An oil tanker ran aground on the Galapagos island of San Cristóbal on 17 January 2001, spilling roughly three million litres of diesel and bunker oil. The slick started to spread westwards¹ and was dispersed by strong currents, so only a few marine animals were killed immediately as a result. Here we draw on long-term data sets gathered before the spill to show that a population of marine iguanas (Amblyrhynchus cristatus) on Santa Fe island suffered a massive 62% mortality in the year after the accident, due to a small amount of residual oil contamination in the sea. Another population on the more remote island of Genovesa was unaffected.

1. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA
2. Department of Psychiatry, University of Wisconsin Medical School, Madison, Wisconsin 53719, USA
3. Charles Darwin Research Station, Galapagos, Ecuador, and Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA

Correspondence to: Martin Wikelski¹ e-mail: Email: wikelski@princeton.edu