Population dynamics of animals in unpredictably-changing tropical environments.

ABSTRACT

We studied population dynamics of a solitary phytophagous beetle, Epilachna viqintioctopunctata 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 (≥300mm) 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 populationregulating 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. By an experimental addition of artificial nest sites, colony density of Trigona minangkabau increased 2·5 times the original density of natural colonies. One-half of artificial nest sites were occupied by arboreal ants and thus competition for nest sites with ants suppressed further increase of Trigona minangkabau. Intermediate rainfall was favourable for Trigona minangkabau because the rate of colony foundation decreased both during dry El Nino-Southern Oscillation months and months with heavy rain. Colony death was independent from rainfall. Many colonies that survived for 6 months persisted for >2 years and colony density was quite stable. Trigona minangkabau colonies could survive even under unfavourable periods, by hoarding resources in the nest. There was no significant ecological crunch during the study period and colony density almost always tracked the carrying capacity of the habitat, which was basically determined by nest-site abundance. 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.