Ecological Consequences of an Exceptionally Strong El Nino Event on Darwin's Finches

We studied the effects of heavy rainfall associated with the El Nino event of 1982-1983 on the demography of Darwin's Ground Finch (Geospizinae) populations and their resources on Isla Daphne Major, Galapagos. Total rainfall during this period (1359 mm) was 10 times the previously recorded wet season maximum. Most plant species responded by dramatically increasing seed production; total seed biomass increased by an order of magnitude, and small seed biomass increased in relative abundance, from 20% of total seed biomass before 1983 to >80% at the peak of the event. Caterpillars, the main food used by finches while breeding, also became extremely abundant. Ground finch species, mainly the Medium Ground Finch (Geospiza fortis) and the Cactus Finch (G. scandens), responded to the increase in food supply by breeding repeatedly. Females of both species produced up to 10 clutches of eggs; normally they produce 1-5 clutches. The most productive female fortis and scandens individuals fledged 25 and 19 young, respectively. Some birds born at the beginning of the season bred successfully before the end. Clutch sizes were scarcely larger than before and overall fledging success per nest was lower than in a normal breeding season. The large number of young produced resulted in (1) increases in population sizes of both species by a factor of four, (2) a dramatic skew in the age structure of both species, with birds born in 1982-1983 making up >75% of each population in January 1984, and (3) >50% of the lifetime fledgling production of females born in 1978 occurring in this breeding season. Individuals of both species immigrated from nearby islands, and a nonresident species, the Large Ground Finch (G. magnirostris) immigrated and bred on the island. Severe El Nino events have important ecological and evolutionary effects on these birds. For example, the high proportion of lifetime reproductive success contributed under exceptionally favorable conditions may have influenced the evolution of life history characteristics in the finches. Overall, our results support the view that rare events can have a major influence on key population processes in long-lived birds living in temporally varying environments, and that long-term studies are necessary to detect such effects.