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  3. CROWN-OF-THORNS STARFISH AND CORAL REEFS (1) The crown-of-thorns starfish, Acanthaster planet, is large, 25 to 35 centimeter

CROWN-OF-THORNS STARFISH AND CORAL REEFS (1) The crown-of-thorns starfish, Acanthaster planet, is large, 25 to 35 centimeter

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问题                                         CROWN-OF-THORNS STARFISH AND CORAL REEFS
    (1) The crown-of-thorns starfish, Acanthaster planet, is large, 25 to 35 centimeters in diameter, and has 7 to 21 arms that are covered in spines. It feeds primarily on coral and is found from the Indian Ocean to the west coast of Central America, usually at quite low population densities. Since the mid-1950s, population outbreaks at densities four to six times greater than normal have occurred at the same time in places such as Hawaii, Tahiti, Panama, and the Great Barrier Reef. The result has often been the loss of a 50% to nearly 100% of the coral cover over large areas.
    (2) [A] A single Acanthaster can consume 5 to 6 square meters of coral polyps per year, and dense populations can destroy up to 6 square kilometers per year. [B] Acanthasters show a preference for branching corals, especially Acroporids. [C] After an outbreak in a particular area, it is common to find that Acroporids have been selectively removed, leaving a mosaic of living and dead corals. [D] In places where Acroporids previously dominated the community devastation can be almost complete, and local areas of reefs have collapsed.
    (3) Areas of dead coral are usually colonized rapidly by algae and often are later colonized by sponges and soft corals. Increases in abundance of plant-eating fish and decreases in abundance of coral-feeding fish accompany these changes. Coral larvae settle among the algae and eventually establish flourishing coral colonies. In 10 to 15 years the reefs often return to about the same percentage of coral cover as before. Development of a four-species diversity takes about 20 years.
    (4) Two schools of thought exist concerning the cause of these outbreaks. One group holds that they are natural phenomena that have occurred many times in the past, citing old men’s recollections of earlier outbreaks and evidence from traditional cultures. The other group maintains that recent human activities ranging from physical coral destruction through pollution to predator removal have triggered these events.
    (5) One theory, the adult aggregation hypothesis, maintains that most species are more abundant than we realize when a storm destroys coral and causes a food shortage. The adult Acanthasters converge on remaining portions of healthy coral and feed hungrily. Certainly there have been outbreaks of Acanthasters following large storms, but there is little evidence that the storms have caused the enough reef damage to create a food shortage for these starfish.
    (6) Two other hypotheses attempt to explain the increased abundance of Acanthasters after episodes of high terrestrial runoff following storms. The first hypothesis is that low salinity and high temperatures favor the survival of the starfish larvae. The second hypothesis emphasizes the food web aspect, suggesting that strong fresh water runoff brings additional nutrients to the coastal waters, stimulating phytoplankton production and promoting more rapid development and better survival of the starfish larvae.
    (7) Those favoring anthropogenic (human influenced) causes have pointed to the large proportion of outbreaks that have been near centers of human populations. It has been suggested that coral polyps are the main predators of the starfish larvae. Destruction of coral by blasting and other bad land use practices would reduce predation on the starfish larvae and cause a feedback in which increases in Acanthaster populations cause still further coral destruction. Unfortunately, there are too few documented instances of physical destruction of coral being followed by outbreaks of Acanthasters for these hypotheses to be fully supported.
    (8) Another group of hypothesis focuses on removal of Acanthaster predators. Some have suggested that the predators might have been killed off by pollution whereas others have suggested that the harvesting of vertebrate and invertebrate predators of Acanthasters could have reduced mortality and caused increased abundance of adults. The problem with this group of hypothesis is that it is difficult to understand how reduced predation would lead to sudden increases in Acanthaster numbers in several places at the same time in specific years. It seems probable that there is no single explanation but that there are elements of the truth in several of the hypotheses. That is there are natural processes that have led to outbreaks in the past, but human impact has increased the frequency and severity of the outbreaks.
According to paragraph 1, all of the following statements about crown-of-thorns starfish are true EXCEPT________.
选项 A、crown-of-thorns starfish usually have several arms covered in spines.
B、crown-of-thorns starfish’s main food is coral.
C、crown-of-thorns starfish normally live in high population densities.
D、crown-of-thorns starfish have caused a significant reduction in coral cover over large areas.
答案C
解析 本题要求根据原文第1段排除下列四项中有关棘冠海星的不正确说法，属否定事实信息题。原文第1段第2句提到，棘冠海星的种群密度通常较低(usually at quite low population densities)，故C项“棘冠海星种群密度通常较高”的说法与原文不符，符合题意。A项“棘冠海星通常有几个被棘刺覆盖的腕”可定位到原文第1段第1句“约有7—21个腕，腕表面布满棘刺”。B项“棘冠海星的主要食物是珊瑚”定位至原文第1段第2句话“它主要以珊瑚为食”。D项“棘冠海星已造成大面积地区的珊瑚覆盖率显著下降”对应原文第1段最后一句“结果往往导致大范围内50％到将近100％的珊瑚死亡”。因此，A、B、D三项符合原文说法。不符合题意，均予以排除。
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