The cane toad in Australia is regarded as an exemplary case of an invasive species. Australia's relative isolation prior to European colonisation and the Industrial Revolution, both of which dramatically increased traffic and import of novel species, allowed development of a complex, interdepending system of ecology, but one which provided no natural predators for many of the species subsequently introduced. The sudden inundation of foreign species has led to severe breakdowns in Australian ecology, after overwhelming proliferation of a number of introduced species, for which the continent has no efficient natural predators or parasites, and which displace native species; in some cases, these species are physically destructive to habitat, as well. Cane toads have been very successful as an invasive species, having become established in more than 15 countries within the past 150 years. In the Environment Protection and Biodiversity Conservation Act 1999, the Australian government listed the impacts of the cane toad as a "key threatening process".
History Introduction and spread
Native to South and mainland Middle America, imported cane toads had been used in Puerto Rico to control sugar cane pests since 1920, and an influential 1932 research paper by Raquel Dexter showed that they largely ate beetle larvae that in turn ate sugar cane. Based on her findings, they were introduced to Hawaii by Cyril Pemberton in the early 1930s, and then introduced to Australia from Hawaii in June 1935 by the Bureau of Sugar Experiment Stations, now Sugar Research Australia, in an attempt to control the native grey-backed cane beetle (Dermolepida albohirtum) and French's beetle (Lepidiota frenchi).Clarke, G. M., Gross, S., Matthews, M., Catling, P. C., Baker, B., Hewitt, C. L., Crowther, D., & Saddler, S. R. 2000, Environmental Pest Species in Australia, Australia: State of the Environment, Second Technical Paper Series (Biodiversity), Department of the Environment and Heritage, Canberra. Those beetles are native to Australia and they are detrimental to sugarcane crops, which are a major source of income for Australia. Adult cane beetles eat the leaves of the crop, but the main problem is the larvae, which feed on the roots. Adult cane beetles have a heavy exoskeleton and their eggs and larvae are often buried underground, making them difficult to exterminate. Furthermore, conventional methods of pest control, such as pesticide use, would have undesirable effects like eradicating harmless species of insects as well. Cane toads were to replace the use of pesticides, such as arsenic, pitch, and copper. The success of using the moth Cactoblastis cactorum in controlling prickly pears in Australia led to the hope that the cane toad would perform a similar function.
In June 1935, 102 cane toads (Rhinella marina, formerly ICZN Bufo marinus) were imported to Gordonvale from Hawaii, with one dying in transit due to dehydration. By March 1937, some 62,000 toadlets were bred in captivity and then released in areas around Cairns, Gordonvale, and Innisfail in northern Queensland. More toads were released around Ingham, Ayr, Mackay, and Bundaberg. Releases were temporarily limited because of environmental concerns, but resumed in other areas after September 1936.
Since their release, toads have rapidly multiplied. By 2011 they were estimated to number over 200 millionThousands killed in 'Toad Day Out' The Sydney Morning Herald, 27 March 2011. Retrieved 19 December 2023. and have been known to spread diseases, thereby affecting local biodiversity. Not only has the introduction of the toads caused significant environmental detriment, but there is no evidence that they have affected the number of cane beetles which they were introduced to prey upon.
The spread of cane toads was slow at first but, by 1959, they had colonised most of Queensland's east coast. In 1964, they appeared in the Gulf of Carpentaria. By 1978, they had reached the border of New South Wales and, by 1984, they had reached the Queensland/Northern Territory border. In March 2001, the invasion front entered the wetlands of the heritage-listed Kakadu National Park and, by 2009, the toads were close to the Northern Territory/Western Australian border, and by 2011 had become established in irrigation areas around Kununurra, in WA's north.Hitchhiker cane toad found in WA southwest/ AdelaideNow, 2 February 2011.
They have also spread south into northern New South Wales, with one isolated community in Port Macquarie. An isolated colony found in 2010 in the south of Sydney, at Taren Point, was later eradicated.Cane toads beware Press release, NSW Department of Planning, Industry and Environment, 15 April 2014. Retrieved 19 December 2023. By 2019, they had become a pest in the Torres Strait Islands, probably carried there by boat. By 2022 cane toads had been sighted as far south as Bankstown, southwest of Sydney. In 2023 cane toads were found in Dural, northwest of Sydney.
The toads on the western frontier of their advance have evolved larger legs, which is thought to be related to their ability to travel farther. As a consequence of their longer legs, larger bodies, and faster movement, about 10% of the leading-edge cane toads have also developed arthritis. The toads were estimated to be advancing the frontier at a rate if per year in 1994, and in 2014 the rate of advance was estimated to have risen to per year on the western front.
In 2023, rangers discovered a female cane toad in Conway National Park in north Queensland which, recorded unofficially at 25 cm and 2.7 kg and dubbed 'Toadzilla', may be the largest ever seen.Knight, Mia Giant cane toad found in Conway National Park in north Queensland weighs 2.7kg ABC News, 20 January 2023. This specimen has been preserved for display at the Queensland Museum.Knight, Mia; Naunton, Jessica, and Stephen, Adam Toadzilla to join other giant cane toads at Queensland Museum ABC News, 4 February 2023.
