Alien Invasion

Examining the rapid takeover of Australia by the cane toad and its invasive brethren.

Cane toads were introduced onto the continent of Australia for one simple reason in 1935 — to control the cane beetle population. It was easy to see why farmers clamored for a natural solution to the cane beetle predicament as cane beetles were destructive pests to sugarcane crops. Adult cane beetles ate the leaves of the crops while cane beetle larvae ate the roots, potentially killing the entire plant. At the time cane toads seemed like a logical, and less destructive, alternative to harmful chemical pesticides. But those who decided on the introduction of the large, toxic toads failed to take into account two glaring problems which thwart their ability to effectively serve as pest control agents against cane beetles — The stocky toads do not climb (making it difficult to feed on the arboreal cane beetles) and cane toads are nocturnal, while cane beetles are diurnal. Now Australia is stuck with more than 200 million introduced toads and cane beetles.

Cane toads (Rhinella marina) are native from South America up to the southern U.S. In their native ranges, the toads populations are kept in check by an assortment of predators capable of tolerating the toad’s poison that is produced in two large large parotoid glands behind the toad’s ears. This poison is spread throughout their bodies and can be secreted from the parotoid glands as a milky, white substance. The deadly cocktail of toxins produced by the cane toad targets the functioning of the heart. When these toads were introduced to Australia, this new poison knocked every Australian predator, from venomous snakes to large goanna lizards and even beloved pet dogs and cats, back on their heels.

In addition to being a hopping toxin-producing machine, cane toads possess several other tools that make them a particularly destructive invasive species. The toads are voracious eaters, and will eat anything they can get their wide mouths around (except cane beetles, of course) and have outcompeted several insectivores, such as native skinks, in addition to depleting populations of native insects. They breed quickly and in almost any source of water. Even their tadpoles and eggs are poisonous to most animals.

Around 3,000 cane toads were released in northern Queensland in 1935 and they have spread like wildfire throughout the eastern and northern coasts of Australia. Moreover, their range is continuing to expand as they order their populations forth. Cane toads have even been found on the far-flung southwest corner of the continent. And they are not content with just Australia, either. They have been found in New Guinea and up into the Philippines. With their destructive traits, cane toads may circumvent the globe and arrive back in South America in a matter of decades.

The spread of the durable and leathery toad is not an isolated incident for Australia. The continent has been ripe for the spread of invasive species since Europeans arrived in the late 1700s. Australia’s isolation from most of the world, since its break from Antarctica about 35 million years ago, has produced a variety of incredible endemic species and allowed marsupials to dominate while being outcompeted by placental mammals (more advanced group of mammals including humans) everywhere else on earth. But this isolation has left the continent susceptible to outside invaders completely alien to native wildlife.

The most destructive invasive species have been nondescript species such as feral cats and foxes. Over the two centuries these European animals have been on Australian soil, at least 34 mammal species have gone extinct, giving Australia the dubious distinction as the world’s leader in mammal extinction. Most of these species are smaller-sized marsupials, like the lesser bilby or the beautiful toolache wallaby (who only survived 85 years of European colonization), that are the perfect size for cats and foxes to prey upon. Cane toads are extremely destructive and difficult to eliminate, but feral cats are a whole different story. The cuddly felines are one of the only animals known to kill for sport, creating depressingly steep kill totals of Australia’s small mammalian fauna.

Although these invaders may seem too staggering in numbers and destruction, there are several measures being undertaken by the Australian government to attempt to rid the continent of these dangerous aliens. Large-scale cat culls are underway throughout Western Australia, including an initiative to drop crate-fulls of poisoned sausages to the two to six million cats that live there.*

Intensive fox eradication programs have been undertaken on many islands, whose contained settings offer the only real chance of eliminating the elusive predators. On Phillip Island, off of Victoria, foxes have been successfully removed from the island in recent years. The bridge between the mainland and the island, the only way on and off the island for anything that does not want to get wet, is under constant surveillance to make sure no fox is able to sneak back on the island. Their eradication on Phillip Island has led to the reintroduction of the Eastern barred bandicoot, which has gone extinct in the wild because of foxes. In 2017, 44 captive bred bandicoots were released on the Summerland Peninsula, itself a symbol for effective conservation implementation. Hopefully the bandicoots can recoup their population on the island away from the threat of foxes.

The management of cane toads will take a concerted team effort by people all over Australia. In southern Australia, quarantine checks for the deadly amphibian, as well as education programs, have helped to curb its spread into the region. If a suspected cane toad is seen, the best outcome would be to capture the toad and report it to the nearest Parks office, as 2/3 of suspected cane toads end up being native frog species. Oftentimes these native frog species are harmless and themselves cane toad victims due to diseases the South American toad spreads.

Cane toads, foxes and cats, along with many invasive species of plants, are incredibly well-equipped and destructive invaders. It will take decades to wipe them out and save Australia’s unique wildlife, and even that may not be enough as these animals become more engrained in the Australian ecosystem. But progress is possible and needs to start right now in order to help Australia lose the title of being the global extinction capital and chart a new path towards saving the vulnerable local wildlife under siege by these ruthless invaders.

*Because cats, beloved pets to some and brooding houseguests to others, are the target of this mass culling, many public figures decried the decision. More than 160,000 people signed online petitions to prevent the killings of the cats, even as they wipe out species after species. Educated public outcry can be beneficial, but there clearly was a lack of understanding about the cats’ destructive capabilities. Thankfully, the Australian government persevered and proceeded with the plan, which killed 211,560 cats during the first year of the program and aims to shrink the population by 2 million cats by 2020.

Sources:

https://www.nationalgeographic.com/animals/amphibians/c/cane-toad/

https://australianmuseum.net.au/learn/animals/frogs/cane-toad/

https://phys.org/news/2018-04-australia-mammal-extinction-worsen-scientists.html

https://www.penguins.org.au/conservation/research/eastern-barred-bandicoots/