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 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 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 tow-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.
Paragraph 1: 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 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.
1.The phrase “mechanisms of natural selection” in the passage is closest in meaning to
O types of natural selection
O dangers of natural selection
O problems natural selection solves
O ways natural selection works
2.According to paragraph 1, what is one effect of competition among individuals of different species?
O It results in the eventual elimination of the resource for which they are competing.
O It leads to competition among individuals of the same species.
O It encourages new species to immigrate to an area.
O It controls the number of individuals in the competing populations.
3. The word “indigenous” in the passage is closest in meaning to
O native
O rate
O most
O numerous
4.In paragraph 1, why does the author mention what happened in New Zealand?
O To indicate that Darwin understood the importance of competition
O To illustrate that competition can lead to the extinction of species
O To identify where the idea of competition among species first arose
O To argue against the idea that the process of selection is a natural occurrence
Paragraph 2: 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 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.
5.According to paragraph 2, competition is not usually a significant factor among two coexisting species when
O one of the species has only recently moved into the territory of the other
O the species are closely related to each other
O the population of one species is much larger than that of the other
O both of the species are herbivores
6.The word “graphically” in the passage is closest in meaning to
O vividly
O frequently
O broadly
O typically
7.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?
O To make the point that predation can have as much effect on species survival as competition does
O To show how the ability to switch to an alternative resource can give a species a competitive advantage
O To account for the current species composition of North and South America
O To provide an example of the serious effects of competition between unrelated species
Paragraph 3: 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 tow-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.
8.Paragraph 3 supports the idea that Gause’s experiments were important because they
O provided a situation in which competition could be removed from the interaction between two species
O showed that previous ideas about the extent to which competition determines the composition of a community were completely mistaken
O helped establish that competition will remove all but one species from any given ecological niche
O offered evidence that competition between species is minimal when there is an overabundance of a single food source
9. 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.
O Apparent exceptions to this law usually involves cases in which two species compete for the same major resource but occupy slightly different niches.
O Although it may appear that two species always have different niches, many exceptions show that species compete with each other.
O Cases in which two species not only compete for a shared resource but also occupy similar niches are considered exceptions to this law.
O Cases in which the two species do not occupy that same niche yet still compete for the same resource are believed to be exceptions to this law.
Paragraph 4: 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.
10.According to paragraph 4, how does competition affect evolution?
O It results in the evolution of physical structures that allow the species to compete with each other more effectively.
O It results I the evolutionary extinction of all but one of the competing species.
O It results in the competing species evolving in such a way that they no longer compete for the same resources.
O It results in the competing species evolving to become so much like each other that competition between them eventually disappears.
11.According to paragraph 4, “species selection” is a misleading term because it
O overemphasizes the role of selection pressure in species extinction
O suggests that selection pressures directly influence whole species
O does not make a distinction between species extinction and species evolution
O suggests that extinction always results whenever there is a competition
Paragraph 5: 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.
12.The word “regulating” in the passage is closest in meaning to
O controlling
O explaining
O observing
O stopping
13. 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?
14. 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.
When necessary resources are limited, competition can occur among individuals of the same species or of different species.
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Answer Choices
O Competition can eliminate a species, but since most species do not depend on a single resource, competition is often reduced by switching to alternative resources.
O Investigation of the ecological role of competition is difficult because ordinarily the competition cannot be observed directly and must be inferred from its presumed effects.
O Competition between a pair of species tends to lessen over time because the species tend to evolve to occupy different ecological niches and ranges.
O Competition between individual of the same species is usually for food whereas competition between species is usually for habitat.
O Experiments and field observation have established that competition between species is strong enough to prevent two species from occupying the same ecological niche.
O Competition is usually strongest when the density of the competing populations is the same.
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14. Competition can eliminate…
Competition between individual…
Experiments and field observation…
當同一物種的不同個體或不同的物種都依靠同一有限資源時，這種情況往往會引發(fā)成所謂的競爭。競爭的存在已為生物學家們所熟知，其結果也已被達爾文詳盡敘述過。同一物種不同個體之間的競爭（種內競爭），作為自然選擇的一個主要原理，現(xiàn)在屬于進化生物學。不同物種個體之間的競爭（種間競爭）是生態(tài)學的主要部分。它是控制有競爭關系的種群數(shù)量的一個因素，而其極端情況可能導致競爭的一方滅絕。達爾文在描述新西蘭本土動植物物種在與引進的歐洲物種的競爭中滅絕時講到該情況。
當主要需求的資源能充分供給時，競爭就不會很激烈，正如很多情況下植食動物都能夠共存。而且絕大部分物種并不只依靠一種資源，當某物種的主要資源匱乏時，它們常轉向其它候選資源。如果多個物種競爭同一稀缺資源，他們常會轉向不同的候選資源。近親之間的競爭往往最激烈，因為他們對環(huán)境有相似需求。但激烈競爭也可能發(fā)生在毫不相關卻需要同一資源的物種之間，例如吃種子的嚙齒動物和螞蟻。當一個生態(tài)系統(tǒng)中的所有動植物都參與到競爭中來時競爭的影響將表現(xiàn)得淋漓盡致，比如在兩百萬年前上新世末期當南、北美洲在巴拿馬地峽處聚攏時所發(fā)生的一切。南、北美洲的物種可以穿越地峽而相互競爭。結果是大量南美洲哺乳動物因抵抗不了來自北美洲物種的競爭而滅絕----盡管過度捕殺也是一個很重要的因素。
競爭在多大程度上決定群落的組成和某特定物種的密度一直備受爭議。問題是競爭通常無法直接觀察得到而必須通過某一物種的擴張或增加而另外一種物種同時減少或消失的對比中推測出來。俄羅斯生物學家G. F. Gause進行了大量的兩物種的室內實驗，結果表明當只提供一種資源時其中一個物種將會滅絕�；谝陨蠈嶒灪蛯嵉赜^察，所謂的競爭滅絕法則是成立的，因為兩個物種不可能同時完全占有同一有限資源。不過也發(fā)現(xiàn)了很多例外，但這些情況下兩個物種，盡管會爭奪某一主要資源，但它們爭奪的資源不完全相同。
物種間的競爭對進化至關重要。爭奪同一生態(tài)資源的物種趨向于朝著依賴不同資源的方向進化。相互競爭的物種會逐漸改變他們的活動范圍使其領地不再重疊。競爭對進化的影響稱為“物種選擇”，但這一描述有很大誤導性。只有某一物種中的個體才能面臨自然選擇的壓力。某一物種的繁盛或生存正是其所有個體自然選擇的結果。因此物種選擇實際上是個體選擇的結果。
任何必須的資源都可能引發(fā)競爭。這種資源對動物而言常是食物，對森林植物而言可能是陽光，對地面物種來說可能是空間，比如很多生活在淺海海床上生物。實際上，競爭對象可能是生物所必需的任何生物的或非生物的因素。通常生物密度越大，競爭越激烈。和捕殺一起，競爭是與生物密度相關的調控生物密度增長的重要因素。
