Passage IV

NATURAL SCIENCE: This passage is adapted from an essay by David Quammen that appeared in The Flight of the Iguana: A Sidelong View of Science and Nature (©1988 by David Quammen).

It is one of the central axioms of ecology, held by most (though not all) scientists in the field, that the stability of an ecosystem is directly related to its complexity. The greater the number of species coexisting in one community, and the greater the number of relationships linking different species, so much greater will be the natural resistance to change, perturbation, catastrophe. From diversity comes strength; from variety, steadiness. So goes the axiom, anyway.

... But in the case of tropical coral reefs一generally judged the Earth's most complex marine ecosystems ...—there is lately some reason for skepticism. The complexity-stability equation seems in doubt. The reason is a plague of starfish.

Acanthaster planci is the species in question, an imposing echinoderm that grows up to two feet in diameter, with as many as twenty-one arms. It moves slowly across the sea bottom in shallow Pacific waters. It feeds on live coral. Its common name is the crown-of-thorns starfish, reflecting the tangle of long sharp protective spines that protrude from the dorsal surface. Each spine is tipped with a toxic mucus. Any such two-foot-wide bush of poisonous spines is, understandably, threatened by few natural enemies....

In droves, in swarms, in startling multitudes, crown-of-thorns starfish are gobbling up the Great Barrier Reef. "There is a possibility," one scientist has written, "that we are witnessing the initial phases of extinction of madreporarian (reef-building) corals in the Pacific."

The sudden abundance of A. planci has raised a few interesting questions. First and most controversially: Is this plague of starfish really so bad—and so unprecedented—as it seems? Expert opinion has been divided. Second: Has the plague somehow been caused by human actions ? .... Studies to prove or disprove the human role are continuing at present. Almost everyone agrees that if the starfish plague has been caused by humankind, then firm measures should be taken to control it.

The third question is rather more tricky.... If the plague has not been caused by humankind—if it has been an epochal but naturally triggered catastrophe, like a lightning-caused forest fire howling across Yellowstone Park—in that case, should anything be done to rescue those glorious coral reefs? Or should we let unsentimental nature, and the crown-of-thorns starfish, have their way?

A. planci has been infamous to marine biologists and scuba divers for more than twenty years, since the first public reports of its population explosion at a place called Green Island.... Green Island marks the approximate center of that long chain of individual reefs known collectively as the Great Barrier Reef, the largest formation of coral ever seen on Earth...

Coral, like starfish, are animals. They belong to the same phylum as sea anemones, and in adulthood make their living as sessile creatures, attached permanently to a hard substrate, gathered together in large colonies of a particular species, each individual waving its tiny tentacles to capture planktonic food from the seawater. Their more famous attribute is that they secrete stony skeletons of calcium carbonate (lime) to support themselves. These lime skeletons—both those that still contain living coral and those that stand derelict—form the main structural matrix of coral reefs. Every reef is built up over a vast mass of compacted limestone, but it is a thin layer of live coral that keeps the reef forever renewed as a living ecosystem That layer is precisely what A. planci at Green Island had begun to devour.

From 1962 to 1964 the starfish population grew inexorably. The various species of stony corals were killed off in large swaths and patches. The starfish legions accomplished this at a slow but implacable pace, each single starfish destroying roughly a square yard of coral per month.... Behind the advancing front of starfish, which shuffled along like a herd of headless porcupines, were left dead coral skeletons that remained ghostly white for a few days or weeks, then gradually took on a gangrenous film of algae. The algae penetrated the lime skeletons and eventually weakened them, until normal wave action reduced the whole edifice to rubble. By that time, of course, the reef-dwelling fishes and other members of the coral community, dependent upon that edifice of lime for their cover, were long gone. A. planci, grown abundant beyond proportion, was literally collapsing its own ecosystem.