Toad-ally Out of Control

Out of hundreds of toad species, only one lives in Australia. Don't feel sorry for the people “Down Under,” though. Most Australians will be quick to assure you that one toad is more than enough, thank you very much.

Australia's toad is the cane toad, Bufo marinus, also called the marine toad or giant toad (the largest specimens are the size of dinner plates!). This wart-covered amphibian, described as “ugly, even by toad standards,” is native to South and Central America. It first arrived in Australia in 1935, as a shipment of 102 toads imported from Hawaii. Farmers in Puerto Rico had successfully used cane toads to control beetles that were devastating sugar cane crops. Australian farmers hoped that the cane toads would do the same for them.

Toads Take Hold

However, that's not how it turned out. What no one had realized was that the cane toads in Puerto Rico stuck around the sugar cane fields eating beetles because Puerto Rico is such a small island—there wasn't very far that they could go. In Australia, cane toads were enticed by the vast land, rich in habitat and food, that lay beyond the cane fields. The toads multiplied and expanded their territory, moving into new areas at a “maximum of 40 kilometers per year,” says Dr. Michael Tyler, environmental biologist at Adelaide University. They are currently as far south as Sydney, and halfway across the wildlife-rich north coast. Tyler expects that “in time they will extend to Derby in Western Australia.”

Where cane toads are established in Australia, there are ten times as many per square kilometer as in the cane toad's native Venezuela. After less than seven decades, the cane toad population in Australia has reached hundreds of millions. That's a lot of toads! In urban areas, they are a nuisance, congregating under lampposts to catch insects attracted to the light, invading gardens, getting into pipes, laying eggs, and leaving a roadkill mess.

A Tad of Good News

The good news for native species is that cane toads don't appear to be out-competing them for habitat or food. “Toads are often present in such huge numbers that it is hard to believe that they would fail to displace any native species that were around,” notes Professor Tyler. “But there rarely seem to be many native species competing with them.”

Native frogs have several things going for them that could save them from being wiped out in areas invaded by cane toads. Many occupy different habitat niches. For example, arboreal (tree-dwelling) frogs live above the ground-dwelling cane toad, and cane toads don't have the ability to shut down during the dry season as native frogs do. Also, cane toads can breed year-round, so they are not competing with native species for breeding areas during a limited season.

Double Danger

Cane toads do impact native species in two other ways—by eating them or being eaten. Most animals that attempt to eat the cane toad die from a lethal cocktail of poisons in its skin and concentrated in protruding glands on each shoulder. When the cane toad is handled roughly, such as when caught in a predator's mouth, the toad releases the poison, sometimes squirting it as far as one meter. The venom is highly toxic. The Cairns Frog Hospital reports snakes found dead with whole cane toads still in their mouths, dead within minutes from choosing the wrong meal! (No joke. There really is a frog hospital in Cairns on the coast of Northern Queensland, Australia, that serves as a help and research station for sick frogs and those that are not normal.) Animals that depend on frogs for their diets are particularly susceptible to poisoning—snakes, turtles, goannas (lizard), and jabiru (large stork)…even crocodiles! Biologists are especially concerned about species already in decline, such as the northern quoll and other marsupials.

Terrible Toad Dreams

The situation is an ecological nightmare. In March 2001, the cane toads reached world heritage–listed Kakadu National Park, home to a rich diversity of life. Some species in the park exist nowhere else on Earth. Biologists are monitoring the situation closely as the cane toads reach ideal conditions (year-round water) for the first time.

Why has the cane toad been so successful as an invasive alien species? In a word, “adaptability.” Its natural habitat ranges from tropical to seasonally arid. “For a species of animal to exist in such a remarkable range of environments,” notes Dr. Tyler, “it must be extremely adaptable, and tolerant to a variety of physical conditions.” The cane toad, for example, has the ability to lose and regain up to half its body weight in water without dying.

Cane toads eat whatever is available that fits in their mouths, mostly insects, but also mice, birds, small reptiles and marsupials, frogs, smaller cane toads, and even dog food!

The cane toad is what scientists call an opportunistic breeder. This means that it is not limited to a particular breeding season, but can choose to breed when conditions are right, regardless of the time of year. It can lay its eggs in practically any type of water—even a temporary puddle or swimming pool will do.

A large egg clutch (25,000 to 30,000 eggs once a year), rapid development, a long life span (up to 16 years in captivity), and having no natural predators also contribute to the cane toad's success as an alien species.

Tough Toad Lessons

There is much to learn about how cane toads affect native species, where the toads will spread, and how to control them (see sidebar). Despite millions of dollars spent on research, the cane toad remains unstoppable. But things are not all gloom and doom in this losing battle. Parts of Australia will remain toad-free due to unsuitable temperature, water availability, altitude, and other factors at some locations. And a few animals appear to be adapting. The keelback snake seems to have the ability to detoxify cane toad venom, and the water rat, ibis, and crow, among others, have learned to flip the toads over to avoid the poison.

