托福EXTRA3阅读生物学 Crown of Thorns Starfish and Coral Reefs 题目解析

Crown of Thorns Starfish and Coral Reefs
The crown of thorns starfish, Acanthaster Tlanci, is large, twenty-five to thirty-five centimeters in diameter, and has seven to twenty-one 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 fifty percent to nearly one hundred percent of the coral cover over large areas.

A single Acanthaster can consume five to six square meters of coral polyps per year, and dense populations can destroy up to six square kilometers per year and move on rapidly. Acanthasters show a preference for branching corals, especially Acroporids. 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. In places where Acroporids previously dominated the community devastation can be almost complete, and local areas of reefs have collapsed.

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 ten to fifteen years the reefs often return to about the same percentage of coral cover as before. Development of a four-species diversity takes about twenty years.

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.

One theory, the adult aggregation hypothesis, maintains that most species is 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 Acanthaster 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.

Two other hypotheses attempt to explain the increased abundance of Acanthaster 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.

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 Acanthaster for these hypotheses to be fully supported.

Another group of hypothesis focuses on removal of Acanthaster's 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 Acanthaster 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.

1.According to paragraph 1, all of the following statements about crown of thorns starfish are true EXCEPT

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首先,根据题干中的“EXCEPT”,可以判断这是一道否定事实信息题。该题型要从4个选项中分别找定位词,再回到原文进行定位,逐一排除。一般来说,选项会按文章顺序来出,一旦一个选项被锁定,那么就要留意附近的其他选项。下面我们逐一来看选项:   A选项中,找到定位词“arms”,“spines”,定位到原文第一句话的后半句“and has seven to twenty-one arms that are covered in spines”,意思是“(棘冠海星)有7到21只被刺所覆盖的触手”。这句话中“seven to twenty-one”对应选项中的“several(几个)”,所以A选项符合原文,故排除。   B选项中,找到定位词“coral(珊瑚)”,定位至原文第二句话“It feeds primarily on coral”,意思是“(棘冠海星)主要以珊瑚为食物”。B选项中的“main food”恰好为“feed primarily on”的同义改写,所以B选项符合原文,故排除。   C选项中,找到定位词“population densities”,定位至第二句话后半段“usually at quite low population densities”,意思是“(棘冠海星)通常种群密度非常低”而选项中的“high”与原文中的“low”是一对明显的反义词,故C选项与原文矛盾,正确选项为C。   D选项中,“coral cover over large areas”在原文最后一句原样出现,我们很容易锁定最后一句话“The result has often been the loss of a fifty percent to nearly one hundred percent of the coral cover over large areas.”意思是“结果往往导致大范围内50%到将近100%的珊瑚死亡”。选项中“a significant reduction”为原文“the loss of a fifty percent to nearly one hundred percent”的同义改写,其它部分与原文一一对应,D选项符合原文,故排除。





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