When an oil tanker runs aground, the impact is immediately obvious: seabirds soaked in petroleum, sick marine mammals, and dead fish washed ashore by the bushel. But the subtle, long-term effects of even small amounts of spilled oil can be devastating as well, according to research published in the 6 June issue of Nature.
Zoologist Martin Wikelski of Princeton University and colleagues had been studying marine iguanas (Amblyrhynchus cristatus) for 20 years on the Galápagos islands of Santa Fe and Genovesa when disaster struck. On 16 January 2001, the tanker Jessica hit a reef off neighboring San Cristóbal Island. Within days, the foundering vessel leaked some 3 million liters of oil. Wikelski is now suing the captain, owners, and insurers of the Jessica for wrecking his Santa Fe iguana studies; the charge is part of a $14 million suit filed by the Galápagos National Park in Ecuadorian court.
At the same time, Wikelski has been busy documenting the spill's effect. Strong currents and a quick cleanup kept oil levels on Santa Fe to a relatively low 1 liter per meter of beach, while northerly Genovesa emerged unscathed. No dead iguanas were found on Santa Fe or Genovesa immediately after the spill. But populations on the two islands experienced markedly different fates over the next 11 months. While all the tagged iguanas remained accounted for on Genovesa, a shocking 62% disappeared on Santa Fe.
Contaminated food didn't seem to be the problem. Santa Fe's algae pastures--the iguanas' main food source--appeared healthy and free of oil just 2 weeks after the spill, and the iguanas continued to forage normally. So the researchers suggest that the oil could have poisoned the hindgut bacteria the iguanas depend on for digestion, causing the animals to starve to death.
That may not have been the only trouble, says zoologist Dee Boersma of the University of Washington, Seattle. "My guess is that ... the oil also affected their digestive efficiency by causing stomach lesions"--an affliction she has seen in Magellanic penguins caught in an oil spill off the coast of Argentina. Still, without the long-term data collected by Wikelski and colleagues, researchers might not have even detected the die-off, says zoologist Robert Paine of the University of Washington. "Proof of these low-level chronic problems ... is extraordinarily difficult to pin down in natural populations."