The Australian Cane Toad Essay, Research Paper
THE AUSTRALIAN CANE TOAD
The cane toad, Bufo marinus, or giant toad, was introduced to Australia by the sugar cane industry with government sanction, in order to control two specific pests of sugar cane. The grey backed cane beetle and the frenchie beetle. Native to Central and South America, the cane toad has been introduced to several Pacific islands as well. One hundred and one toads arrived at Edmonton in North Queensland in June 1935. About 11 sugar growing locations in northern and central coastal Queensland received authorized shipments. People at Normanton and Burketown, and in northern New South Wales deliberately released the cane toad into the wild. Scientists warned the farmers not to bring the cane toad to Australia but the farmers did not listen and brought them in anyway.
Did the cane toad have any impact on the two cane beetles it was introduced to control? Apparently not. The cane toad ate beetles when they were available, but as a control agent, it had no impact at all. Instead of controlling certain insect populations, the cane toad ate large numbers of bees and other beneficial insects. Within 5 years, an effective insecticide became available and the sugar industry lost interest in the cane toad.
Although not native to Australia, the cane toad has one of the widest ranges of any living toad. The species lives in a wide variety of habitats, but is restricted mainly by the availability of water, since water is a vital element in the breeding cycle. However, toads can survive near very small pools, or steams in arid regions. During the dry or cold seasons, they remain inactive in shallow ground excavations beneath ground cover.
Cane toads are very large and heavily built amphibians (up to 15 cm long) with warty skin. The skin is strong, tough, and durable. Females tend to be larger and smoother-skinned than males. Cane toads are olive-brown to reddish-brown on top, with a paler white or yellowish belly. The underside is usually spotted with brown. The toad is characterized by a stout body, which is heavier than that of frogs.
The most distinctive features of the cane toads are bony ridges over each eye and a pair of enlarged glands, one on each shoulder. These glands are able to ooze venom. A pronounced angular ridge runs between the eyes and snout.
Giant toads can tolerate temperatures of 0 degrees Celsius to 41 degrees Celsius and are able to survive high levels of dehydration. They can also adapt to different temperatures. Their temperature and moisture tolerances may limit their distribution. However, they do occur in warm temperate to semi-arid climates and are abundant in the wet and dry tropics. A prediction, based on their ability to tolerate a variety of climates, is that they will become established in Darwin early next century and eventually spread over much of the coastal seaboard of Australia.
The call of the male cane toad is a high-pitched “brrrr” which sounds like a telephone dial tone. The cane toad also has a distinctive stance and hop. It sits upright in an almost vertical position and moves in a series of fast, short hops rather than long ‘frog like’ hops. Also cane toads do not have webs between their toes.
Cane toads will eat almost any small creature they can catch. They eat whatever is available. They often eat bees and dung beetles, small amphibians, reptiles, and mammals. In fact, they eat any animal they can swallow. Unlike other amphibians, giant toads eat things which do not move. They have also been known to steal food from dog and cat bowls.
They have few predators native to Australia. The common Fresh Water Snake, (Tropidonophis mairii), is the only Australian snake known to be able to feed on small cane toads without dying as a result. Other native animals such as the estuarine crocodile, the water rat and species of ibis are believed to feed on toads or on their internal organs.
Behavior and Breeding
Cane toads are highly adaptable, both in terms of survival and reproduction. They are much more tolerant than other Australian frogs and can survive and breed in somewhat salty water. In Australia, giant toads normally breed from June to January, but they have been found in breeding condition throughout the year. Cane toads usually begin breeding in their second summer, when they are about 75 mm long. The cane toad needs only a small pool of water for breeding. The males fertilize the eggs as they are laid. Male toads will attempt to mate with anything resembling a female toad – living or dead. As many as 35,000 eggs may be produced by each female, thus giving the species a high breeding potential. Cane toad eggs are blackish in color and are deposited in long jelly like strings onto plants, rocks or debris near water. The spawn consists of long double chains of black eggs about 1 mm in diameter enclosed in a transparent cover.
Embryos begin hatching within 48 hours; after several days, the tadpoles begin feeding, and the tail grows proportionately larger and hind limbs develop. In three days, the eggs hatch into small (3 cm) jet black tadpoles – unlike those of any native frog. These tadpoles become toadlets unusually early, so they are out of the water and hopping around faster than most other frogs. Cane toad tadpoles differ further from most species in that they occur in massive numbers and frequently form dense aggregations in shallow water.
