Toxic Invaders: The Australian Cane Toad Dissemination

The year 1935 saw the beginnings of a seemingly beneficial plan to combat a pest threatening Australia’s sugar cane industry, the cane beetle (Dermolepida alborhirtum). The introduction of the cane toad (Bufo marinus) to Australia from Hawaii was expected to reduce the impact of the destructive cane beetle population as it was assumed that the toads would prey on the beetles. In the end, nothing could be farther from the truth. The introduced cane toads generally ignored the beetles they were supposed to eat, and developed a taste for other creatures. The frogs also thrived in their new environment, making their homes in the swamps and other bodies of water throughout Australia’s northern territory. Poisonous to many of Australia’s native species, the consumption of these frogs has killed many animals which mistake them for the native frogs they normally prey on. The impact this invasive species has had on Australian wildlife is devastating, with certain native species now threatened with extinction due to the baneful effects of the cane toad invasion. With no common threats and the ability to propagate quickly, the cane toad has undoubtedly become one of the greatest dangers to the ecology of Australia.

The cane toad is a fairly large toad which can reach a length of 4-6 inches and weigh as much as 2.9 lbs.[1]The toad’s coloration can vary from gray to yellow to brown. It is large and bulky with dry, warty skin as well as bony ridges above its eyes.[2] These toads also possess parotoid glands located over their shoulders (noticeable in adults as large swellings) which secrete a deadly white liquid that is actually a mixture of different toxins. Powerful enough to kill many animals which dare to attack the toad, the toxic mixture primarily targets the functioning of the heart. Though it can be secreted from the parotoid glands, the toxin is also present throughout the amphibian’s body.[3] Juvenile cane toads resemble many of Australia’s native frogs, with their lack of toxin-secreting glands and smooth, dark skin. This leads to many predators mistaking the toads for a safe food source and consuming them, a detrimental error. Cane toad tadpoles are much darker in coloration, consisting of a shiny black color on their upper bodies and a plain dark belly. At less than 3.5 cm long, they are quite tiny even in comparison to most native tadpoles.[4] The toads are poisonous throughout the entirety of their life cycles.[5]

Cane toads are native to the southern United States, Central America, and tropical South America. Their numbers within this range are manageable due to the presence of other species designed to prey on these poisonous toads. Such is not the case in Australia, where threats to the toads are much less common.[6] More than 3000 individuals were originally released into the sugar cane plantations of northern Queensland in 1935. Since then, the species has rapidly spread throughout northern Australia. They were first sighted within the Northern Territory 1984, and by 2001 the toad population managed to spread into Kakadu National Park as well as the river catchments surrounding the Park. Around 3-4 years later, the hazardous amphibians reached Darwin, alerting many to the species’ rapidly expanding population. The species also spread south from Queensland into New South Wales and has most recently been detected over the border in Western Australia. Though there have been many attempts to stop the population from spreading deeper into Western Australia, the advancement of the toads has only been lessened in speed. The cane toads have also managed to colonize the offshore islands of the Northern Territory. It is believed that the toads were washed out to sea by the flooding of the McArthur River in 2001, with many of these toads eventually reaching areas such as the Sir Edward Pellew Islands in the Gulf of Carpentaria (which had their first recording of the toads shortly after the McArthur River flood).[7] It almost seems as if the toads are unstoppable as they continue to spread throughout Australia.

They have proven to be very adaptable, capable of surviving in a variety of habitats throughout their Australian range, from the margins of rainforests and mangroves to sand dunes and coastal heath. Adults are generally active at night during warm months. During daytime hours or whenever they encounter cold or dry weather, the toads will shelter in moist crevices and hollows. They may also excavate depressions under logs, rocks, and debris. They can easily adapt to cold and dry weather, and have been known to survive the loss of more than 50% of their body water.[8]

These toads are absolutely voracious eaters that will consume nearly anything that they can fit into their stomachs. Beetles, honey bees, ants, winged termites, crickets, and other insects make up the bulk of their diet. However, they will also feed on marine snails, smaller toads and native frogs, small snakes, and small mammals when given the chance. They will even accept pet food, household scraps, and carrion. During their time as tadpoles, cane toads will mainly rely on algae and other aquatic plants as their food sources, though they will also filter organic material from the water and larger tadpoles may occasionally consume the eggs of other cane toads.[9]

Cane toads generally do not have to worry about many predators. In fact, most animals which attempt to eat one are usually putting their lives in danger due to the toad’s powerful toxins. Some of the animals negatively affected or killed by cane toad consumption include Northern quolls (carnivorous marsupials), goannas, frogs, snakes, fish, freshwater crocodiles, and egrets. Pets such as dogs and cats have also been known to bite the toads and suffer from the ingested toxins. Indeed, in parts of Queensland, many native animal populations were dramatically decreased when these dangerous toads were first introduced. However, it appears that many species have recovered by learning to avoid the toads as a food source.[10] In fact, some animals have taken this behavior a step forward. Both young and adult cane toads are preyed on by freshwater crayfish, saltwater crocodiles, crows, white-faced heron, kites, bush-stone curlews, tawny frogmouths, water rats, and giant white-tailed rats. Some of these predators only eat the toads’ tongues, while others attack their bellies and consume only the mildly poisonous organs. Cane toad tadpoles are also preyed on by dragonfly nymphs, water beetles, saw-shelled turtles, and keelback snakes. Keelback snakes can also kill and eat young toads, with laboratory tests showing that the snakes can handle low levels of toad toxins.[11] Despite these cases of animals adapting to cane toad predation or predators with naturally high tolerance to the invasive amphibians’ toxins, the cane toad population continues to increase.

Apart from the lack of enough predators to maintain their population numbers in check, the cane toad’s rapid dissemination is due to the fact that it is a prolific breeder. For these creatures, breeding takes places twice a year between June and January, though they can breed throughout the year under the right conditions. Each female can lay anywhere from 8,000-35,000 eggs at a time. The eggs hatch after two or three days and the newborn tadpoles can change into toads in as early as 17 days, though this depends on temperature and food availability (under less favorable conditions, it can take tadpoles as long as 180 days to change into toads). The toads can reach sexual maturity within a year, and the process can then take place again.[12] The combination of bountiful reproduction and rapid growth makes this threatening species incredibly difficult to manage.

Managing the spread of the cane toad is normally handled by creating barriers which halt the advancement of the species into certain areas, such as ponds or dams. Quarantine checks and public awareness and response have also been of aid, though the toad populations still continue to grow over time. Scientists at the CSIRO Animal Health Laboratory in Victoria are currently investigating gene technology as a possible mechanism of control. Also, scientists at the University of Adelaide have managed to isolate a sex pheromone in a native Australian frog. If a similar pheromone is discovered in the cane toad, then scientists may be able to use it to disrupt the cane toad’s breeding cycle.[13] For now, however, only time will tell how the invasive toads are dealt with.

The cane toad is one of the deadliest of all invasive species. With its ability to grow rapidly, breed prolifically, adapt to new challenges, and secrete toxins that are dangerous to most of Australia’s native wildlife, it has proven itself to be an ecological nightmare for the continent’s environment. There are currently no solid methods for combating these creatures, and their numbers continue to grow as time passes by. The Australian cane toad invasion is an important example of how the introduction of a foreign species into a new environment can lead to unexpected and disastrous consequences. Whether the cane toad is ever eliminated from Australia or not, it is crucial to remember that ecosystems are complex structures that cannot be carelessly tampered with. Bearing this in mind, it is possible to avoid similar disasters in the future.