POPULATION-DYNAMICS OF ANIMALS IN UNPREDICTABLY-CHANGING TROPICAL ENVIRONMENTS

We studied population dynamics of a solitary phytophagous beetle, Epil achna vigintioctopunctata and a social stingless bee, Trigona minangka bau, in Sumatra, Indonesia for 5 years from 1981. Population increase of Epilachna vigintioctopunctata was suppressed in months of normal ra infall (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 ha tchability and the time of adult's residence on host plants. When dry weather continued for more than three generations, the Epilachna vigin tioctopunctata population reached a density at which food shortage due to defoliation occurred. Although parasitism of immature stages was h igh, it was not a population-regulating factor. Thus, there were two t ypes of ecological crunch: competition for food resources at the end o f favourable dry periods and high mortality during heavy rainfall peri ods 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 nat ural colonies. One-half of artificial nest sites were occupied by arbo real ants and thus competition for nest sites with ants suppressed fur ther increase of Trigona minangkabau. Intermediate rainfall was favour able for Trigona minangkabnu because the rate of colony foundation dec reased both during dry El Nino-Southern Oscillation months and months with heavy rain. Colony death was independent from rainfall. Many colo nies that survived for 6 months persisted for >2 years and colony dens ity was quite stable. Trigona minangkabau colonies could survive even under unfavourable periods, by hoarding resources in the nest. There w as no significant ecological crunch during the study period and colony density almost always tracked the carrying capacity of the habitat, w hich was basically determined by nest-site abundance. Climatic conditi ons, especially rainfall, changed with various periodicities, 4-5 year s for El Nino-Southern Oscillation, and 2 years for the monsoon and ot her shorter periods. The contribution of periodicities of 1 and 0.5 ye ars, that were linked to movement of the sun, were weak, indicating th at animals could not use seasonal changes of environments, e.g. daylen gth, to predict environmental changes. We discuss traits adaptive to s uch unpredictably-changing tropical environments. Separation of predic tability of temporal environmental change and synchronous changes amon g patches improves our understanding. Low oviposition;ate and resultin g prolonged life-span of Epilachna vigintioctopunctata, usually associ ated with K-selected traits of life history, seem to be adaptations fo r unpredictable environmental changes.