Directions: Read the passage. Then answer the questions. Give yourself 20 minutes to complete this practice set.

WHAT IS A COMMUNITY?

The Black Hills forest, the prairie riparian forest, and other forests of the western United States can be separated by the distinctly different combinations of species they comprise. It is easy to distinguish between prairie riparian forest and Black Hills forest—one is a broad-leaved forest of ash and cottonwood trees, the other is a coniferous forest of ponderosa pine and white spruce trees. One has kingbirds; the other, juncos (birds with white outer tail feathers). The fact that ecological communities are, indeed, recognizable clusters of species led some early ecologists, particularly those living in the beginning of the twentieth century, to claim that communities are highly integrated, precisely balanced assemblages. This claim harkens back to even earlier arguments about the existence of a balance of nature, where every species is there for a specific purpose, like a vital part in a complex machine. Such a belief would suggest that to remove any species, whether it be plant, bird, or insect, would somehow disrupt the balance, and the habitat would begin to deteriorate. Likewise, to add a species may be equally disruptive.
One of these pioneer ecologists was Frederick Clements, who studied ecology extensively throughout the Midwest and other areas in North America. He held that within any given region of climate, ecological communities tended to slowly converge toward a single endpoint, which he called the "climatic climax." This "climax" community was, in Clements's mind, the most well-balanced, integrated grouping of species that could occur within that particular region. Clements even thought that the process of ecological succession—the replacement of some species by others over time—was somewhat akin to the development of an organism, from embryo to adult. Clements thought that succession represented discrete stages in the development of the community (rather like infancy, childhood, and adolescence), terminating in the climatic "adult" stage, when the community became self-reproducing and succession ceased. Clements's view of the ecological community reflected the notion of a precise balance of nature.
Clements was challenged by another pioneer ecologist, Henry Gleason, who took the opposite view. Gleason viewed the community as largely a group of species with similar tolerances to the stresses imposed by climate and other factors typical of the region. Gleason saw the element of chance as important in influencing where species occurred. His concept of the community suggests that nature is not highly integrated. Gleason thought succession could take numerous directions, depending upon local circumstances.
Who was right? Many ecologists have made precise measurements, designed to test the assumptions of both the Clements and Gleason models. For instance, along mountain slopes, does one life zone, or habitat type, grade sharply or gradually into another? If the divisions are sharp, perhaps the reason is that the community is so well integrated, so holistic, so like Clements viewed it, that whole clusters of species must remain together. If the divisions are gradual, perhaps, as Gleason suggested, each species is responding individually to its environment, and clusters of species are not so integrated that they must always occur together,
It now appears that Gleason was far closer to the truth than Clements. The ecological community is largely an accidental assemblage of species with similar responses to a particular climate. Green ash trees are found in association with plains cottonwood trees because both can survive well on floodplains and the competition between them is not so strong that only one can persevere. One ecological community often flows into another so gradually that it is next to impossible to say where one leaves off and the other begins. Communities are individualistic.
This is not to say that precise harmonies are not present within communities. Most flowering plants could not exist were it not for their pollinators—and vice versa. Predators, disease organisms, and competitors all influence the abundance and distribution of everything from oak trees to field mice. But if we see a precise balance of nature, it is largely an artifact of our perception, due to the illusion that nature, especially a complex system like a forest, seems so unchanging from one day to the next.

单选题 in paragraph 1, why does the author distinguish between prairie riparian forest and Black Hills forest?

• A. To highlight the difference between the views of various ecologists about the nature of ecological communities
• B. To illustrate why some ecologists tended to view ecological communities as highly integrated
• C. To demonstrate that one forest has a greater variety of species than the other
• D. To show how these two forests differ from others in the United States

单选题 According to paragraph 1, what was a common claim about ecological communities before the early twentieth century?

• A. Every species in a community has a specific role in that community.
• B. It is important to protect communities by removing certain species.
• C. A precise balance is difficult to maintain in an ecological community.
• D. It is necessary for new species to be added quickly as ecological communities develop.

单选题 The word "clusters" in the passage is closest in meaning to

• A. models
• B. categories
• C. examples
• D. groups

单选题 According to paragraph 1, the belief in a balance of nature suggests that removing a species from an ecological community would have which of the following effects?

• A. It would reduce competition between the remaining species of the community.
• B. It would produce a different, but equally balanced, community.
• C. It would lead to a decline in the community.
• D. It would cause more harm than adding a species to the community.

单选题 The word "ceased" in the passage is closest in meaning to

• A. succeeded
• B. balanced
• C. ended
• D. advanced

单选题 Which of the following best represents the view of ecological communities associated with Frederick Clements in paragraph 2?

• A. Only when all species in a community are at the reproductive stage of development is an ecological community precisely balanced.
• B. When an ecological community achieves "climatic climax," it begins to decline.
• C. All climates have similar climax communities.
• D. Ecological communities eventually reach the maximum level of balance that is possible for their region.

