The crown of thorns starfish, Acanthaster Tlanci, is large, twenty-five to thirty-five centimeters indiameter, and has seven to twenty-one arms that are covered in spines. It feeds primarily on coral and isfound from the Indian Ocean to the west coast of Central America, usually at quite low populationdensities. Since the mid-1950s, population outbreaks at densities four to six times greater than normalhave occurred at the same time in places such as Hawaii, Tahiti, Panama, and the Great Barrier Reef. Theresult has often been the loss of a fifty percent to nearly one hundred percent of the coral cover over largeareas.
■A single Acanthaster can consume five to six square meters of coral polyps per year, and densepopulations can destroy up to six square kilometers per year and move on rapidly. ■ Acanthasters show apreference for branching corals, especially Acroporids. ■After an outbreak in a particular area, it iscommon to find that Acroporids have been selectively removed, leaving a mosaic of living and deadcorals. ■In places where Acroporids previously dominated the community devastation can be almostcomplete, and local areas of reefs have collapsed.
Areas of dead coral are usually colonized rapidly by algae and often are later colonized by sponges andsoft corals. Increases in abundance of plant-eating fish and decreases in abundance of coral-feeding fishaccompany these changes. Coral larvae settle among the algae and eventually establish flourishing coralcolonies. 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 arenatural phenomena that have occurred many times in the past, citing old men's recollections of earlieroutbreaks and evidence from traditional cultures. The other group maintains that recent human activitiesranging from physical coral destruction through pollution to predator removal have triggered these events.Two schools of thought exist concerning the cause of these outbreaks. One group holds that they arenatural phenomena that have occurred many times in the past, citing old men's recollections of earlieroutbreaks and evidence from traditional cultures. The other group maintains that recent human activitiesranging 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 werealize when a storm destroys coral and causes a food shortage. The adult Acanthasters converge onremaining portions of healthy coral and feed hungrily. Certainly there have been outbreaks of Acanthasterfollowing large storms, but there is little evidence that the storms have caused the enough reef damage tocreate a food shortage for these starfish.
Two other hypotheses attempt to explain the increased abundance of Acanthaster after episodes of highterrestrial runoff following storms. The first hypothesis is that low salinity and high temperatures favorthe survival of the starfish larvae. The second hypothesis emphasizes the food web aspect, suggesting thatstrong fresh water runoff brings additional nutrients to the coastal waters, stimulating phytoplanktonproduction and promoting more rapid development and better survival of the starfish larvae.
Those favoring anthropogenic (human influenced) causes have pointed to the large proportion ofoutbreaks that have been near centers of human populations. It has been suggested that coral polyps arethe main predators of the starfish larvae. Destruction of coral by blasting and other bad land use practiceswould reduce predation on the starfish larvae and cause a feedback in which increases in Acanthasterpopulations cause still further coral destruction. Unfortunately, there are too few documented instancesof physical destruction of coral being followed by outbreaks of Acanthaster for these hypotheses to befully supported.
Another group of hypothesis focuses on removal of Acanthaster's predators. Some have suggested thatthe predators might have been killed off by pollution whereas others have suggested that the harvestingof vertebrate and invertebrate predators of Acanthaster could have reduced mortality and caused increasedabundance of adults. The problem with this group of hypothesis is that it is difficult to understand howreduced predation would lead to sudden increases in Acanthaster numbers in several places at the sametime in specific years. It seems probable that there is no single explanation but that there are elements ofthe truth in several of the hypotheses. That is there are natural processes that have led to outbreaks in thepast, 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
A. brief appearance
B. sudden increase
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
B. speed up
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
B. come together
C. spread out
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 healthy 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 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.