abundance, spatial distribution and diurnal activities were monitored at Cardwell (18'16's) during. 1989-92, with particular emphasis placed on three species of ...
Aust. J. Zool., 1995,43,209-29
Seasonal Changes in Relative Abundance and Spatial Distribution of Australian Lowland Tropical Satyrine Butterflies M. F. Bmby Department of Zoology, James Cook University of North Queensland, Townsville, Qld 481 1, Australia. Present address: CSIRO Division of Entomology, GPO Box 1700, Canberra, ACT 2601, Australia.
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 (18'16'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.
Introduction Tropical ecosystems show tremendous seasonal variability in the amount and distribution of rainfall, from the relatively aseasonal moist lowland rainforests where rain may fall all year round, to the harsher arid savannas where rain is both low and unpredictable and the dry season is highly pronounced (Young 1982; Denlinger 1986). The ways in which tropical insect populations respond to such seasonality is fundamental to understanding their phenological patterns and life-cycle strategies. Many tropical insects, like their temperate counterparts, exhibit seasonal and annual variations in abundance (Phipps 1968; Janzen 1973; Buskirk and Buskirk 1976; Wolda 1978a, 1978b, 1988 and references therein; Denlinger 1980; Young 1982; Levings and Windsor 1982;
M. F. Braby
Frith and Frith 1985; Spitzer et al. 1993). For many tropical species, activity periods are longer, multivoltinism is common and adult numbers usually peak during the wet season, presumably in response to increased availability of larval food (Janzen and Schoener 1968; Owen and Chanter 1970; Fogden 1972; Wolda 1978b; Lowman 1982; Frith and Frith 1985). However, the duration of the activity period and the timing and magnitude of the seasonal peak vary greatly among species (Owen et al. 1972; Young 1980; Wolda 1980; Wolda and Fisk 1981), and several workers have pointed out that the life-history patterns, and especially the mechanisms underlying seasonal phenology, are still very poorly known (Young 1982; Wolda 1988). Seasonality in tropical butterflies has been investigated in west Africa (Owen et al. 1972; Owen 1971; Owen and Chanter 1972, Central America (Emmel and Leck 1970; Brown and Benson 1974; Young 1981), and to a lesser extent in the Bahamas and West Indies (Ehrlich and Gilbert 1973; Clench 1977), east Africa (Owen 1971; Denlinger 1980) and south-east Asia (Spitzer 1983; Spitzer et al. 1993). In Australia, the seasonal patterns of our tropical butterflies are poorly known, and only two detailed studies of temporal changes have been made. Jones and Rienks (1987) monitored changes in abundance of four species of Eurema (Pieridae), and Hill (1992) examined populations of two subtropical lycaenids of the genus Hypochrysops. A third, but less detailed, account is that of Moore (1985). Taken together, these studies have shown that the timing of peak activity varies between taxa: some butterfly species are more abundant during the wet season, others during the drier months, while a few display no obvious seasonal pattern. In most cases the variables responsible for the observed patterns of fluctuation are not known, although rainfall and availability of larval food resources have been implicated (Owen 1971; Owen et al. 1972; Young 1982; Jones and Rienks 1987). The present study deals with seven species of satyrines (Lepidoptera : Nymphalidae) from a lowland region in the wet tropics of north-eastern Australia, which experiences a monsoonal climate. Because &heiradults are common, conspicuous, relatively sedentary and fly slowly close to the ground (
