Seasonal-Changes in Relative Abundance and Spatial-Distribution of Australian Lowland Tropical Satyrine Butterflies

Abstract

Seven species of grass-feeding satyrine butterflies (Lepidoptera : Nymphalidae) coexist in lowland regions of the wet tropical zone of north-eastern Queensland, Australia. Their seasonal changes in relative abundance, spatial distribution and diurnal activities were monitored at Cardwell (18 degrees 16'S) during 1989-92, with particular emphasis placed on three species of Mycalesis. The climate at Cardwell is monsoonal, being characterised by high summer rainfall and an annual winter dry season that lasts about seven months on average (although usually some rain falls during this period). Rainfall is quite variable in terms of both the timing and magnitude of the wet season. In general, relative abundance of adult Mycalesis species, increased during the early dry season, peaked during the dry winter months, decreased in the late dry season and then reached very low levels with the first significant wet-season rainfall. The pattern of seasonal abundance was broadly synchronous with seasonal changes in grass moisture content, which in turn was linked with rainfall regime. Ypthima arctous, Hypocysta irius and H. adiante showed seasonal fluctuations similar to those of Mycalesis but numbers of Melanitis leda peaked at the end of the dry season before the first significant rainfall. The seven satyrines also showed pronounced spatial and temporal differentiation in habitat distribution and timing of peak flight activity in the late dry season. Mycalesis terminus predominated in rainforest edge and adults were most active from late morning to early afternoon, whereas My. perseus and My. sirius occurred in the more open areas, favouring open eucalypt forest and paperbark woodland (Melaleuca swampland), respectively. Peak activity of My. perseus and My. sirius was confined largely to early morning and late afternoon, while Me. leda was strictly crepuscular. Peak activity of Y. arctous coincided with that of My. terminus but, unlike H. irius, which occurred only in rainforest edge, Y. arctous favoured the less shaded habitat of paperbark woodland. It is likely that adults of all species move seasonally and contract to moist refugia in the late dry season. The patterns of seasonality in Mycalesis may be influenced by variation in rainfall, and hence larval food quality, but other factors likely to influence fluctuation in abundance are briefly discussed.