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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.
An introduction sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Attempts have been made to explain the sudden increases in Acanthaster populations since the mid-1950s and their impact on coral reefs.
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Answer Choices
(A) Larvae Acanthaster populations have preyed on coral reefs and caused the loss of coral cover over large areas and the collapse of some coral reef communities.
(B) Acanthaster outbreaks have been attributed to natural phenomena such as storms that have created favorable conditions for the rapid development of the starfish larvae.
(C) Acanthaster outbreaks are the results of natural processes that have led to some increases in the past, as well as of human impact that has sped up the frequency and severity of the outbreaks.
(D) There are several hypotheses trying to explain the Acanthaster outbreaks but none have elements of the truth because they contradict each other.
(E) The crown-of-thorns starfish, Acanthaster planci, is large, 25 to 35 centimeters in diameter, and has 7 to 2I arms that are covered in spines.
(F) The favorable anthropogenic causes, such as destruction of coral by blasting and other bad land use practices, have provided Acanthasters more food sources and therefore stimulate its population outbreaks.
选项
答案A,B,C
解析 文章总结题。A项“棘冠海星幼体种群以珊瑚礁为食，并造成大范围珊瑚覆盖面积的减少和部分珊瑚礁群落的崩塌”对应原文第1段第2句、最后一句和第2段的说法。B项“棘冠海星数量的爆发式增长可以归因于风暴等自然现象，因为风暴能为棘冠海星幼体提供良好的生长环境”是原文第6段的整合。C项“棘冠海星数量的爆发式增长在过去是自然过程的结果，而人类的影响也加快了爆发式增长的频率，加剧了爆发式增长的严重性”对应原文最后一段最后一句。从原文的行文来看，第1段是对棘冠海星的整体介绍，并进而引出其食性。第2、3段谈论棘冠海星对鹿角珊瑚的破坏以及珊瑚礁的恢复，即棘冠海星对珊瑚礁的影响，A项与此对应。第4—8段尝试解释棘冠海星数量激增的原因，B项和C项为其主要思想。因此，A、B、C三项是文章主要的观点。D项“有几种假说试图解释棘冠海星数量呈爆发式增长的原因，但没有一个有合理性，因为它们相互矛盾”，原文第8段倒数第2句提到，这几个假说都有合理性，故D项与原文内容不符。E项为原义第1段第1句。但该句是细节信息，后文没有与此相关的信息，不是本文主题内容。F项“爆破等不良的土地利用方式增加了棘冠海星的食物，使其数量增长”，原文第7段第3句说的是，因为爆破等方式减少了棘冠海星的‘捕食者”，其数量才得以增加。
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