B. marinus adapted well to the Australian environment. So well that they are moving closer to the wetlands of Kakadu National Park, which includes the Katherine Gorge. District park manager John De Koning says wildlife like crocodiles, goannas, and snakes will be threatened by the arrival of the non-native pest, even though they have only found a few so far. “We came across one large female, this was right on the very southern eastern edge of the Nitmiluk National Park. We came across one female and we could hear several males in the distance.” The natural rate of spread of the cane toad is now 30-50 km/year in the Northern Territory and about 5 km/year in northern New South Wales.
Finally, because their diet is so variable, they do not need to expend much energy searching for food. They can just sit in a convenient spot, and gobble up anything that wanders by. In urban areas, they are often seen gathered around street lamps eating insects attracted by the light
One of the most important factors in the success of the cane toad is that they are highly poisonous to eat, at every stage of their life cycle. All frogs and toads may have enlarged chemical-secreting glands at particular points on their bodies, or small glands spread over the whole skin. The cane toad is one such amphibian. These secrete white venom when the animal is handled or threatened. The eggs and tadpoles are also poisonous and can cause cardiac arrest and death.
A cane toad’s reaction to a threat is to turn side-on to its attacker so that the venom glands face them. The glands on the cane toad’s shoulders are also capable of oozing venom or even squirting it over a distance of up to 2m. Animals picking up a cane toad and receiving a dose of venom may die within fifteen minutes. This venom is composed mainly of cardioactive (affecting the heart) substances.
The biggest danger to humans is that the venom could come in contact with the eyes, where it causes intense pain and temporary blindness. Under pressure cane toads can shoot their venom a short distance. This substance may be splashed into a person’s mouth or eyes as they attempt to kill the toad. Since the poison can be absorbed into the system through mucus membranes, without ever being swallowed. Therefore, the mouth, eyes, and nose should always be rinsed thoroughly if contact with venom occurs.
Experiments and observations indicate that a variety of native animal life are extremely susceptible to the many poisons in the cane toad’s venom. These include avid frog eaters such as the Tiger and Red bellied Black Snakes and the quolls. In areas where toads appear, there has been a subsequent decline in populations of these types of native animals, although more research is needed in this field.
Research and Control
“The CSIRO Australian Animal Health Laboratory (AAHL) at Geelong is a high security microbiological facility, purpose designed and operated to undertake research into viruses, bacteria, fungi and parasites which are exotic and do not occur in Australian domestic or wild animals.” It is the only laboratory of its kind in Australia where studies on these exotic micro-organisms can be undertaken. Recently, the laboratory has been commissioned and funded to begin research into the control of the cane toad, using viruses or other microbial agents found overseas.
The major concerns about the toad involve its “prodigious” appetite, and the toxicity of all its life stages to native animals. There are firmly held beliefs that these characteristics of the cane toad are responsible for the deaths of Australian wildlife including herpetofauna, mammals, and fish. The toad will almost certainly establish itself throughout the sensitive wetlands of northern Australia. The Australian Government has provided significant funds to gather data to determine whether the toad has an impact on the Australian environment and whether a biological control agent is required. The funding also encompasses the search for and assessment of possible control agents.
Funding of the project is distributed through the CSIRO Division of Wildlife Ecology, Canberra, upon the advice of the Cane Toad Research Advisory Committee. Current work to investigate the control of the cane toad by biological means has evolved from “extensive studies over the past decade which have gathered basic ecological and disease data for the species.” Such studies have been conducted in Australia, Venezuela, and Brazil. A search for “microbial” agents with potential for control of toads has recently been concluded in Venezuela. Research into the potential of viruses to control cane toads involved isolating and purifying viruses from cane toads in their native habitats of Venezuela, in South America. The effects of the viruses on cane toads and native frog species were then tested in the secure “biocontainment” facilities at the CSIRO Australian Animal Health Laboratory. Dr Alex Hyatt from CSIRO Animal Health says viruses isolated from Venezuelan cane toads were compared with other viruses of the same family from around the world, and are believed to fall within a new species of virus. While the viruses proved effective in killing cane toad tadpoles, they also killed one species of Australian frog in the trial. The team also found a small percentage of Australian cane toads in the wild had been exposed to a virus similar to the Venezuelan viruses, which are known to cause disease and death in fish and amphibian populations in Australia and overseas. “This adds another dimension to the potential impact of cane toads on the Australian environment,” Dr Hyatt says.
