文勇的新托福黄金&ccedil

enormouschangesinmaterialsandtechniquesofconstruction

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Unformatted text preview: rganisms than an environment that is uniform. A local population that goes extinct is quickly replaced by 126 Mail@liuwenyong.com 在在在在在在 www.liuwenyong.com 在在在在在在在在在在在在在在在,在在在在在在在在在在 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. Paragraph 1: 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 long­lived 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. 1. The word “particular” in the passage is closest in meaning to ○Natural ○Final ○Specific ○Complex 2. According to paragraph 1, which of the following is NOT true of climax communities? ○They occur at the end of a succession. ○They last longer than any other type of community. ○The numbers of plants in them and the mix of species do not change. ○They remain stable for at least 500 years at a time. Paragraph 2: 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. 3. According to paragraph 2, which of the following principles of ecosystems can be learned by studying a pond? ○Ecosystem properties change more slowly than individuals in the system. ○The stability of an ecosystem tends to change as individuals are replaced. ○Individual organisms are stable from one year to the next. ○A change in the members of an organism does not affect an ecosystem’s properties Paragraph 3: 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 d...
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This note was uploaded on 09/17/2013 for the course LANGUAGE 13DL208 taught by Professor Wang during the Fall '13 term at East China Normal University.

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