Roger Downie, University of Glasgow and Froglife
I first encountered cane toads in Trinidad (Rhinella marina, previously Bufo marinus, known locally as crapaud, the French for toad) during my initial visit there aimed at studying the variety of tropical amphibians and their life histories. The visit began in the dry season and cane toads were one of the few species around at that time of year. These large toads (a record-breaker has recently weighed in at 2.7kg in Australia) could often be found sitting in dog food dishes in back yards after consuming the food, with the hungry dog warily watching nearby, having learned that attacking these toads is not a good plan: they are well protected by the toxins in their massive parotoid glands. I also saw cane toad tadpoles – small, jet black, highly numerous and often in shoals – in slow-moving drainage ditches and low dry season rivers: the toads can breed at any time of year, their prolonged rumbling calls audible over a considerable distance. Linnaeus conferred the specific marinus on the basis of the Dutch naturalist Albertus Seba claiming they could live on land or in the sea. This was a partial error: they do not enter the sea, but can inhabit sandy beaches, feeding on the abundant invertebrates present there, and can spawn successfully in rivers close to the sea, where salinity levels may be significantly higher than in normal freshwater.
Cane toads are native to the Americas, ranging from southern Texas through central America and down into Peru and Amazonia; the islands of Trinidad and Tobago, adjacent to Venezuela, are considered part of their native range. However, the reason that cane toads are one of the most studied amphibian species is a consequence of their deliberate introduction to other parts of the world. Books and hundreds of scientific papers have been written about this species, and it must be the only anuran to feature in two full length documentary films (Cane toads: an unnatural history, 1988; Cane toads: the conquest, 2010). The toads tend to be found in open grassland areas, rather than forest, and the reason for their introductions elsewhere relates to their usual common name, cane toad: they are considered to predate the pests of sugar cane plantations. During the early 19th century, cane toads were introduced to several Caribbean islands as a control agent for pests such as rats and sugar cane beetles. Although the success of this action was patchy, positive results on the island of Puerto Rico were influential, and other introductions soon followed, to Hawaii and other Pacific islands.
The most consequential introduction was to the continent of Australia: in 1935, 102 toads sourced in Hawaii were brought to Queensland, and used to generate a population of 62,000 toadlets, which were then released into northern Queensland. The aim was to control a native beetle, considered a pest of the growing sugar cane industry. The toad’s colonisation of Australia was initially slow: not until 1959 was the coastal region of Queensland fully colonised; by 1978, the toads were moving south into New South Wales; by 1984 they were into the Northern Territories, and are now fully into Western Australia. Research by Rick Shine’s group has demonstrated rapid evolutionary changes in the characteristics of cane toads in the migration front: longer limbs, larger bodies, faster movement- allowing the front to advance now at 60 km per year.
The effects of cane toads on Australian native species are varied and changeable: they mainly relate to the toxicity of adults and tadpoles, reducing the effects of predators. Some Australian species have shown steep population declines following the arrival of cane toads: examples are the marsupial carnivore, the Northern quoll, and the varanid lizard known as the goanna. However, some predators appear to have adapted to the presence of cane toads and have found ways to avoid their toxins. In the longer term, we might expect that to progress. Overall, although there is considerable alarm over the effects of cane toads in Australia, they remain uncertain.
Many control methods have been tried, but none has been rated as successful so far. The annual Great Cane Toad Bust, lasting a week, has teams of local volunteers capturing adult toads and killing them by freezing. This is unlikely to have a significant effect on a population estimated at around 200 million. A feature that needs more research is what keeps cane toad populations in check within their native range: Lampo has estimated that native population densities are only 1-2% of those reached in Australia.
Finally, how effective were cane toads in controlling sugar cane pests in Australia? Shine’s analysis shows that the effects of cane toads have been complex, but not overall beneficial to sugar cane production. Sugar production did not increase following cane toad introduction. One reason for this is, that as a generalist predator, cane toads consume not only the beetles that are a pest of the cane, but also the ants which help control the beetles. They also poison the varanid lizards, another beetle control. Another factor is that the rodent population is unaffected by toad toxin, allowing them to consume toads and therefore grow their populations, thereby increasing their harmful effects on sugar cane. So despite the toads eating many beetles, their overall effects made matters worse for sugar cane production.
This cautionary tale should be at the forefront of thinking, whenever anyone advocates the introduction of an alien species as the answer to an ecological problem: the consequences are hard to predict.
Hinchliffe (2023). Public enemy no.1: on the hunt with Queensland’s army of volunteer toad busters. The Guardian 28/1/2023.
Lampo et al. (1998). The invasion ecology of the cane toad from South America to Australia. Ecological Applications 8, 388-396.
Phillips et al. (2010). Evolutionarily accelerated invasions: the rate of dispersal evolves upwards during the range advance of cane toads. Journal of Evolutionary Biology 23, 2595-2601.
Shine (2010). The ecological impact of invasive cane toads in Australia. Quarterly Review of Biology 85, 253-291.
Shine et al. (2020). A famous failure : why were cane toads an ineffective biocontrol in Australia? Conservation Science and Practice 2, e296.
Wikipedia articles on: The cane toad; the cane toad in Australia.