As part of the work, the researchers also identified two fungal pathogens that are lethal to cane toads and other amphibians. One fungus is thought to be responsible for frog fatalities in Australia and Panama. Research also shows a small number of Australian cane toads may be carrying a virus similar to the Venezuelan viruses, which could affect Australian wildlife.
CSIRO scientists have ruled out the use of viruses from Venezuela to control cane toads in Australia because laboratory trials show that the viruses can also kill native Australian frogs as well as the toads.
At AAHL, a specialized group has been formed bringing together expertise in virology, aquatic animal pathology, electron microscopy, and molecular biology. Expertise in the group has resulted in the isolation of previously unknown disease causing agents in Queensland.
The objective of the current project is to find “exotic, infectious microbial” agents which may spread throughout cane toad populations in Australia and decrease their numbers. The project is also assessing the effects of these agents on adult, metamorphic, and juvenile life stages of the toad, since it is likely that different life stages have differing weaknesses.
Further research is being undertaken by CSIRO. Giant toads are often transported in shipments of fruit and other commodities. Until effective control methods are available, quarantine checks and the destruction of any accidental releases of toads are essential to reduce their rate of spread.
Challenge experiments have commenced to evaluate the effects of viruses on toads. These experiments are conducted under maximum “microbiological security” to ensure that escape of the viruses cannot occur. Toads are maintained in “laminar flow” cabinets, within sealed rooms. The air pressure of the rooms is lower than atmospheric pressure; thus ensuring all air movement is into the room. Air leaving the room is double filtered to eliminate the “smallest viral particle from escaping.” Water from the room is heated to kill any infectious agents. Entrance to the room is through an air-lock and exit of personnel from the room requires a full three minute shower.
In association with the challenge experiments is a spectrum of “microbiological, serological, and molecular studies to characterize and compare the agents under investigation, to establish information on the host’s resistance to infection, and to gather information related to similar agents in toads and other amphibian populations.” Should an agent be found which offers the potential for control of the toad, an extensive series of subsequent studies is planned. To achieve the objectives of the project, amphibian populations from around the world are being studied for possible infectious agents. An international network of scientists, scientific institutions, interest groups and interested individuals is being developed for information exchange relating to diseases and population declines of amphibians.
Potential for biological control of the toad is considered good, as the toad is the only representative of the bufonid family in Australia and is distinct from other Australian amphibian species. In addition, many species of the genus Bufo exist overseas, offering exciting prospects that an infectious or parasitic agent from these may cause disease in Australian Bufo marinus, without affecting native species.
It is hoped that these studies will provide valuable information on “the causes of, and initiating factors behind, the recent declines in frog populations in Australia, Britain and elsewhere.” Also, to contribute to the possibility that an infectious agent might be found that would control cane toads in Australia. They are constantly looking for possible pathogens for consideration for the biological control of cane toads.
Finally, Chinese medicine manufacturers have been using the toads for centuries in the treatment of Cardio Vascular Diseases and Cancer treatment. It is highly likely that the cane toad will one day be farmed in Queensland for production of therapeutic medicines.
There is still much work to be done to fully understand what effects cane toads have on native wildlife, and just how far they can spread. There are some reasons for optimism. In the areas where cane toads have been around for the longest time, their populations have declined after the initial population explosion. It is also possible that some native animals are learning to avoid eating them. Other animals have shown they can eat the toad. The Keelback Snake can detoxify the venom and Water Rats, Ibis, Crows and other birds turn the toads over and eat only the non-poisonous internal organs. Opinion is divided concerning their current status, some think the native wildlife is starting to recognize them as a threat and they stay away.
Australia is still a long way from controlling cane toad numbers or putting a stop to their expansion. Scientific evidence suggests that this “imported animal represents a nuisance to man and an ecological threat to the Australian environment.” The rapid growth of the species may have consequences in areas considered irrelevant at the time of its introduction. The cane toad has provided a painful lesson in what can happen to native species when an exotic species is introduced to a new habitat.