Spatial distribution and range expansion of the Tawny Coster butterfly, Acraea terpsicore (Linnaeus, 1758) (Lepidoptera: Nymphalidae), in South-East Asia and Australia

Documenting the range size and range boundaries of species, and understanding the factors determining changes in these spatial components, is crucial given current rates of anthropogenic climate change and habitat loss. Here, we document the establishment of the acraeine butterfly, Acraea terpsicore, in South-East Asia (Indonesian islands south of Malay Peninsula, and Timor) and Australia for the first time. We estimate its rate of colonisation and potential for further range expansion in the Indo-Australian region according to bioclimatic niche models. We modelled the potential distribution of the species in the Indo-Australian region under current climatic conditions and in 2050 following climate change. The bioclimatic niche models were based on five different modelling techniques, three global circulation models (GCMs) and two CO₂ emission scenarios (SRES), yielding 30 individual models that were combined in a consensus model. Acraea terpsicore became established in Indo-China (Thailand) during the 1980s and since that time it has spread to other parts of South-East Asia. It was first recorded on the Australian mainland in the Northern Territory in April 2012 and within a few months of detection was found to occur at six locations, with an estimated extent of occurrence of 4000 km². Thus, the range size of A. terpsicore has expanded by approximately 6000 km across the equator (c. 32 degrees latitude) in 28 years, with an average rate of colonisation (from Thailand to Australia) of 200 km year^-1 (range: 170-230 km year^-1). The bioclimatic niche models identified additional regions with favourable climatic conditions, and within Australia it is likely to occupy coastal and subcoastal savannah woodlands of the entire monsoon tropics, indicating potential for further range expansion. Moreover, the species' potential range is likely to increase with climate change. We hypothesise that habitat modification, particularly rapid deforestation of tropical forest in South-East during the past three decades, is a major factor accounting for the range expansion given the species' habitat preference for disturbed and open degraded areas. Climate change may be a contributing factor but is unlikely the sole determinant given the spatial area involved and rate of spread.