The El Niño Southern Oscillation, Variable Fruit Production, and Famine in a Tropical Forest

We tested the hypothesis that the El Niño Southern Oscillation influences forest-wide fruit production, which, in turn, limits frugivorous and granivorous mammals on Barro Colorado Island (BCI), Panama. Observations of BCI mammals have been compiled for 49 years. Frugivorous mammals experienced famine between September and January in 1931–1932, 1958–1959, 1970–1971, and 1993–1994. The most recent famine is evident from an 11-yr record of natural deaths of mammals and a 2-yr record of population densities. Famine occurred every time a mild dry season followed an El Niño event in the 49-yr record. This coincidence is statistically improbable, as demonstrated by a randomization test. A 2-yr cycle of high, then low community-level fruit production has been observed twice for BCI when a mild dry season followed an El Niño event. We used 260 litter traps to monitor communityand species-level fruit production from 1 January 1987 through 30 June 1996. Community-level fruit production was greatest during the 1992 El Niño event and lowest one year later, after the mild 1993 dry season. We also reinterpret an earlier 2-yr record of fruit production in light of our 9.5-yr record of fruit production. Communitylevel fruit production was elevated during the 1969 El Niño event and was very low one year later, after the mild 1970 dry season. We hypothesize that (1) El Niño conditions enhance fruit production; (2) high fruit production consumes stored reserves, limiting the next reproductive event; and (3) mild dry seasons reduce fruit production. Each plant species may respond to any combination of the three components of this hypothesis. Communitylevel fruit production is extremely low when species sensitive to components 1 and 2 are entrained with species sensitive to component 3, or when a mild dry season follows one year after an El Niño event. El Niño events bring dry, sunny conditions to BCI and a large portion of the wet tropics. Drought and sun may both favor fruit production in wet tropical forests. Drought is known to synchronize flowering, and sunny conditions may relieve light limitation. The El Niño Southern Oscillation has a strong 24-mo periodicity. This creates a strong tendency for dry, sunny years to alternate with wet, cloudy years in Central America and elsewhere in the tropics. We present evidence that this alternately enhances and reduces fruit production on BCI. Terborgh hypothesized that predators regulate frugivorous mammals in tropical forests. As a corollary, he further hypothesized that famines occur on BCI because several large predators are absent and frugivores escape predation. We extended censuses to the nearby mainland to evaluate this hypothesis. Poachers, who are active on the mainland, reduce the abundances of game species below levels maintained in the presence of large felids and raptors. There was evidence for famine in poached, mainland populations, and we rejected the hypothesis that reduced predation pressure is a prerequisite for famine.