An invasion in slow motion: the spread of invasive cane toads (Rhinella marina) into cooler climates in southern Australia

Geographical variation in abiotic and biotic conditions can significantly affect the rate that an invasive species expands its range. The colonisation of Australia by cane toads (Rhinella marina) has attracted extensive research, but mostly in tropical regions rather than cooler climatic zones. We assembled multiple datasets to characterise the historical spread of toads at their southern (cool-climate) invasion front in north-eastern New South Wales (NSW). Perhaps because toads are relatively easy to find, visual and acoustic surveys appear to be as effective as eDNA-based surveys in detecting the species’ presence. Expansion of the toads’ range in NSW has occurred through the establishment of satellite populations as well as by growth of the range-core. Overall rates of spread have been more than tenfold lower than on the tropical front (means of < 5 km vs. > 50 km per year), and in some decades, the toads’ southern range has declined rather than expanded. Overall rates of spread since 1970 have accelerated to the south (through coastal habitats), but not to the west (into montane areas). The toads’ range has expanded most rapidly in decades with dry, warm weather conditions, but predicted future changes to climate are likely to have only minor effects on rates of toad spread. Understanding historical patterns of toad invasion in NSW can clarify probable future spread, and hence identify priority areas for control programs.