Starfish Swarm Devouring Corals in Indonesia
Dave Hansford in Wellington, New Zealand
for National Geographic News
|January 17, 2008|
Predatory starfish are swarming over one of the world's most diverse coral reef ecosystems, researchers announced, threatening the health of reefs throughout the Indo-Pacific region.
Crown-of-thorns starfish, named for the long spines covering their bodies, feed on corals by spreading their stomachs over the animals living inside, then secreting enzymes that liquefy the corals' tissue.
"They prefer certain species and take them first, then they'll eat the others later," said Alison Green, a marine scientist with the nonprofit Nature Conservancy.
The starfish are found naturally throughout the Indo-Pacific. But a recent survey of reefs off the Indonesian island of Halmahera revealed that the numbers of the predators in some areas are double those that exist in a healthy reef.
Halmahera, the largest island in Indonesia's Maluku group, lies within the "coral triangle," which has been described as a global center of marine biodiversity.
The triangle spans eastern Indonesia, parts of Malaysia, the Philippines, Papua New Guinea, Timor, and the Solomon Islands (see a map of the region).
The localized starfish outbreak, experts say, could be an early warning of more widespread reef decline.
"Imagine the most beautiful coral reef with lots of three-dimensional structure, lots of color, and lots of fish," Green said.
"Then [imagine] the same place, except that it is dead, covered in black algae, and the fish are gone. Crown-of-thorns can do that."
Andrew Baird is a scientist with the Australian Research Council's (ARC) Centre of Excellence for Coral Reef Studies and James Cook University.
He was part of the starfish survey team, jointly led by the ARC center and the New York-based Wildlife Conservation Society.
The team found between 300 and 400 crown-of-thorns starfish within a hectare (2.5 acres) of reef around Halmahera, Baird said.
"On [Australia's] Great Barrier Reef, we define an outbreak as 200 animals per hectare," Baird said.
High nutrient levels due to agricultural fertilizer runoff were most likely responsible for the population boom, he added.
"It stimulates blooms of microalgae—plankton—and the larvae of the crown-of-thorns starfish, under those conditions, survive very well," he said.
"In normal years, perhaps one in a million [starfish larvae] might survive. In one of these years, maybe a hundred in a million survive. You get huge recruitment."
Overfishing of the starfish's natural predators, such as triggerfish and the giant triton mollusk, likely worsened the situation.
Survey teams also found evidence of reef blasting—a practice that uses explosives to stun fish or collect coral as construction material.
"A lot of people rely on the reefs for their livelihood and their food. Without healthy reefs, it could result in serious economic hardship," Baird said.
(Related news: "Coral Reefs Vanishing Faster Than Rain Forests" [August 7, 2007].)
For now, the experts noted, certain species on the reef show healthy enough populations that the ecosystem could recover.
"I think the answer lies in good management to prevent the outbreaks in the first place," the Nature Conservancy's Green said.
"We need to be particularly careful about how we manage the land and fisheries in those areas."
A network of marine protected areas, she said, combined with land-use and fisheries reforms, would ensure the survival of the coral triangle.
"Reefs can cope with periodic disturbances if they are healthy. If they've got good fish populations, good water quality, and good coral, they can bounce back within 10 to 15 years."
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