Population dynamics of animals in unpredictably-changing tropical environments

We studied population dynamics of a solitary phytophagous beetle, Epilachna vigintioctopunctata and a social stingless bee, Trigona minangkabau, in Sumatra, Indonesia for 5 years from 1981. Population increase of Epilachna vigintioctopunctata was suppressed in months of normal rainfall ( greater than or equal to 300 mm) but was released in the 1982-1983 El Nino-Southern Oscillation when rainfall dropped to 50% of the long-term average. Mechanisms might be direct; rainfall lowered egg hatchability and the time of adult's residence on host plants. When dry weather continued for more than three generations, the Epilachna vigintioctopunctata population reached a density at which food shortage due to defoliation occurred. Although parasitism of immature stages was high, it was not a population-regulating factor. Thus, there were two types of ecological crunch: competition for food resources at the end of favourable dry periods and high mortality during heavy rainfall periods that usually followed El Nino-Southern Oscillation dry conditions. Climatic conditions, especially rainfall, changed with various periodicities, 4-5 years for El Nino-Southern Oscillation, and 2 years for the monsoon and other shorter periods. The contribution of periodicities of 1 and 0.5 years, that were linked to movement of the sun, were weak, indicating that animals could not use seasonal changes of environments, e.g. daylength, to predict environmental changes. We discuss traits adaptive to such unpredictably-changing tropical environments. Separation of predictability of temporal environmental change and synchronous changes among patches improves our understanding. Low oviposition rate and resulting prolonged life-span of Epilachna vigintioctopunctata, usually associated with K-selected traits of life history, seem to be adaptations for unpredictable environmental changes.