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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.
Why does the author conclude in paragraph 8 about the causes of sudden population increases of Acanthasters?
选项 A、A sudden population increase in the Acanthaster population could be attributed to natural process as well as human activities.
B、There are no elements of truth in the hypothesis proposed to explain sudden population increases of Acanthasters.
C、It is possible to offer a single explanation for sudden population increases of Acanthasters.
D、Natural processes have been responsible for sudden population increases of Acanthasters in the past while present outbreaks are due to human activities.
答案A
解析 本题要求回答作者为什么在第8段中总结“棘冠海星群落突增的原因”。第8段最后一句提到，在过去，自然过程已导致棘冠海星数量爆发式增长；而如今，人类的影响加快了爆发式增长的频率，加剧了爆发式增长的严重性。这说明，是自然过程和人类活动导致了棘冠海星群落的突增，故A项“棘冠海星群落的突增是因为自然的过程和人类活动”正确，B项的there are no elements of truth与原文第8段倒数第2句中的but that, there are elements of the truth in several of the hypotheses矛盾，故B排除。C项“这可能会为棘冠海星群落的突增提供单一的解释”与原文第8段倒数第2句“这似乎没有单一的解释”相矛盾。D项“在过去，自然过程是导致棘冠海星群落突增的原因，而目前的暴增是由人类活动造成的”为最大的干扰项，但是文章中只提到人类活动“加快了”棘冠海星数量爆发式增长的频率，“加剧了”爆发式增长的严重性，并没有指出现在的棘冠海星数量骤增完全是由人类活动引起的，所以现在自然过程依然有可能导致棘冠海星数量激增，故排除D项。
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