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Many theories have been formulated to explain therole of grazers such as zooplankton in controlling theamount of planktonic algae (phytoplankton) in lakes.The first theories of such grazer control were merely5 based on observations of negative correlationsbetween algal and zooplankton numbers. A low numberof algal cells in the presence of a high number ofgrazers suggested, but did not prove, that the grazershad removed most of the algae. The converse obser-10 vation, of the absence of grazers in areas of highphytoplankton concentration, led Hardy to proposehis principle of animal exclusion, which hypothe-sized that phytoplankton produced a repellent thatexcluded grazers from regions of high phytoplankton15 concentration. This was the first suggestion of algaldefenses against grazing. Perhaps the fact that many of these first studiesconsidered only algae of a size that could be collectedin a net (net phytoplankton), a practice that over-20 looked 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 subse-quent research. Increasingly, as in the individualstudies of Lund, Round, and Reynolds, researchers25 began to stress the importance of environmentalfactors such as temperature, light, and water move-ments in controlling algal numbers. These environ-mental factors were amenable to field monitoring andto simulation in the laboratory. Grazing was believed30 to have some effect on algal numbers, especially afterphytoplankton growth rates declined at the end ofbloom periods, but grazing was considered a minorcomponent of models that predicted algal populationdynamics.35 The potential magnitude of grazing pressure onfreshwater phytoplankton has only recently beendetermined empirically. Studies by Hargrave andGeen estimated natural community grazing rates bymeasuring feeding rates of individual zooplankton40 species in the laboratory and then computing com-munity grazing rates for field conditions using theknown population density of grazers. The high esti-mates of grazing pressure postulated by theseresearchers were not fully accepted, however, until the45 grazing rates of zooplankton were determined directlyin the field, by means of new experimental techniques.Using a specially prepared feeding chamber, Haneywas able to record zooplankton grazing rates in naturalfield conditions. In the periods of peak zooplankton50 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 6.6percent and 114 percent of daily phytoplankton production.Cladocerans had higher grazing rates than55 copepods, usually accounting for 80 percent of thecommunity grazing rate. These rates varied seasonally,reaching the lowest point in the winter and early spring.Haney‘s thorough research provides convincing fieldevidence that grazers can exert significant pressure on60 phytoplankton population.(461 words)
1. It can be inferred from the passage that one wayin which many of the early researchers on grazercontrol could have improved their data wouldhave been to_____
(A) emphasize the effects of temperature, ratherthan of light, on phytoplankton
(B) disregard nannoplankton in their analysis ofphytoplankton numbers
(C) collect phytoplankton of all sizes beforeanalyzing the extent of phytoplanktonconcentration
(D) recognize that phytoplankton other than netphytoplankton could be collected in a net
(E) understand the crucial significance of netphytoplankton in the diet of zooplankton
2. According to the passage, Hargrave and Geendid which of the following in their experiments?
(A) They compared the grazing rates ofindividual zooplankton species in thelaboratory with the natural grazing ratesof these species.
(B) The hypothesized about the populationdensity of grazers in natural habitats byusing data concerning the populationdensity of grazers in the laboratory.
(C) They estimated the community grazing ratesof zooplankton in the laboratory by usingdata concerning the natural communitygrazing rates of zooplankton.
(D) They estimated the natural communitygrazing rates of zooplankton by usingdata concerning the known populationdensity of phytoplankton.
(E) They estimated the natural communitygrazing rates of zooplankton by usinglaboratory data concerning the grazingrates of individual zooplankton species.
3. It can be inferred from the passage that the “first theories” ofgrazer control mentioned in the first paragraph would have beenmore convincing if researchers had been able to_____
(A) observe high phytoplankton numbers undernatural lake conditions
(B) discover negative correlations betweenalgae and zooplankton numbers from theirfield research
(C) understand the central importance ofenvironmental factors in controlling thegrowth rates of phytoplankton
(D) make verifiable correlations of cause andeffect between zooplankton andphytoplankton numbers
(E) invent laboratory techniques that wouldhave allowed them to bypass their fieldresearch concerning grazer control
4. Which of the following, if true, would call intoquestion Hardy‘s principle of animal exclusion?
(A) Zooplankton are not the only organismsthat are affected by phytoplanktonrepellents.
(B) Zooplankton exclusion is unrelated tophytoplankton population density.
(C) Zooplankton population density is higherduring some parts of the year than duringothers.
(D) Net phytoplankton are more likely toexclude zooplankton than arenannoplankton.
(E) Phytoplankton numbers can be stronglyaffected by environmental factors.For the following question, consider each of the choices separately and select all that apply
5. The author would be likely to agree withwhich of the following statements regardingthe pressure of grazers on phytoplanktonnumbers?
A Grazing pressure can vary according to theindividual type of zooplankton.
B Grazing pressure can be lower in nutrient-poor lakesthan in bog lakes.
C Grazing tends to exert about the samepressure as does temperature.
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