Climate change and thermal responses across diel and seasonal cycles in subtropical lepidoptera (butterflies and moths)

Abstract

Climate change is warming global temperatures and can interact with large-scale land use changes to pose multiple threats to organisms. Tropical ectotherms (living between latitudes 23.5°N and 23.5°S) are particularly vulnerable, as they are physiologically adapted to limited thermal variation and are living close to their thermal optimum. Organismal adaptations to diel and seasonal environmental cycles can show distinct climate responses that can be recognized from the individual to community levels. In this thesis, I used various field sampling, experimental, and data analysis methods to understand the thermal ecology of subtropical Lepidoptera (butterflies and moths) across different timescales in Hong Kong (22.3°N).

In Chapter 2, I investigated the variation of heat tolerance under starvation across seasonal morphs and life stages in a tropical butterfly Mycalesis mineus through full-factorial treatments on reared individuals. While starvation and temperature significantly affected developmental traits, heat tolerance was found to be consistent across all treatments. This suggested that some insects may be capable of coping with extreme heat under simultaneous stresses in food resources and seasonality disruption.

In Chapter 3, I evaluated the thermoregulatory needs, strategies, and challenges in diurnal and nocturnal lepidopteran insects (butterflies and moths) across seasons, through the comparison of field body temperatures, operative temperatures, and preferred temperatures. Subtropical diurnal insects experienced hot environments, showing a heat threshold at 31.8 to 36.5°C above which they buffered against further increases in temperature. Nocturnal insects experienced favourable temperatures, but they may have limited opportunities to thermoregulate at night. Distinct strategies between diurnal and nocturnal insects may translate into distinct challenges under climate change that shows a diel asymmetry in warming trends.

In Chapter 4, I examined community responses to large-scale climate and habitat changes using two long-term butterfly monitoring datasets spanning 13 to 21 years. Community temperature index (CTI) has increased significantly in some surveys (+0.034 to 0.061 °C/year) with no evidence of decline across communities, indicating a restructuring of butterfly communities to contain more warm-adapted species. A consistent influx and surge of tropical species was also detected across sites. An interplay of climate, habitat, and range shifters can be important for contemporary community transition in the subtropics.

In Chapter 5, I studied the aggregation, movement, and orientation behaviours of multiple danaid butterfly species (subfamily Danainae) in autumn and winter across two years. Tagging of 4,814 butterflies revealed movements in 12 individuals with the help of citizen scientists. In flight release experiments, Euploea butterflies oriented southwest (201.0°), but no species or thermal effects were detected. A better understanding of movement patterns, destinations, and reproductive status are important to recognize the winter migration in this multi-species system.

Through these chapters, my thesis highlights the sensitivity of subtropical ectotherms across different timescales to global changes in climate and habitat. In further studies, I suggest a stronger focus on tropical ecology and temporal cycles, and provide methodological suggestions to further understand species responses under mixed environmental pressures.