The lessons learned from the cane toad experience have come with a price, though. Life has changed forever in Australia. As Professor Tyler predicts, “Cane toads will become the dominant form of life in Australia.”

Made the List!

The Invasive Species Specialist Group of The World Conservation Union (IUCN) maintains a list of “100 of the World's Worst Invasive Alien Species.” Both the cane toad (Bufo marinus) and the American bullfrog (Rana catesbeiana) made the list.


Another Alien Invasion

Green, moist-skinned aliens with big, bulging, gold eyes are invading the west coast of North America. No, it's not some science fiction tale. It's a true story. The alien is a frog—the American bullfrog, to be exact. And it's a space invader extraordinaire in wetland habitats from British Columbia to California.

The bullfrog (Rana catesbeiana) is native to eastern North America, but is an alien, or non-native, species on the West Coast. It was introduced from the 1890s to 1940s to meet the demand for frog legs on restaurant menus.

The bullfrog has flourished and spread. Native species such as red-legged and yellow-legged frogs have been eliminated or seriously reduced in many locations, their habitats taken over by bullfrogs.

Doctoral student Purnima Price at the University of Victoria is studying the impact of introduced bullfrogs on red-legged frogs and Pacific tree frogs on Vancouver Island, British Columbia. She sees evidence that bullfrogs are out-competing and displacing these native species. “Once bullfrogs get established, they pretty much clean out the competition,” she says.

Certainly, as the largest frog in North America (8 to 20 cm), the bullfrog has a physical advantage over native species at all life stages. Bullfrogs lay about 20 times as many eggs, and bullfrog tadpoles and adults prey on native frogs and tadpoles.

There is little doubt that the bullfrogs are running rampant in some areas, but even in these areas scientists are cautious about blaming native species declines on bullfrogs alone. Human disturbance also plays a major role.

More study and close monitoring is required to better understand the relationship between the invasive bullfrog and native frogs. Meanwhile, the invasion continues.

Biological Control for the Biological Control?

Researchers at the Commonwealth Scientific & Industrial Research Organisation (CSIRO) in Australia have spent years looking for biological methods to control invasive cane toads. It's proving to be a difficult task. The trick is finding a way to do it without killing off native species.

CSIRO researchers looked into the possibility of infecting toads with a fatal, exotic ranavirus—a non-native virus that infects amphibians, reptiles, and fish. They abandoned the idea in 1998, when they found that it killed a native frog species as well as cane toads.

Gene technology also looks promising. The hunt is on for a gene critical to toad development that could be “switched off” to prevent metamorphosis—the changing of a tadpole into an adult frog. “If the toads don't reach adulthood,” says CSIRO's Dr. Tony Robinson, “they won't be able to reproduce.”

Dr. Alex Hyatt at CSIRO's Australian Animal Health Laboratory thinks that a ranavirus inserted with the development-controlling gene can be used as a biological “taxi” to deliver the toad-specific gene. “The cane toad should see the gene product as a foreign entity and initiate an immune response against it,” says Robinson. “Such a response should interfere with metamorphosis.” Keeping the safety of other species in mind, Dr. Hyatt notes, “We will weaken the viruses so that other amphibians and fish will not suffer from the normal effects of the virus, if infected.”

Another possible idea involves pheromones—chemicals used by some animals to attract mating partners. In 1999, Paul Wabnitz of Adelaide University discovered that at least one species of frog uses pheromones. “There's a possibility that cane toads also excrete a sex pheromone,” he said. If a pheromone chemical can be isolated, it could be used to disrupt the toad breeding cycle by confusing or trapping toads to prevent them from mating. However, there has been no funding to test the theory.

Whichever way you look at it, it'll be years before biological controls are available for use. As Dr. Hyatt says, there's no “quick fix.”

The distinctive calls of alien bullfrogs and cane toads sound nothing like those of the native frog species around them. The cane toad sounds sort of like a telephone dial tone: “brrrrr.”

Vocabulary

marsupial:
Mammal of the order Marsupialia, which includes kangaroos, bandicoots, and wombats, and whose members are found principally in Australia and the Americas.

protruding:
Sticking out from a surface.

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Activity

  1. How would you describe the relationship that cane toads have with the Australian environment–mutualistic, commensalistic, or parasitic? Why? Write a sentence or two to explain your answer.
    [anno: Cane toads have a parasitic relationship with the Australian environment. The cane toads eat many of the native plants and animals and often poison predators trying to eat them, so they don't offer much benefit to the greater food web.]
  2. If the Australian government could find a natural predator for the cane toad, do you think the government should import that predator to Australia? What might be the benefits and the consequences of importing a natural predator for the cane toad? Write a few sentences to explain your answer.
    [anno: Answers will vary but could include that the benefit of bringing in a natural predator would mean a reduction in the cane toad population. This would help other species to increase their population. A consequence of bringing in a natural predator might be the creation of another situation like the current situation with the cane toad, in which the animal that was supposed to fix a problem causes another problem.]