Eighteen months ago, a grounded tanker spilled 150,000 gallons of diesel and bunker fuel into the waters around the famed Galapagos Islands. Luckily, shifting winds sent most of the fuel out to sea rather than into shore, so sea lion and bird deaths numbered in the dozens rather than the hundreds. At the time, biologists and conservationists breathed a sigh of relief, believing the islands and their inhabitants had been largely spared. Now, though, a long-term study of the unique Galapagos marine iguana has found that the small amount of oil that did reach the islands wrought a disproportionately large amount of havoc. On the island of Santa Fe, where the spill left about a quart of oil per each yard of the windward shore, the iguana population declined from 25,000 to 10,000. Scientists theorize that the oil killed the bacteria naturally present in iguanas’ guts that allows them to digest seaweed; in the absence of the bacteria, the animals starved to death. The findings provide new evidence that even small spills can have subtle yet far-reaching environmental effects.