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 healt="2018年11月11日托福阅读机经精准预测"hy 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
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.
2.The word "outbreak" in the passage is closest in meaning to
3.What can be inferred from paragraph 2 about a coral reef that has a small number of Acroporids
A.It will collapse completely after Acanthaster consumes the Acroporids.
B.It will lose the Acroporids before the other corals when Acanthaster attack it.
C.It will have a mixture of living and dead Acroporids after an Acanthaster attack.
D.It will have a dense population of Acanthaster for several years.
4.The word "accompany" in the passage is closest in meaning to
A.occur along with
5.Which of the following is NOT mentioned in paragraph 3 as a consequence of the destruction of an area of a coral by Acanthaster
A.Algae colonize the dead area.
B.Plant-eating fish increase in number.
C.Coral larvae disappear.
D.Species diversity recovers fully over time.
6.The word "converge" in the passage is closest in meaning to
7.According to paragraph 5, what is an important weakness of the adult aggregation hypothesis
A.It is based on studies of the deeper parts of coral reefs.
B.It fails to explain the abundance of Acanthaster in the deeper parts of coral reefs.
C.It fails to explain why Acanthaster feed hungrily on healt="2018年11月11日托福阅读机经精准预测"hy coral after a storm.
D.It is not supported by evidence that storms result in food shortages for Acanthaster.
8.What can be inferred from paragraph 6 about the diet of Acanthaster larvae
A.Phytoplankton is an important food source for them.
B.Storm water runoff washes away important nutrients they need.
C.When water temperature rises they have difficulty finding enough food.
D.Storms add needed salt="2018年11月11日托福阅读机经精准预测" to their diet.
9.Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
A.Blasting and other bad land use practices would cause the destruction of coral and increase the number of predators that feed on Acanthaster larvae.
B.Bad land use practices would reduce predation on Acanthaster larvae and increase the starfish population which would cause more coral destruction.
C.A reduction of bad land use practices would reduce coral destruction by increasing the survival of Acanthaster larvae and their predators.
D.The destruction of coral through land use activities would cause a decrease in the number of predators that feed on Acanthaster larvae.
10.Why does the author state 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"?
A.To indicate the difficulty in proving that increases in Acanthaster numbers occur at the same time in
different areas of the world.
B.To identify a problem with a hypothesis that associates the increase in the population of Acanthaster with a removal of its predators.
C.To argue against the hypothesis that human activities cause sudden population outbreaks of Acanthaster in different places at the same time.
D.To argue against the view that the predators of Acanthaster decline because of pollution and harvesting by humans.
11.Why does the author conclude in paragraph 8 about the causes of sudden population increases of Acanthaster?
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 Acanthaster.
C.It is possible to offer a single explanation for sudden population increases of Acanthaster.
D.Natural processes have been responsible for sudden population increases of Acanthaster in the past while present outbreaks are due to human activities.
12.The word "severity" in the passage is closest in meaning to
13.Look at the four squares that indicate where the following sentence can be added to the passage.
The crown of thorns starfish is a coral reef predator that preys on coral polyps.
Where would the sentence best fit?
14.Directions: An introductory 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 answer choices 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.