The cane toad's (Chaunus [Bufo] marinus) increasing ability to invade Australia is revealed by a dynamically updated range model

Abstract

Invasive species threaten biological diversity throughout the world. Understanding the dynamics of their spread is critical to mitigating this threat. In Australia, efforts are underway to control the invasive cane toad (Chaunus [Bufo] marinus). Range models based on their native bioclimatic envelope suggest that the cane toad is nearing the end of its invasion phase. However, such models assume a conserved niche between native and invaded regions and the absence of evolution to novel habitats. Here, we develop a dynamically updated statistical model to predict the growing extent of cane toad range based on their current distribution in Australia. Results demonstrate that Australian cane toads may already have the ability to spread across an area that almost doubles their current range and that triples projections based on their native distribution. Most of the expansion in suitable habitat area has occurred in the last decade and in regions characterized by high temperatures. Increasing use of extreme habitats may indicate that novel ecological conditions have facilitated a broader realized niche or that toad populations at the invasion front have evolved greater tolerance to extreme abiotic conditions. Rapid evolution to novel habitats combined with ecological release from native enemies may explain why some species become highly successful global invaders. Predicting species ranges following invasion or climate change may often require dynamically updated range models that incorporate a broader realization of niches in the absence of natural enemies and evolution in response to novel habitats.

Figure 1

Current and projected range of invasive cane toads in Australia. (a) Cane toads now inhabit approximately 1.2 million km² of Australia (dark grey). (b) The predicted distribution of cane toads (black) based on annual maximum and minimum temperature, their squared terms, annual precipitation, precipitation×topographical variation, elevation, annual evaporation, minimum moisture index per cent built-up area and paved road density as estimated by model-averaged logistic regression at a data-derived threshold of 0.505. The future range area of cane toads predicted by this model (2.0 million km²) is almost triple the projections from the most recent model (0.7 million km²) based on its native limits (Sutherst et al. 1995).

Figure 2

Projected range area and associated 95th percentile maximum temperatures for cane toad invasion dynamics. Range models were produced for nested subsets of toad occurrences collected prior to 1975, 1985, 1995 and 2005. (a) After a period of relative stasis, the model predicted a substantial expansion of projected cane toad suitable habitat area from 1995 to 2005. (b) Over time, cane toads increasingly have colonized warmer regions of Australia, as indicated here by the 95th percentile maximum temperature of all habitats colonized by toads through time.

Figure 3

Predicted future distribution of cane toads in Australia over time: (a) 1975, (b) 1985, (c) 1995 and (d) 2005. Prediction maps are based on a logistic prediction model as in figure 1 for all data collected prior to the date indicated.