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当同一物种或几个不同物种的几个个体依赖于相同的有限资源时，可能会出现一种称为竞争的情况。

When several individuals of the same species or of several different species depend on the same limited resource, a situation may arise that is referred to as competition.

自然学家早就知道竞争的存在，达尔文对竞争的影响进行了相当详细的描述。

The existence of competition has been long known to naturalists; its effects were described by Darwin in considerable detail.

同种个体之间的竞争（种内竞争）是进化生物学关注的问题，也是自然选择的主要机制之一。

Competition among individuals of the same species (intraspecies competition), one of the major mechanisms of natural selection, is the concern of evolutionary biology.

不同物种个体之间的竞争（种间竞争）是生态学关注的主要问题。

Competition among the individuals of different species (interspecies competition) is a major concern of ecology.

这是控制竞争种群大小的因素之一，在极端情况下，它可能会导致一个竞争物种的灭绝。

It is one of the factors controlling the size of competing populations, and in extreme cases it may lead to the extinction of one of the competing species.

达尔文在描述那些引进欧洲竞争物种后灭绝的新西兰本土动植物时，提及了这种情况。

This was described by Darwin for indigenous New Zealand species of animals and plants, which died out when competing species from Europe were introduced.

当主要所需资源供应过剩时，不存在严重的竞争，就像在大多数情况下，草食动物（草食者）共存。

No serious competition exists when the major needed resource is in superabundant supply, as in most cases of the coexistence of herbivores (plant eaters).

此外，大多数物种并不完全依赖于单一的资源。

Furthermore, most species do not depend entirely on a single resource.

如果一个物种的主要资源变得稀缺，该物种通常可以转向替代资源。

If the major resource for a species becomes scarce, the species can usually shift to alternative resources.

如果不止一个物种在争夺稀缺的资源，相互竞争的物种通常会转向不同的替代资源。

If more than one species is competing for a scarce resource, the competing species usually switch to different alternative resources.

在对环境有相似要求的近亲之间，竞争通常是最激烈的。

Competition is usually most severe among close relatives with similar demands on the environment.

但它也可能发生在完全不相关的物种之间，它们争夺相同的资源，比如吃种子的啮齿动物和蚂蚁。

But it may also occur among totally unrelated forms that compete for the same resource, such as seed-eating rodents and ants.

当一个生态系统中的所有动物或所有植物都参与竞争时，这种竞争的影响就形象生动地展示出来了，就像200万年前上新世末期发生的那样，当时北美和南美洲被巴拿马的地峡连接在一起。

The effects of such competition are graphically demonstrated when all the animals or all the plants in an ecosystem come into competition, as happened 2 million years ago at the end of the Pliocene, when North and South America became joined by the Isthmus of Panama.

横跨地峡迁徙的北美和南美物种现在开始相互竞争。

North and South American species migrating across the Isthmus now came into competition with each other.

其结果是很大一部分南美哺乳动物灭绝，它们显然无法承受来自北美入侵物种的竞争——尽管额外的捕食也是一个重要因素。

The result was the extermination of a large fraction of the South American mammals, which were apparently unable to withstand the competition from invading North American species—although added predation was also an important factor.

竞争在多大程度上决定着一个群落的组成和特定物种的密度，一直是引起争议的根源。

To what extent competition determines the composition of a community and the density of particular species has been the source of considerable controversy.

问题是，竞争通常不能直接观察，而必须从一个物种的扩散或增加以及另一个物种的同时减少或消失中推断出来。

The problem is that competition ordinarily cannot be observed directly but must be inferred from the spread or increase of one species and the concurrent reduction or disappearance of another species.

俄罗斯生物学家高斯在实验室进行了无数的两种物种实验，其中一种物种在只有一种资源可用的情况下灭绝。

The Russian biologist G.F. Gause performed numerous two-species experiments in the laboratory, in which one of the species became extinct when only a single kind of resource was available.

在这些实验和野外观察的基础上，基于任何两个物种都不能占据相同的生态位，形成了所谓的竞争排斥定律。

On the basis of these experiments and of field observations, the so-called law of competitive exclusion was formulated, according to which no two species can occupy the same niche.

自那以后，人们发现了许多看似不符合这一定律的例外情况，但通常可以解释为，尽管这两个物种竞争着一个主要的共同资源，但实际上并没有占据完全相同的生态位。

Numerous seeming exceptions to this law have since been found, but they can usually be explained as cases in which the two species, even though competing for a major joint resource, did not really occupy exactly the same niche.

物种之间的竞争在进化上具有相当重要的意义。

Competition among species is of considerable evolutionary importance.

在同一生态位中争夺资源的物种的物理结构倾向于向以允许它们占据不同生态位的方式逐渐演变。

The physical structure of species competing for resources in the same ecological niche tends to gradually evolve in ways that allow them to occupy different niches.

相互竞争的物种也倾向于改变它们的活动范围，以便它们的领土不再重叠。

Competing species also tend to change their ranges so that their territories no longer overlap.

