Rates of expansion of invasive cane toads in New South Wales

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.

Methods
Data on the distribution of invasive cane toads in New South Wales was collated from all available sources, to quantify rates of expansion and to identify correlates of that rate of spread. We also conducted pilot studies to comapre alternative emthods of detecting invasion-front populations of toads in the field.

非生物与生物环境的地理差异，可显著影响外来入侵物种的分布范围扩张速率。海蟾蜍（蔗蟾，Rhinella marina）入侵澳大利亚并定殖的相关研究已得到广泛开展，但相关研究多集中于热带区域，而非气候较为凉爽的地带。本研究整合多组数据集，以刻画海蟾蜍在澳大利亚新南威尔士州（New South Wales, NSW）东北部区域内的南部凉气候入侵前沿的历史扩散过程。由于海蟾蜍相对易于观测，目视调查与声学调查在检测该物种存在与否方面，似乎与基于环境DNA（environmental DNA, eDNA）的调查方法同样有效。海蟾蜍在新南威尔士州的分布范围扩张，既通过建立卫星种群实现，也依赖于分布范围核心区域的扩张。整体扩散速率较热带前沿低一个数量级以上（年均扩散距离不足5千米，而热带前沿年均超50千米）；部分十年间，海蟾蜍的南部分布范围甚至出现缩减而非扩张。1970年以来，海蟾蜍向南（沿沿海生境）的扩散速率有所加快，但向西（进入山地区域）的扩散并未出现类似加速。海蟾蜍的分布范围在温暖干燥的十年间扩张最快，但未来预估的气候变化对其扩散速率的影响或将十分有限。明晰海蟾蜍在新南威尔士州的历史入侵模式，有助于阐明其未来潜在的扩散趋势，进而确定防控项目的优先区域。

研究方法
本研究从所有可获取的数据源中整合了新南威尔士州入侵海蟾蜍的分布数据，以量化其扩散速率并识别该扩散速率的相关影响因子。此外，我们还开展了试点研究，以对比野外探测海蟾蜍入侵前沿种群的多种替代检测方法。