Australia's most unwelcome guest
It seemed like a good idea at the time. In 1935, two types of beetles were chewing through Queensland's sugar-cane fields. In desperation, growers turned to cane toads to battle the insects. They'd heard glowing reports about the warty, fist-sized amphibians from growers at a conference in the Caribbean two years earlier, and successfully lobbied to import them.
Australia would come to rue that day.
Instead of concentrating on beetles, the voracious toads began munching on almost everything in site: insects, bird eggs, and even pet food. Their poison killed predators - even pets - who tried to eat them. And instead of staying put in cane fields, they began to spread along a broad swath of the country.
In recent years, the cane toad has become a poster child for the problem of invasive species here, forcing the government to embark on a multimillion-dollar campaign to stop them.
Invasives are a major problem worldwide. Plants, animals, and even pathogens arrive in a new ecosystem, often wreaking environmental and economic damage in their newfound homes. The World Conservation Union (WCU) lists the planet's 100 worst invasive species, but that's just the tip of the iceberg.
In Australia, the problem is magnified. The continent is biologically rich, hosting a broad range of species found nowhere else on the planet. Australia is one of the world's top 12 "megadiverse" countries, according to the Australian Academy of Science. The other 11 are too poor to implement comprehensive long-term conservation programs.
Yet with a population of only 20 million people, the biggest current threats to native species in many areas are bush fires and invasive species, notes Kent Wommack, program director for the Nature Conservancy's projects in Australia.
Cane toads - a.k.a. giant toads, marine toads, or more formally, Bufo marinus - have the dubious distinction of joining the Caribbean tree frog and the bullfrog as the only amphibians on the WCU's list of worst invasive species. They have spread from the northeast corner of New South Wales to the tropical rain forests of the Northern Territory. They've invaded the ecologically sensitive Kakadu National Park and now are hopping toward the outskirts of Darwin.
If the projected effects of climate change hold true for Australia's part of the globe, specialists expect the toads to reach far into West Australia and, in the east, as far south as Sydney within the next 20 years.
Introducing them "was not an inspired idea," says Ross Alford, professor of biology at James Cook University here in Townsville.
The toads' successful invasion is all the more intriguing, he says, because they are so poorly adapted to the country's climate. They lose enormous amounts of body moisture to evaporation during the dry season; many die of dehydration. Many others starve when they've exhausted the food supply around dry-season water holes. Those food-free regions can extend for more than 100 yards from the water hole.
"But they breed like crazy and are toxic," he adds. "Those cover up a lot of faults in other areas."
Ironically, the toads are not viewed as vile interlopers in some of the areas they've reached outside their native northern Venezuela and Guyana. Florida has put up with them for years. Indeed, one gardening company's website holds that they should be destroyed only if they threaten young children who mishandle them or pets. Otherwise, the site counsels, the toads make great garden companions because they eat an enormous number of insects.
"No one in Florida seems to care," says Professor Alford, who attended the University of Florida and received his master's degree there. "People figure the state is so full of introduced species that one more or less doesn't make any difference."
Indeed, complacency is one of the big hurdles facing international efforts to stem the flow of invasive species, according to Faith Campbell, the North American representative to the Invasive Species Specialists Group, which compiled the 100-worst list for the WCU.
Education is another. Many people, she says, don't sufficiently appreciate the importance of biodiversity in ecosystems. And they don't know enough about their natural surroundings to realize what's out of place. By the time an alien species has encroached sufficiently to attract wide attention, it's too late.
"Outside of Australia and New Zealand, no one takes this stuff seriously," she says. Except for Hawaii, which is struggling to save its native species from mainland invaders, "in the US, it's generally not seen as a major issue."
Australia and New Zealand have been more successful building public support for control measures partly because they've stressed the threat to agriculture, notes Richard Mack, an ecologist at Washington State University.
Reasonable estimates for the economic cost of invasive species globally are hard to come by, he says. But for the US, one study published three years ago by Cornell University's David Pimentel put the cost at $137 billion a year. That represents a drain comparable to Iran's or Thailand's annual economic output.
A few other countries also are making progress on the issue, notes Dr. Mack, a member of the board that oversees the Global Invasive Species Program under the international Convention on Biodiversity. South Africa, for example, has been felling pines, acacias, and other thirsty species of trees introduced along its rivers and streams, he says. The trees soak up enormous amounts of water in a land where building new dams is too expensive. In a program reminiscent of Work Progress Administration projects in the US prior to World War II, residents are replacing the trees they remove with native vegetation. Once-dry streams are flowing again. "They're literally working for water," he says.
Overall, while he and Dr. Campbell give the world a "D" for efforts during the past decade to control invasive species, Australia earns one of the few "A's."
"They're serious about stopping invasives at the border. They are adopting regulations that close pathways. And they rely very heavily on inspections," Campbell explains.
But these approaches still leave Australia struggling to deal with a legacy of ecosystem-ravaging arrivals that predate the tighter controls. One up-and-coming invader is the red-eared slider turtle, which some researchers suggest could one day rival the cane toad in its destructive habits.
This is not the first time that Australia has tried to thwart the toads' spread. In the mid-1980s, the government financed an effort to search for diseases that could be introduced to control the toads, as it did for rabbits. But James Cook University's Alford notes that the money ran out before the program could bear fruit.
Now, the government is talking in terms of spending between $5 million and $7 million over 15 years to deal with cane toads, he adds. Last year, Australia established a National Cane Toad Task Force. And it has sponsored a design contest that aims to develop a trap that catches toads with less "by-catch."
At this stage, cane-toad control efforts are largely local, with individuals or groups of volunteers hunting them and killing them immediately or freezing them to death.
The Northern Territory has established an Island Ark program to save ecologically critical species such as cat-like quolls from cane toads by moving the quolls to nearby islands.
Meanwhile, researchers are exploring several other approaches to controlling the amphibious pests.
Over the long term, scientists with Australia's Commonwealth Scientific and Industrial Research Organization are looking for genetic approaches to cane-toad control. One approach might be to infect cane-toad tadpoles with designer viruses that immunize them against a protein their bodies make when they become adults. As they mature and make the protein, their immune systems kick in and attack the protein, which would kill the toads. In addition, teams are looking for diseases in other toad species that they can enlist against cane toads.
Alford's group is working on approaches that may have payoffs in the short term. The strategy is to fool a toad's nose and attract it to a trap or poison.
He notes that only within the past few years have scientists realized that frogs and toads have a sense of smell and use it to find food and, presumably, mates. Previously, they were thought to use sight alone.
Alford suggests that the right scent could bring the toads to the traps more efficiently, where they could dine on bait laced with poison or a tailored virus. The ultimate would be a dab of something "that would cause toads to drop what they are doing and shoot hundreds of meters" to the waiting trap, he says. But for now, he'll be happy with identifying the right scents to use.
The World Conservation Union has listed 100 of the world's worst invasive species. Among them:
• Crazy ants: Their native origin is unknown, but their impact has been felt from Hawaii to Zanzibar. On Christmas Island in the Indian Ocean, they've damaged the rain forest canopy, decimated a key native crab, and could drive an endangered bird from its only known home.
• Brown tree snakes: Originally from Oceania, they've bitten people, caused major power outages, and nearly exterminated the native forest birds of Guam. The snake has reached the mainland United States and Spain.
• Caulerpa seaweed: Introduced to the Mediterranean some 20 years ago, this tropical plant has smothered underwater habitats and threatens native marine flora and fauna.
• Feral pigs: Found throughout much of the world, they damage crops, dig up large areas of native vegetation, and spread weeds. They also eat turtles, seabirds, and reptiles.