Editor's note: In an effort to better understand how other countries are managing their parklands, and to compare and contrast U.S. efforts to those from abroad, Traveler on occasion runs items from beyond U.S. borders.
From large Burmese pythons in the Everglades to tiny quagga mussels at Lake Mead, exotic animals are a growing problem in national parks. A recent story from Australia demonstrates that combating exotics doesn't always have to be complicated or expensive. Some ingenious Down Under scientists have discovered a low-tech combination of weapons that's effective against one deadly invader: cat food and ants.
Although it seemed like a good idea at the time, Australia's woes with cane toads offers a classic example of the risks of importing an exotic species to combat a pest—or to use as pets. The war against these toads also offers some very good reasons to keep these critters out of the mainland U.S.
Cane toads (Bufo marinus) were introduced to Australia in 1935 from South America via Hawaii in an attempt to control cane beetles, a pest plaguing the nation's sugar cane industry. The toads have no natural enemies in Australia, so the result was probably predictable: the original 101 toads have now burgeoned into a multitude and are spreading across large areas of the country.
These amphibians pose a major threat to native animals, including those in Kakadu National Park, "a World Heritage area well known for its spectacular wilderness, nature conservation values, rich diversity of habitats, flora and fauna, and wetlands."
What can be so terrible about a seemingly innocuous creature like a toad? According to CaneToadsinOz.com, cane toads "are among the biggest frog species in the world." Although most females are about three to five inches in length, "one recently caught specimen, dubbed “Toadzilla” by the media, was 8 inches long and weighed 1.9 lb."
Even so, the size of these animals isn't the issue. Every life stage of these amphibians, from eggs to adults, is poisonous to most other animals. A government website says, "Their toxin can kill most native animals that normally eat frogs."
That makes cane toads a threat to both wildlife and domestic animals like dogs and cats, and these small pests can even spell the end of large predators. A University of Sydney study found a "75 per cent reduction in the numbers of freshwater crocodiles in the Victoria River District of the Northern Territory after the toads arrived in the river system."
I know what at least some of you are thinking, so let's put this idea to rest right now: these toads are not the solution to the python problem in Florida!
According to the government of New South Wales, cane toads thrive in a wide variety of habitats in both urban and disturbed areas, have a voracious appetite, eat many different foods, and breed rapidly. They are reported to be expanding their range by up to ten miles per year—an impressive pace indeed for amphibians of this size.
As is often the case when dealing with exotic species, it's been difficult to find control measures that are effective against the toads but not detrimental to other species. Solutions to such problems don't always have to be high-tech, and researchers from the University of Sydney's School of Biological Sciences have come up with an ingenious but effective suggestion for controlling cane toads. Professor Rick Shine and his colleagues found that native insects called meat ants were already feeding on young toads, and they began to wonder how this natural predation might be increased.
One answer is... cat food.
The scientists found that placing a small amount of cat food near the edge of ponds when the young frogs were emerging from the water attracted a larger number of ants to those locations. Once the ants were drawn to the area, they discovered something even tastier than cat food—young toads.
According to information from the University, "When cat food was introduced as bait, ant numbers grew and cane toad numbers declined more quickly. The research reveals that meat ants can be used with low risk of collateral damage to native wildlife. The approach is also logistically feasible, low technology and inexpensive."
"Unlike many previous efforts at pest control in Australia, like the cane toad itself, the use of meat ants promises to be a useful component of a broadly-based ecological approach," says Professor Shine.
"We can look at an interaction that's already happening, meat ants are already killing millions of cane toads," Shine explains. "We're just looking to make it a bit easier for them."
Shine's findings were recently published in the British Ecological Society's Journal of Applied Ecology.
Cane toads are easy targets for meat ants because unlike their native counterparts, they don't try to make a quick escape. When faced with an enemy, the young toads are more likely to freeze than flee. This is definitely one time when the "if I don't move, I'm invisible" theory doesn't work…at least for the prey.
So, in a contest between meat ants and young toads, how do the odds stack up?
"The study found 98 per cent of metamorph toads were encountered by meat ants and 84 percent were attacked within a very brief (two minute) period. Over 50 per cent of attacks were immediately fatal, while 88 per cent of 'escapee' toads died within 24 hours."
Those are encouraging numbers in the toad wars, and the technique certainly seems to be both inexpensive and eco-friendly, but there is one small complication: the technique doesn't sit well with a well-intentioned animal-rights group.
Australia's RSPCA says it recognizes that cane toads must be controlled, but urges researchers to "concentrate on identifying effective methods that do not cause unnecessary pain or distress [for the toads.]"
Encouraging predation by ants apparently fails that test, although the group does suggest an alternative that requires catching the toads one at a time and "ensures the animals are rendered unconscious (stunned) prior to killing."
Dr. Shine commends the efforts by a variety of groups to physically capture and dispose of toads, but he points out one of the challenges of dealing with any exotic animal that reproduces rapidly:
"… physical removal can never stop the toad invasion. The reason is simple mathematics—to reduce the number of toads in an area, you have to take them out quicker than they can replace themselves …unfortunately, cane toads reproduce at an amazing rate. They can reach maturity in just a few months, and a female can produce up to 30,000 eggs in a single clutch. So even if you catch 98 out of 100 toads around a pond, the two you missed can produce 30,000 new toads the next night—so even allowing for many of the eggs and tadpoles dying, you may have ten times as many toads there next week …"
So, are ants and cat food the answer to the problem? While Professor Shine points out the potential of the ant-baiting technique, he also cautions, "No single control will be a silver bullet to eradicate the cane toad from the Australian landscape."
The native ants can be part of solution. "If we understand the biology of cane toads and their interactions with Australian fauna we'll be in a much better position to control them," he says. It is hoped the ant-baiting approach will form part of a multi-pronged attack on cane toads.
Australia's battle against cane toads is also a good reminder for all of us: The best way to control exotic species is to keep them out of the country in the first place.