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Rabbits and Toads—Invaders of a Continent

BY AWAKE! WRITER IN AUSTRALIA

THE battlefield is a scarred, barren wasteland. The once lush field is now pitted with deep holes. The bodies of combatants litter its surface. These soldiers are not clad in camouflage green with boots and bayonets but with soft coats of fur and sharp teeth. They are Australia’s plague of feral rabbits.

Rabbits, Rabbits, Everywhere

European rabbits launched their incursion on the southeastern tip of Australia in 1859. Imported for the amusement of local sportsmen, they were soon hunted, not for sport, but in a desperate bid to control their numbers.

While the European rabbit took 900 years to colonize Britain, in just 50 years it overran an area of Australia more than half the size of Europe. With adult females producing up to 40 young a year, rabbits pushed the front line of assault across the continent at a rate of up to 60 miles [100 km] a year. A report from the Bureau of Rural Sciences (BRS) states: “It was the fastest rate of any colonising mammal anywhere in the world.” The effects were devastating.

Rabbits consume the fodder of native animals and commandeer their burrows; they are blamed for the localized extinction of numerous species. They are even held responsible for clearing forests. As one researcher explains, “they eat tree seedlings so that when the adult trees die there are no young ones to replace them.” When they invade a small island, the results can be catastrophic. “Rabbits introduced onto Laysan Island in 1903 had by 1936 eliminated three endemic bird species and 22 of the 26 plant species. . . . In 1923 the island was a barren waste of sand with a few stunted trees,” states a BRS report.

Deploying Weapons of Mass Destruction

In Australia rabbits were shot, trapped, and poisoned. The famous Rabbit Proof Fence—stretching 1,139 miles [1,830 km] across the state of Western Australia—was built in an attempt to halt their advance.a But nothing seemed to be able to hold back the invading army.

Then, in 1950 a counterattack was launched using a biological weapon—the myxomatosis virus. This virus dramatically reduced the rabbit population, estimated by that time to be a staggering 600 million. Carried by mosquitoes and fleas, myxomatosis affects only rabbits, and it killed 500 million of the invaders in just two years. However, rabbits rapidly developed resistance to the disease, and survivors reproduced with a vengeance. Thus, by the 1990’s their numbers had soared to about 300 million. Another defense was desperately needed.

Bad News—Good News

In 1995 a second biological weapon, rabbit hemorrhagic disease (RHD), was released in Australia. RHD first appeared in China in 1984. By 1998 it had spread to Europe and shortly afterward wiped out 30 million domestic rabbits in Italy. RHD was bad news for the European rabbit industry but good news for Australian farmers, as ten million rabbits were destroyed in the first two months following its release. The virus seems to confine itself to rabbits, which die 30 to 40 hours after infection, with no visible signs of distress. By 2003, RHD had reduced rabbit numbers in many drier parts of Australia by 85 percent or more.

Without rabbits to nibble their foliage, native orchids in one South Australian national park increased eightfold in less than five years. In other parts of that state, there has been “significant early regeneration of native shrubs . . . in areas where regular outbreaks of the disease had occurred,” states Ecos magazine. Introduced predators, such as foxes and feral cats, have also declined in some areas because of the lack of rabbits. Both ecologists and farmers are pleased by the effectiveness of this new weapon, since rabbits have cost the Australian economy up to $600 million a year. However, the long-term effect of this disease on Australia’s resilient rabbit population is yet to be determined.

From Shining Knight to Ugly Toad

While scientists may have won the day against feral rabbits, they seem to have met their match with a more recent invader—the cane toad. Like the rabbit, this villain did not sneak into the country but was deliberately imported. Why?

Early in the 20th century, two species of sugarcane beetle threatened the existence of Australia’s sugarcane industry, currently worth two billion dollars annually to the economy. In 1935, Bufo marinus, or the cane toad, a fist-sized amphibian with a reputation for being a voracious beetle eater, was thought to be the savior of cane growers. Despite the misgivings of some scientists, the toad was imported from South America via Hawaii and was released into Queensland’s cane fields.

Upon release the cane toad ignored the cane beetles and became a renegade. These creatures are toxic in every stage of their development from egg to adult. As they transform from tadpoles to toads, special glands grow under their skin, which exude a highly poisonous milky slime when the toads become annoyed. Cane toads are known to kill native lizards, snakes, wild dogs, and even crocodiles that are foolish enough to ingest them. They are prolific breeders and have now spread more than 600 miles [900 km] from their original release points. Population densities reach up to ten times that found in their native country, Venezuela. Like a Biblical plague, they invade fields, infest homes, and lurk in toilet bowls. Advancing at a rate of 15 miles [30 km] a year, they have now entered an area that could be described as toad paradise—the world-heritage-listed Kakadu National Park in the Northern Territory. The Australian government has spent millions of dollars funding research aimed at halting the toads’ progress, yet no effective method has been found. The battle is not over, but so far the toads are winning.

Why the Conflict?

In an undisturbed ecosystem, organisms have their own natural regulators that keep their population numbers restrained. However, when liberated from the checks and balances of their native environments, seemingly harmless creatures may reproduce so fast that they wreak havoc.

The enormous damage resulting from uncontrolled populations of introduced animals and plants was not foreseen by the first European settlers in Australia. True, many imported species have proved beneficial. In fact, Australians are now completely dependent on introduced species of plants and animals—sheep, cattle, wheat, rice, and other staples. However, the rabbit and the cane toad are sobering reminders of the need for caution when humans choose to manipulate earth’s dazzlingly complex web of life.

[Footnote]

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A hero turned villain—the cane toad invasion continues

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U.S. Geological Survey/photo by Hardin Waddle

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Thirsty invaders at a water hole on Wardang Island, Spencer Gulf, South Australia

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By courtesy of the CSIRO

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Rabbits: Department of Agriculture, Western Australia; toad: David Hancock/© SkyScans