Ecological effects
The long-term effects of toads on the Australian environment are difficult to determine, but some effects include "the depletion of native species that die eating cane toads; the poisoning of pets and humans; depletion of native fauna preyed on by cane toads; and reduced prey populations for native insectivores, such as skinks."
Precipitous declines in populations of the northern quoll, Mertens' water monitor, and Mitchell's water monitor have been observed after toads have invaded an area, causing them to become endangered. A number of cases of declines in goanna and snake populations have been reported after the arrival of toads. For example, local populations of yellow-spotted monitor dropped by up to 90% when their habitat was invaded by cane toads.J. S. Doody et al (2007): A Preliminary Assessment of the Impacts of Invasive Cane Toads (Bufo marinus) on Three Species of Varanid Lizards in Australia. Mertensiella 16 (Advances in Monitor Research III), S. 218–227 The preliminary risk assessment of cane toads in Kakadu National Park stated that the predation of the cane toad by native wildlife is the greatest risk to biodiversity. Other factors, such as competition with native wildlife for resources, and the predation of the cane toad on native wildlife, were considered much lower risk factors, but requiring further study. In the Northern Territory, goanna deaths resulting from poisoning after predation on cane toads has been linked to a rise in the number of undamaged saltwater crocodile eggs. Cane toads were present within a few days of the crocodiles hatching in April 2007.
Numerous native species have been reported as successfully preying on toads. Some birds, such as the black kite (Milvus migrans), have learned to attack the toad's belly, avoiding the poison-producing glands on the back of the head. There are numerous reports of Ibis picking up cane toads in their beaks, flicking them about, washing them and then eating them. Anecdotal reports in the Northern Territory suggest that a native frog, Dahl's aquatic frog (Litoria dahlii), is able to eat the tadpoles and live young of the toad without being affected by the poison that often kills other predators. This may account for slower than expected infestations of toads in certain areas of the Northern Territory, although later research carried out jointly by several Australian Universities casts doubt on these reports.. Some snake species have been reported to have adapted smaller jaws so that they are unable to swallow large cane toads, which have large quantities of poison.
Another study, however, notes that the cane toad is adapting to a wider environmental range and may in the future be spreading into habitats currently not available.
In 2009, the native meat ant was found to be immune to the toad's poison and can successfully prey upon young cane toads. Whereas native frogs and toads have natural reflexes to avoid the meat ants, the cane toads do not tend to try to escape the ants, rather standing still when attacked waiting for the toxin to kill the attacker.
New research has indicated that cane toads prey on dung beetles by nestling in cow pats and waiting for the beetles, eating up to 150 in one meal. In areas where cane toads have free access to water in dams, the dung beetles have been decimated. This indicates that the cane toad has the potential to economically affect the cattle industry through increased disease in cattle.
Predator effects
Predators in Australia are not adapted to the cane toad's toxin, which is its main defense mechanism. Because of this, toads do not tend to hide and are usually targeted by predators, which then expose themselves to the toxic effects. One study suggests Australian reptiles are greatly threatened by invasion of the cane toad, more so than any other group. Two species of crocodiles and around 70 species of freshwater turtles were found to be at risk of invasion, and all species studied were found capable of eating a toad large enough to kill them. All freshwater turtles and crocodiles are predicted to share part of their future distribution with the invasive species by 2030. Australia's varanids and agamids are also at a great risk.
One native freshwater turtle species, Myuchelys latisternum (saw-shelled turtle), which ranges along rivers and streams from Cape York Peninsula to northern New South Wales, is reported to be one of the few native animals that is a successful predator of cane toads.Ryan, Michelle, General Editor. (2000). Wildlife of Tropical North Queensland: Cooktown to Mackay. Queensland Museum. . p. 198. The larger the animal, or predator, is the better chance it has of survival, as its body weight effectively dilutes the concentration of the toxin in its body. There are reports of one native species, the Torresian crow, eating cane toads without ingesting the poison by flipping it onto its back and eating its non-toxic innards via its underside. In 2014, researchers found evidence that the Australian freshwater crocodile (Crocodylus johnstoni) had learned to eat just the hind legs of cane toads. One research study concluded that in less than 75 years, the red-bellied black snake had evolved in toad-inhabited regions of Australia to have increased resistance to toad toxin and decreased preference for toads as prey.
Evidence exists for native predator species adapting to the presence of the cane toad, through learning or evolutionary selection, but the initial drop in population is often steep and can reduce biodiversity on a population level. One proposed solution is to use "teacher toads", or smaller toads that are less likely to kill predators. These toads would allow predators to learn not to eat the toads while mitigating mortality. Some have even proposed adding some chemical to make the toads distasteful to further discourage predation. These efforts have shown some promising results so far.