Marine iguanas (Amblyrhynchus cristatus) on Santa Fé lay their eggs during January/February and the young hatch in April/May. This population suffered from the immigration of indigestible brown algae during El Niño (Laurie, 1988) and decreased by 70% during the disturbance (Laurie, 1988). The 1982 hatchlings were almost completely exterminated, and the cohorts of 1981, 1980, and 1979 suffered progressively lower mortality. Although breeding was normal in January/February 1983 almost no breeding took place in the 1983-84 season, after the El Niño. Apparently females still had enough reserves for egg production early in the El Niño event, while after the event had ended they were unable to accumulate new reserves for reproduction during the few normal months (approximately from August to December 1983) prior to the breeding season. Due to fast recovery of the normal algal flora and low intraspecific competition for food in 1984, the growth and reproduction of the survivors was very high during 1984 and in early 1985. The observed drastic decrease of this population, coupled with an equally dramatic change in the age structure, is going to remain noticeable for at least a decade after the event (Laurie, 1988).