熟（到达顶级群落）后，林地上的物种种类和个体数量都会减少。

Once a redwood forest matures, for example, the kinds of species and the number of individuals growing on the forest floor are reduced

一般来说，多样性本身并不能确保稳定。

In general, diversity, by itself, does not ensure stability

生态系统的数学模型同样表明，多样性并不能保证生态系统的稳定性，实际上恰恰相反。

Mathematical models of ecosystems likewise suggest that diversity does not guarantee ecosystem stability—just the opposite, in fact

一般来说，更复杂的系统比简单系统更容易崩溃。

A more complicated system is, in general, more likely than a simple system to break down

（与儿童三轮车相比，一辆15英里速度行进赛车自行车更容易发生故障。）

(A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle.)

因为全世界的顶级群落都正受到人类活动的严重破坏或毁灭，生态学家特别想知道是什么因素有助于提高群落的恢复力。

Ecologists are especially interested to know what factors contribute to the resilience of communities because climax communities all over the world are being severely damaged or destroyed by human activities

例如，在美国西北部，圣海伦斯山的火山爆炸造成的破坏与人类造成的破坏相比则黯然失色。

The destruction caused by the volcanic explosion of Mount St Helens, in the northwestern United States, for example, pales in comparison to the destruction caused by humans

我们需要知道群落在哪些方面对于其抵抗破坏及其恢复最为重要。

We need to know what aspects of a community are most important to the community’s resistance to destruction, as well as its recovery

如今许多生态学家认为，顶级群落的相对长期稳定性不是来自多样性，而是来自环境的“板块分布性”。与单一环境相比，差异化板块状分布的环境可以支持的生物种类更多。

Many ecologists now think that the relative long-term stability of climax communities comes not from diversity but from the “patchiness” of the environment; an environment that varies from place to place supports more kinds of organisms than an environment that is uniform

濒临灭绝的当地物种会迅速被邻近群落的移民取代。

A local population that goes extinct is quickly replaced by immigrants from an adjacent community

即使新的种群属于不同的物种，它也可以填补已灭绝种群腾出的空缺，并保持食物网络完整。

Even if the new population is of a different species, it can approximately fill the niche vacated by the extinct population and keep the food web intact.

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

According to paragraph 1, which of the following is NOT true of climax communities?

According to paragraph 2, which of the following principles of ecosystems can be learned by studying a pond?

According to paragraph 3, ecologists once believed that which of the following illustrated the most stable ecosystems?

According to paragraph 4, why is the question of ecosystem stability complicated?

According to paragraph 4, which of the following is true of climax communities?

Which of the following can be inferred from paragraph 5 about redwood forests?

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

In paragraph 5, why does the author provide the information that “(A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle.)”?

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

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 word “adjacent” in the passage is closest in meaning to

Look at the four squares  that indicate where the following sentence could be added to the passage. In fact, damage to the environment by humans is often much more severe than damage by natural events and processes.  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.

The Long-Term Stability of Ecosystems Plant communities assemble themselves flexibly, and their particular structure depends on the specific history of the area. Ecologists use the term “succession” to refer to the changes that happen in plant communities and ecosystems over time. The first community in a succession is called a pioneer community, while the long-lived community at the end of succession is called a climax community. Pioneer and successional plant communities are said to change over periods from 1 to 500 years. These changes—in plant numbers and the mix of species—are cumulative. Climax communities themselves change but over periods of time greater than about 500 years. An ecologist who studies a pond today may well find it relatively unchanged in a year’s time. Individual fish may be replaced, but the number of fish will tend to be the same from one year to the next. We can say that the properties of an ecosystem are more stable than the individual organisms that compose the ecosystem. At one time, ecologists believed that species diversity made ecosystems stable. They believed that the greater the diversity the more stable the ecosystem. Support for this idea came from the observation that long-lasting climax communities usually have more complex food webs and more species diversity than pioneer communities. Ecologists concluded that the apparent stability of climax ecosystems depended on their complexity. To take an extreme example, farmlands dominated by a single crop are so unstable that one year of bad weather or the invasion of a single pest can destroy the entire crop. In contrast, a complex climax community, such as a temperate forest, will tolerate considerable damage from weather or pests. The question of ecosystem stability is complicated, however. The first problem is that ecologists do not all agree what “stability” means. Stability can be defined as simply lack of change. In that case, the climax community would be considered the most stable, since, by definition, it changes the least over time. Alternatively, stability can be defined as the speed with which an ecosystem returns to a particular form following a major disturbance, such as a fire. This kind of stability is also called resilience. In that case, climax communities would be the most fragile and the least stable, since they can require hundreds of years to return to the climax state. Even the kind of stability defined as simple lack of change is not always associated with maximum diversity. At least in temperate zones, maximum diversity is often found in mid-successional stages, not in the climax community. Once a redwood forest matures, for example, the kinds of species and the number of individuals growing on the forest floor are reduced. In general, diversity, by itself, does not ensure stability. Mathematical models of ecosystems likewise suggest that diversity does not guarantee ecosystem stability—just the opposite, in fact. A more complicated system is, in general, more likely than a simple system to break down. (A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle.) Ecologists are especially interested to know what factors contribute to the resilience of communities because climax communities all over the world are being severely damaged or destroyed by human activities.  The destruction caused by the volcanic explosion of Mount St. Helens, in the northwestern United States, for example, pales in comparison to the destruction caused by humans.  We need to know what aspects of a community are most important to the community’s resistance to destruction, as well as its recovery.  Many ecologists now think that the relative long-term stability of climax communities comes not from diversity but from the “patchiness” of the environment; an environment that varies from place to place supports more kinds of organisms than an environment that is uniform. A local population that goes extinct is quickly replaced by immigrants from an adjacent community. Even if the new population is of a different species, it can approximately fill the niche vacated by the extinct population and keep the food web intact.