Low-Level Oil Spills Can Cause Serious Damage; Study Tracks Deaths Of Iguanas In The Galapagos

In a report published in the June 6 issue of Nature, biologist Martin Wikelski and colleagues reported that 62 percent of the marine iguanas on the Galapagos island of Santa Fe died within a year after a grounded tanker dumped nearly 800,000 gallons of oil into nearby waters.

The consequences of the spill had been thought to be relatively mild because strong currents dispersed the oil. In the immediate aftermath, it seemed that the lives of all but a few marine animals were spared. The researchers' findings suggest that the iguanas died because oil killed off a beneficial microorganism that lives in the animals' guts and helps them digest their diet of seaweed. The iguana populations on other Galapagos islands that were not affected by the spill did not suffer declines during the same period.

"Our results illustrate the severe effects that low-level environmental contamination can have on wild animal populations," Wikelski and colleagues wrote. "Our findings warn against complacency over apparently low-impact contamination after environmental disasters in other wildlife areas, such as the Arctic National Wildlife Refuge in Alaska."

Wikelski's research confirmed a dire prediction he had made based on results of his earlier work with the iguanas. In an ongoing study, Wikelski and colleagues had shown that iguanas that died because of famine had increased blood levels of the stress hormone corticosterone in the weeks before their death. By coincidence, the researchers had tested the corticosterone levels in the iguanas just three days before the tanker Jessica ran aground on San Cristóbal Island on Jan. 17, 2001. They returned after the accident and found that the levels had increased dramatically, leading Wikelski to predict that many iguanas would die.

"In this context, corticosterone levels are a reliable indicator of the induction of life-threatening stress," the authors conclude in their current paper. This simple blood test could be valuable to biologists who need to estimate the severity of environmental disasters well before the animals start to die, Wikelski said.

Wikelski's co-authors include graduating Princeton students Vanessa Wong and Brett Chavalier, who helped conduct assessments in the Galapagos, as well as Niels Rattenborg of the University of Wisconsin and Howard Snell of the Charles Darwin Research Station in the Galapagos.

Wikelski's investigation of the effects of oil contamination was an unexpected turn in a long-term study designed to shed light on the interactions between animal physiology and natural environmental changes. Wikelski began tagging and tracking the iguanas in 1987. Because the spill killed many of the animals and altered the natural conditions, Wikelski has filed suit against Petroecuador, the national oil company of Ecuador, for the loss of data. Wikelski has pledged to use any settlement money to support young Ecuadorian scientists studying the Galapagos ecosystems.