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Adaptations of Cane Toad in Australia

Introduction to Cane Toads

Cane toads (Rhinella marina; formerly Bufo marinus) are tough, adaptable, and poisonous toads that have become an invasive species in Australia affecting its ecology and environment (Fig. 1). This toad sp. is native to Mexico and Hawaii (Kosmala et al., 2017). The toad was first introduced in Queensland in the 1930s, Australia as a biological control for the beetles present in the sugarcane farms. A characteristic feature of the invasive species is to adapt fast to the conditions and be able to perpetuate successfully in geographical locations (Doody et al., 2019). Cane toads have been dominant invasive species of Australia since their introduction as they do not have any natural predators along with effective adaptations to ensure their survival. This report aims to find the structural, physiological, and behavioural adaptations of cane toad that have allowed it to survive successfully in Australia (Ducatez et al., 2016). This report will also compare the importance of the acquired adaptations against the predator of these toads, the Keelback snakes. The tadpoles of the toad are also consumed by the dragonfly nymphs, water beetles, and saw shelled turtles. Further, this report will also investigate the impact of the introduction of cane toads in the Australian environment.

Physical Adaptations

The successful feature of the cane toads that have allowed it to be an invasive species is its highly plastic morphology. The amount of toxin that is produced by each toad is dependent on the size of toad and the relative size of its paratoid glands (Southwell et al., 2017). The toads exhibit phenotypic lability that allows the toads to exert their maximum impact on the native predators in the area. However, the level of impact may vary. The larger individuals that colonise an area are few in number and exist in lower density than the smaller ones (Hudson et al., 2016). The paratoid glands of the toad are highly toxic and are known to kill animals with the powerful poison. The Australian fauna has not been adapted to the toad as it is non-native. This results in a severe impact on the native fauna of Australia that are toad eaters like snakes and results in their elimination from the population (Doody et al., 2019). Since the toads require moisture and lack membrane to retain it for long, they can absorb moisture through their belly from minimally wet surfaces like dew, moist sand, and any other form of moist material. This allows for their long survival and helps them perpetuate (Ducatez et al., 2016).

Physiological Adaptations

The toad can survive in extremely dry environments that have helped them thrive even in inhospitable conditions for the other toad species. The cane toad has adapted itself to survive with less than 50% of the water in the body (Southwell et al., 2017). The toads have also adopted to the heat and water stress in the regions that have assisted them in colonizing the previously inhabitable areas. They are opportunistic feeders and can digest anything they swallow (Guber et al., 2017).

Behavioural Adaptations

The cane toads are active in the night during the warm months of the year and the day in the cold or dry weathers. The toad is, therefore, able to hunt and survive in both light and dark conditions throughout the year. The species is characteristically nocturnal but has adapted to behave as a diurnal species for its survival (Brannelly et al., 2018). Further, the toad can survive at a wide range of temperature ranging from 5ºC – 40 ºC allowing it to survive in a wide range of temperature conditions (Guber et al., 2017). The organisms depict a nomadic behaviour change and have been known to move about 200m in a single night increasing their geographical spread.

Impact of introduction

Economic Impacts

The cane toads do not represent immediate economic impacts on Australian agriculture. However, some indirect costs can be estimated. The loss in fisheries due to the poisonous nature of the toad is one of the losses associated (Brannelly et al., 2018).

Ecological Impact

Acne toads have been identified as a major environmental threat. And have been identified as “key threatening process” under Australia’s Environment Protection and Biodiversity Conservation Act 1999 (The National Australian Museum, 2020). The native species have been impacted through predation, competition, and by the toxic poison produced by cane toad. The native animals of Australia are highly susceptible to the poison of the toad. The cone toad has been associated with extinction of several native species of the Northern Territory and Queensland in Australia. These include species like Northern quoll (Clarke et al., 2019).

Social Impacts

The toad is considered to be a hazard due to its ability to poison a large number of animals including humans and pets. All the stages of the life cycle of a cane toad are poisonous to humans and affect the heart (Trumbo et al., 2016). This has also affected the movement of humans with large populations and has been a threat to the native Australians living in remote locations.

Conclusion on Adaptations of Cane Toad in Australia

This report presents a brief outline of the impact of the introduction of cane toads in Australia with tremendous ecological and social impacts. The survival of the cane toads has been in Australia can be attributed to their wide array of physical, physiological, as well as behavioural adaptations along with the absence of natural predators on the Australian continent promoting their spread. The toads are toxic and kill the native animals of Australia making them a major menace in the Australian ecology. These toads have been competing with the existing fauna and driving their extinction and deaths by their toxin release interfering with the natural food web of the Australian ecosystem.

References for Adaptations of Cane Toad in Australia

Brannelly, L. A., Martin, G., Llewelyn, J., Skerratt, L. F., & Berger, L. (2018). Age-and size-dependent resistance to chytridiomycosis in the invasive cane toad Rhinella marina. Diseases of Aquatic Organisms, 131(2), 107-120.

Clarke, G. S., Shine, R., & Phillips, B. L. (2019). Fight or flight: the competitive ability of cane toad larvae is lower on the invasion front:The evolution of competitive ability across a biological invasion: a study of cane toads in tropical Australia, Molecular Ecology 44,2

Doody, J. S., McHenry, C. R., Rhind, D., & Clulow, S. (2019). Novel habitat causes a shift to diurnal activity in a nocturnal species. Scientific Reports, 9(1), 1-10.

Ducatez, S., Crossland, M., & Shine, R. (2016). Differences in developmental strategies between long‐settled and invasion‐front populations of the cane toad in Australia. Journal of Evolutionary Biology, 29(2), 335-343.

Gruber, J., Brown, G., Whiting, M. J., & Shine, R. (2017). Is the behavioural divergence between range-core and range-edge populations of cane toads (Rhinella marina) due to evolutionary change or developmental plasticity?. Royal Society Open Science, 4(10), 170789.

Hudson, C. M., McCurry, M. R., Lundgren, P., McHenry, C. R., & Shine, R. (2016). Constructing an invasion machine: the rapid evolution of a dispersal-enhancing phenotype during the cane toad invasion of Australia. PloS One, 11(9).

Kosmala, G., Christian, K., Brown, G., & Shine, R. (2017). Locomotor performance of cane toads differs between native-range and invasive populations. Royal Society Open Science, 4(7), 170517.

National Australian Museum (2020). Cane toad. Retrieved from: https://australianmuseum.net.au/learn/animals/frogs/cane-toad/

Southwell, D., Tingley, R., Bode, M., Nicholson, E., & Phillips, B. L. (2017). Cost and feasibility of a barrier to halt the spread of invasive cane toads in arid Australia: incorporating expert knowledge into model‐based decision‐making. Journal of Applied Ecology, 54(1), 216-224.

Trumbo, D. R., Epstein, B., Hohenlohe, P. A., Alford, R. A., Schwarzkopf, L., & Storfer, A. (2016). Mixed population genomics support for the central marginal hypothesis across the invasive range of the cane toad (Rhinella marina) in Australia. Molecular Ecology, 25(17), 4161-4176.

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