Aseasonality of benthic invertebrates in a tropical stream on Bougainville Island, Papua New Guinea

Abstract

The benthic invertebrate fauna of Konaiano Creek, a small, aseasonal, tropical, mountain rainforest stream on Bougainville Island, Papua New Guinea, was quantitatively studied over a period of 22 months. Konaiano Creek supported a diverse, mostly undescribed, fauna dominated by two closely related species of Simuliidae (55 % of the total specimens collected), and two species of Hydropsychidae. Over 182 taxa were recorded, more than half of which were Diptera. The community exhibited no pattern of seasonal change in faunal abundance or species composition. The life cycles of 15 species were examined (8 Trichoptera: Hydropsychidae spp. 1 and 2, Chimarra sp., Philopotamidae sp., Nyctiophylax sp., Anisocentropus sp., nr Triaenodes sp., Apsilochorema sp.; 3 Ephemeroptera, Platybaetis sp., Barba sp., Prosopistoma sedlaceki; one Zygoptera, Lieftinckia kimminsi; one Blephariceridae, Apistomyia sp.; and two Simuliidae, Simulium (Morops) spp. 1 and 2). They all exhibited asynchronous aseasonal life cycles with continuous hatching and growth. This strategy appeared to be in response to the lack of environmental cues to enable synchronization of hatching, growth and emergence, since the climate of Bougainville Island is remarkably equable throughout the year with regard to temperature, rainfall and humidity, and there is a yearly variation in photoperiod of only 36 min. It is further suggested that asynchrony of life histories would be promoted by the frequent intense but unpredictable spates in Konaiano Creek since the presence of all life stages at any one time (particularly flying adults) would enhance a species' survival.