竞争对物种的进化影响被称为“物种选择”；然而，这种描述有潜在的误导性。

The evolutionary effect of competition on species has been referred to as “species selection;” however, this description is potentially misleading.

只有物种中的个体才会受到自然选择的压力。

Only the individuals of a species are subject to the pressures of natural selection.

物种福祉和生存的影响正是该物种所有个体选择影响的结果。

The effect on the well-being and existence of a species is just the result of the effects of selection on all the individuals of the species.

因此，物种选择实际上是个体选择的结果。

Thus species selection is actually a result of individual selection.

竞争可能会发生在任何所需资源（争夺）中。

Competition may occur for any needed resource.

对于动物来说，它通常是食物；对于森林植物来说，它可能是光；对于底栖生物来说，它可能是空间，就像许多浅水底栖海洋生物一样。

In the case of animals it is usually food; in the case of forest plants it may be light; in the case of substrate inhabitants it may be space, as in many shallow-water bottom-dwelling marine organisms.

事实上，它可能是任何一个物理的或者生物的，即对有机体来说必不可少的因素。

Indeed, it may be for any of the factors, physical as well as biotic, that are essential for organisms.

竞争通常越激烈，种群越密集。

Competition is usually the more severe the denser the population.

它与捕食一起，是调节种群增长的最重要的密度制约因子。

Together with predation, it is the most important density-dependent factor in regulating population growth.

The phrase “mechanisms of natural selection” in the passage is closest in meaning to

According to paragraph 1, what is one effect of competition among individuals of different species?

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

In paragraph 1, why does the author mention what happened in New Zealand?

According to paragraph 2, competition is not usually a significant factor among two coexisting species when

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

In paragraph 2, why does the author talk about what happened as a result of North and South America becoming joined at the Isthmus of Panama?

Paragraph 3 supports the idea that Gause’s experiments were important because they

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.

According to paragraph 4, how does competition affect evolution?

According to paragraph 4, “species selection” is a misleading term because it

The word “regulating” 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. That is, as the density of a population increases, competition has a greater impact and leads to greater mortality.  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.

Competition When several individuals of the same species or of several different species depend on the same limited resource, a situation may arise that is referred to as competition. The existence of competition has been long known to naturalists; its effects were described by Darwin in considerable detail. Competition among individuals of the same species (intraspecies competition), one of the major mechanisms of natural selection, is the concern of evolutionary biology. Competition among the individuals of different species (interspecies competition) is a major concern of ecology. It is one of the factors controlling the size of competing populations, and in extreme cases it may lead to the extinction of one of the competing species. This was described by Darwin for indigenous New Zealand species of animals and plants, which died out when competing species from Europe were introduced. No serious competition exists when the major needed resource is in superabundant supply, as in most cases of the coexistence of herbivores (plant eaters). Furthermore, most species do not depend entirely on a single resource. If the major resource for a species becomes scarce, the species can usually shift to alternative resources. If more than one species is competing for a scarce resource, the competing species usually switch to different alternative resources. Competition is usually most severe among close relatives with similar demands on the environment. But it may also occur among totally unrelated forms that compete for the same resource, such as seed-eating rodents and ants. The effects of such competition are graphically demonstrated when all the animals or all the plants in an ecosystem come into competition, as happened 2 million years ago at the end of the Pliocene, when North and South America became joined by the Isthmus of Panama. North and South American species migrating across the Isthmus now came into competition with each other. The result was the extermination of a large fraction of the South American mammals, which were apparently unable to withstand the competition from invading North American species—although added predation was also an important factor. To what extent competition determines the composition of a community and the density of particular species has been the source of considerable controversy. The problem is that competition ordinarily cannot be observed directly but must be inferred from the spread or increase of one species and the concurrent reduction or disappearance of another species. The Russian biologist G.F. Gause performed numerous two-species experiments in the laboratory, in which one of the species became extinct when only a single kind of resource was available. On the basis of these experiments and of field observations, the so-called law of competitive exclusion was formulated, according to which no two species can occupy the same niche. Numerous seeming exceptions to this law have since been found, but they can usually be explained as cases in which the two species, even though competing for a major joint resource, did not really occupy exactly the same niche. Competition among species is of considerable evolutionary importance. The physical structure of species competing for resources in the same ecological niche tends to gradually evolve in ways that allow them to occupy different niches. Competing species also tend to change their ranges so that their territories no longer overlap. The evolutionary effect of competition on species has been referred to as “species selection;” however, this description is potentially misleading. Only the individuals of a species are subject to the pressures of natural selection. The effect on the well-being and existence of a species is just the result of the effects of selection on all the individuals of the species. Thus species selection is actually a result of individual selection. Competition may occur for any needed resource.  In the case of animals it is usually food; in the case of forest plants it may be light; in the case of substrate inhabitants it may be space, as in many shallow-water bottom-dwelling marine organisms.  Indeed, it may be for any of the factors, physical as well as biotic, that are essential for organisms.  Competition is usually the more severe the denser the population.  Together with predation, it is the most important density-dependent factor in regulating population growth.