2017-03-13 09:44:00 来源：新东方在线论坛GRE资料下载
Many theories have been formulated to explain the role of grazers such as zooplankton in controlling the amount of planktonic algae (phytoplankton) in lakes. The first theories of such grazer control were merely based on observations of negative correlations between algal and zooplankton numbers. A low number of algal cells in the presence of a high number of grazers suggested, but did not prove, that the grazers had removed most of the algae. The converse observation, of the absence of grazers in areas of high phytoplankton concentration, led Hardy to propose his principle of animal exclusion, which hypothesized that phytoplankton produced a repellent that excluded grazers from regions of high phytoplankton concentration. This was the first suggestion of algal defenses against grazing.
Perhaps the fact that many of these first studies considered only algae of a size that could be collected in a net (net phytoplankton), a practice that overlooked the smaller phytoplankton (nannoplankton) that we now know grazers are most likely to feed on, led to a de-emphasis of the role of grazers in subsequent research. Increasingly, as in the individual studies of Lund, Round, and Reynolds, researchers began to stress the importance of environmental factors such as temperature, light, and water movements in controlling algal numbers. These environmental factors were amenable to field monitoring and to simulation in the laboratory. Grazing was believed to have some effect on algal numbers, especially after phytoplankton growth rates declined at the end of bloom periods, but grazing was considered a minor component of models that predicted algal population dynamics.
The potential magnitude of grazing pressure on freshwater phytoplankton has only recently been determined empirically. Studies by Hargrave and Geen estimated natural community grazing rates by measuring feeding rates of individual zooplankton species in the laboratory and then computing community grazing rates for field conditions using the known population density of grazers. The high estimates of grazing pressure postulated by these researchers were not fully accepted, however, until the grazing rates of zooplankton were determined directly in the field, by means of new experimental techniques. Using a specially prepared feeding chamber, Haney was able to record zooplankton grazing rates in natural field conditions. In the periods of peak zooplankton abundance, that is, in the late spring and in the summer, Haney recorded maximum daily community grazing rates, for nutrient-poor lakes and bog lakes, respectively, of 6.6 percent and 114 percent of daily phytoplankton production. Cladocerans had higher grazing rates than copepods, usually accounting for 80 percent of the community grazing rate. These rates varied seasonally, reaching the lowest point in the winter and early spring. Haney‘s thorough research provides convincing field evidence that grazers can exert significant pressure on phytoplankton population. (461 words)
1. It can be inferred from the passage that the "first theories" of grazer control mentioned in the first paragraph would have been more convincing if researchers had been able to
(A) observe high phytoplankton numbers under natural lake conditions
(B) discover negative correlations between algae and zooplankton numbers from their field research
(C) understand the central importance of environmental factors in controlling the growth rates of phytoplankton
(D) make verifiable correlations of cause and effect between zooplankton and phytoplankton numbers
(E) invent laboratory techniques that would have allowed them to bypass their field research concerning grazer control
2. Which of the following, if true, would call into question Hardy‘s principle of animal exclusion?
(A) Zooplankton are not the only organisms that are affected by phytoplankton repellents.
(B) Zooplankton exclusion is unrelated to phytoplankton population density.
(C) Zooplankton population density is higher during some parts of the year than during others.
(D) Net phytoplankton are more likely to exclude zooplankton than are nannoplankton.
(E) Phytoplankton numbers can be strongly affected by environmental factors.
For the following question, consider each of the choices separately and select all that apply
3. The author would be likely to agree with which of the following statements regarding the pressure of grazers on phytoplankton numbers?
□A Grazing pressure can vary according to the individual type of zooplankton.
□B Grazing pressure can be lower in nutrient-poor lakes than in bog lakes.
□C Grazing tends to exert about the same pressure as does temperature.
4. It can be inferred from the passage that one way in which many of the early researchers on grazer control could have improved their data would have been to
(A) emphasize the effects of temperature, rather than of light, on phytoplankton
(B) disregard nannoplankton in their analysis of phytoplankton numbers
(C) collect phytoplankton of all sizes before analyzing the extent of phytoplankton concentration
(D) recognize that phytoplankton other than net phytoplankton could be collected in a net
(E) understand the crucial significance of net phytoplankton in the diet of zooplankton
5. According to the passage, Hargrave and Geen did which of the following in their experiments?
(A) They compared the grazing rates of individual zooplankton species in the laboratory with the natural grazing rates of these species.
(B) The hypothesized about the population density of grazers in natural habitats by using data concerning the population density of grazers in the laboratory.
(C) They estimated the community grazing rates of zooplankton in the laboratory by using data concerning the natural community grazing rates of zooplankton.
(D) They estimated the natural community grazing rates of zooplankton by using data concerning the known population density of phytoplankton.
(E) They estimated the natural community grazing rates of zooplankton by using laboratory data concerning the grazing rates of individual zooplankton species.
1，"新东方在线"上的内容，包括文章、资料、资讯等， 本网注明"稿件来源：新东方在线"的，其版权 均为"新东方在线"或北京新东方迅程网络科技有限公司所有 ，任何公司、媒体、网站或个人未经授权不得转载、链接、转贴或以其他方式使用。已经得到 "新东方在线"许可 的媒体、网站，在使用时必须注明"稿件来源：新东方"，违者本网站将依法追究责任。
2， "新东方在线" 未注明"稿件来源：新东方"的 文章、资料、资讯等 均为转载稿，本网站转载出于传递更多信息之目的，并不意味着赞同其观点或证实其内容的真实性。如其他媒体、网站或个人从本网站下载使用，必须保留本网站注明的"稿件来源"，并自负版权等法律责任。如擅自篡改为 " 稿件来源：新东方 " ，本网站将依法追究其法律责任。