Appendix A. A description of previous species distribution modeling for cane toads in Australia.
The following is a brief review of species distribution models (SDM) for cane toads in Australia (also see van Beurden 1981; Floyd 1983; Sutherst et al. 1996; Urban et al. 2007; Kearney et al. 2008; reviewed in Phillips et al. 2008). A quasi-mechanistic SDM using moisture, temperature, and stress parameters estimated based on the entire native range predicted a cane toad distribution in Australia was largely consistent with its current range, except for an over-prediction into WA and under-prediction in interior QLD (Sutherst et al. 1996). Toads generally were most successful in hot, wet conditions, but sensitive to conditions too cold for development. The best-fit model had a low-temperature threshold for population growth parameter of 14°C, consistent with the 12-15°C range necessary for development and survival based on physiological studies (R. B. Floyd, unpublished thesis). Stress induced by insufficient warmth for daily metabolism was suggested to be a more important limit for the range prediction than was excessive cold.
A recent correlative SDM using temporal subsets of locality data from the introduced range showed a recent expansion (after 1994) into areas exceeding a minimum annual temperature of 12°C and maximum annual temperature of 37°C (Urban et al. 2007). Toads now occupy regions with minimum monthly temperatures less than 5°C and maximum monthly temperatures greater than 37°C. This study highlighted differences in range expansion dynamics between hotter and colder regions in Australia, and proposed evolutionary niche expansion to tolerate hotter temperatures during the rapid westward expansion in the north (NT). For southward expansion in the east (NSW), although toads inhabit colder regions than expected compared to the native range, expansion has slowed and no evidence exists for increased cold tolerance over the past 30 years. This stasis suggests a lack of an evolutionary response to colder southern conditions, and that toads may be reaching their limit in the south.
Similarly, predictions from a mechanistic SDM largely agree that thermal constraints, specifically limited breeding and adult foraging activity due to cold temperatures, limit the southern spread of cane toads in eastern Australia (Kearney et al. 2008). Toads never occupy areas where potential movement is predicted to be less than 500 m per year. Gradual decreases in temperature together with changes in the seasonal pattern of rainfall and humidity impose these constraints and may present an opportunity for evolution of cold tolerance and further range increase on the southern front (NSW). This SDM suggests that the cane toad’s current range largely fills its predicted fundamental niche, and that no niche expansion is needed to explain tolerance of hotter temperatures in the north (NT), where the cane toad’s range is predicted to be constrained by rainfall and evaporation.
Kearney, M., B. L. Phillips, C. R. Tracy, K. Christian, G. Betts, and W. P. Porter. 2008. Modelling species distributions without using species distributions: the cane toad in Australia under current and future climates. Ecography 31:423–434.
Phillips, B. L., J. D. Chipperfield, and M. R. Kearney. 2008. The toad ahead: challenges of modelling the range and spread of an invasive species. Wildlife Research 35:222–234.
Sutherst, R. W., R. B. Floyd, and G. F. Maywald. 1996. The potential geographical distribution of the cane toad, Bufo marinus L. in Australia. Conservation Biology 10:294–299.
Urban, M. C., B. L. Phillips, D. K. Skelly, and R. Shine. 2007. The cane toad’s (Chaunus [Bufo] marinus) increasing ability to invade Australia is revealed by a dynamically updated range model. Proceedings of the Royal Society B 274:1413–1419.
van Beurden, E. K. 1981. Bioclimatic limits to the spread of Bufo marinus in Australia: a baseline. Proceedings of the Ecological Society of Australia 11:143–149.