单选题 According to paragraph 2, Clements compared the process of ecological succession to

• A. the replacement of animal habitats over time
• B. the development of an organism
• C. self-reproduction
• D. changes in climate

单选题 According to Gleason in paragraph 3, the occurrence of a species in a particular community is influenced by

• A. unpredictable events
• B. how individualistic the species is
• C. the number of other species present
• D. the tolerance of other species to stresses

单选题 What did the ecologists in paragraph 4 hope to determine with their measurements?

• A. Whether different species compete for the same environments
• B. Whether habitats are sharply separated or gradually flow into each other
• C. Whether succession differs in different types of habitats
• D. Whether integrated communities survive better than independent communities

单选题 In paragraph 5, why does the author mention green ash trees and plains cottonwood trees?

• A. To support the current view about how ecological communities develop
• B. To provide an example of species that prefer to live on floodplains
• C. To provide evidence that supports the theory of Clements
• D. To show where one ecological community stops and the other begins

单选题 The word "persevere" in the passage is closest in meaning to

• A. reproduce
• B. fail
• C. expand
• D. continue

单选题 Which of the sentences below best expresses the essential information in the highlighted sentence in paragraph 6? Incorrect choices change the meaning in important ways or leave out essential information.

• A. We see nature as precisely balanced because nature is unchanging.
• B. A precise balance of nature is not possible because of the complexity of natural systems.
• C. Our sense that nature is precisely balanced results from the illusion that it is unchanging.
• D. Because nature is precisely balanced, complex systems do not seem to change.

单选题 Look at the four squares [■] that indicate where the following sentence can be added to the passage.
Their research has helped to decide between the two views because it has focused on questions to which Clements and Gleason would give opposing answers.
Where would the sentence best fit?

• A. Their research has helped to decide between the two views because it has focused on questions to which Clernents and Gleason would give opposing answers. Who was right? ■ Many ecologists have made precise measurements, designed to test the assumptions of both the Clements and Gleason models. ■ For instance, along mountain slopes, does one life zone, or habitat type, grade sharply or gradually into another? ■ If the divisions are sharp, perhaps the reason is that the community is so well integrated, so holistic, so like Clements viewed it, that whole clusters of species must remain together. If the divisions are gradual, perhaps, as Gleason suggested, each species is responding individually to its environment, and clusters of species are not so integrated that they must always occur together.
• B. ■ Who was right? Their research has helped to decide between the two views because it has focused on questions to which Clements and Gleason would give opposing answers. Many ecologists have made precise measurements, designed to test the assumptions of both the Clements and Gleason models. ■ For instance, along mountain slopes, does one life zone, or habitat type, grade sharply or gradually into another? ■ If the divisions are sharp, perhaps the reason is that the community is so well integrated, so holistic, so like Clements viewed it, that whole clusters of species must remain together. If the divisions are gradual, perhaps, as Gleason suggested, each species is responding individually to its environment, and clusters of species are not so integrated that they must always occur together.
• C. ■ Who was right? ■ Many ecologists have made precise measurements, designed to test the assumptions of both the Clements and Gleason models. Their research has helped to decide between the two views because it has focused on questions to which Clements and Gleason would give opposing answers. For instance, along mountain slopes, does one life zone, or habitat type, grade sharply or gradually into another? ■ If the divisions are sharp, perhaps the reason is that the community is so well integrated, so holistic, so like Clements viewed it, that whole clusters of species must remain together. If the divisions are gradual, perhaps, as Gleason suggested, each species is responding individually to its environment, and clusters of species are not so integrated that they must always occur together.
• D. ■ Who was right? ■ Many ecologists have made precise measurements, designed to test the assumptions of both the Clements and Gleason models. ■ For instance, along mountain slopes, does one life zone, or habitat type, grade sharply or gradually into another? Their research has helped to decide between the two views because it has focused on questions to which Clements and Gleason would give opposing answers. If the divisions are sharp, perhaps the reason is that the community is so well integrated, so holistic, so like Clements viewed it, that whole clusters of species must remain together. If the divisions are gradual, perhaps, as Gleason suggested, each species is responding individually to its environment, and clusters of species are not so integrated that they must always occur together.

填空题 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.
Write your answer choices in the spaces where they belong. You can either write the letter of your answer choice or you can copy the sentence.
Over time, a variety of views have been formed on the structure of ecological communities.
·______
·______
·______

A. Clements held that ecological communities were like organisms that compete with each other for dominance in a particular climatic region.
B. Clements saw the community as a collection of thoroughly interdependent species progressing toward a single climax community.
C. Gleason held that within a single climatic region, differing local factors would cause ecological communities to develop in different ways.
D. Gleason believed that sharp divisions would exist between species in different habitats.
E. Today's ecologists recognize that ecological communities must be precisely and permanently balanced.
F. The current thinking is that communities are individualistic and largely accidental collections of species with similar needs and tolerances.