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　　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 denses 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 fect 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.

　　20. The author most likely mentions Hardy’s principle of animal exclusion in order to

　　(A)give an example of one theory about the interaction of grazers and phytoplankton

　　(B)dend the first theory of algal denses against grazing

　　(C)support the contention that phytoplankton numbers are controlled primarily by environmental factors

　　(D)demonstrate the superiority of laboratory studies of zooplankton feeding rates to other kinds of studies of such rates

　　(E)rute researchers who believed that low numbers of phytoplankton indicated the grazing fect of low numbers of zooplankton

　　21. It can be inferred from the passage that the “first theories” of grazer control mentioned in line 4 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 fect between zooplankton and phytoplankton numbers

　　(E)invent laboratory techniques that would have allowed them to bypass their field research concerning grazer control

　　22. 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.

　　23. The author would be likely to agree with which of the following statements regarding the pressure of grazers on phytoplankton numbers?

　　I. Grazing pressure can vary according to the individual type of zooplankton.

　　II. Grazing pressure can be lower in nutrient-poor lakes than in bog lakes.

　　III. Grazing tends to exert about the same pressure as does temperature.

　　(A)I only

　　(B)III only

　　(C)I and II only

　　(D)II and III only

　　(E)I, II, and III

　　24. The passage supplies information to indicate that Hargrave and Geen’s conclusion regarding the grazing pressure exerted by zooplankton on phytoplankton numbers was most similar to the conclusion regarding grazing pressure reached by which of the following researchers?

　　(A)Hardy

　　(B)Lund

　　(C)Round

　　(D)Reynolds

　　(E)Haney

　　25. 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 fects of temperature, rather than of light, on phytoplankton

　　(B)disregard nannoplankton in their analysis of phytoplankton numbers

　　(C)collect phytoplankton of all sizes bore 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

　　26. 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.

　　27. Which of the following is a true statement about the zooplankton numbers and zooplankton grazing rates observed in Haney’s experiments?

　　(A)While zooplankton numbers began to decline in August, zooplankton grazing rates began to increase.

　　(B)Although zooplankton numbers were high in May, grazing rates did not become high until January.

　　(C)Both zooplankton numbers and grazing rates were higher in December than in November.

　　(D)Both zooplankton numbers and grazing rates were lower in March than in June.

　　(E)Both zooplankton numbers and grazing rates were highest in February.

　　A D B C E C E D

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