Pressures on the World Heritage Area - Cane Toads

Arguably one of Australia's worst environmental disasters, the cane toad is not a problem in its native Central America. Australia has provided it with abundant food, abundant space and even free transport to new areas.The cane toad was brought to Australia under the misconception that it would assist cane farmers by eating the destructive cane beetle that was attacking their crops. Other countries had been experimenting with cane toads as beetle controllers and initial reports were optimistic (although erroneous) that the toads were doing the job in Hawaii and Puerto Rico.

Despite the warnings of some experts, 102 cane toads were shipped to Queensland from Hawaii in 1935 and they were released into cane fields in Gordonvale, just south of Cairns. As it turns out, the toads can't climb and their life cycle didn't coincide with that of the cane beetle which never came to the ground. So the cane toads didn't eat the cane beetles after all, but they did eat nearly everything else. Cane toads can reach well over 2kg in weight and achieve a snout to vent length of over 25 cm (10 inches).

The toads also turned out to have a massive reproductive rate. Published accounts range from 20,000 eggs per female per year upwards to 60,000 eggs. Their tadpoles (often called toadpoles) develop faster than most Australian frog tadpoles. Toadpoles, like the toad itself, are toxic so an animal or fish that eats them dies.

Cane toads have spread over Queensland, and crossed the border into New South Wales and are just over the border into the Northern Territory. If they reach the Kakadu National Park it could be disastrous. Cane toads have found their way to Sydney via produce and nursery trucks and are establishing themselves at Homebush Bay and some western Sydney suburbs. Homebush Bay is the 2000 Olympics site and also one of the last remaining viable New South Wales populations of the endangered Green and Golden Bell frog (Litoria aurea). If the cane toad becomes established at Homebush Bay, this may spell the end of the Green and Golden Bell frog at its most famous location.

Cane toads possess two glands behind the eyes that contain a milky poison. Snakes, birds and any other animal that eats a cane toad dies, often before it has even swallowed the offensive toad. There are a few exceptions to this which seem to show how animals somehow learn to adapt to changes in their environment. Certain species of snake (such as the Keelback) can swallow cane toads without any side effects and some birds have learned to eat the toad by turning it over onto its back and devouring everything but the skin and those deadly glands. So far, Butcherbirds, herons and egrets have been seen to use this technique to get their meal and not die for it.

 
Australian frogs are impacted in two ways by cane toads:

• toads compete for available food and breeding sites; and
• some species of Australian ground dwelling frogs are often mistaken for cane toads and killed by people who don't realise what they're doing.

The toads prefer open habitats and are not usually seen deep in the rainforest although they are common along roads and clearings in the Wet Tropics region. It is rare to see one more than 16 cm (6 inches) in length these days as the toad has been established in Tropical North Queensland for more than 60 years. When the toad invades a new area, it eats everything it can find and grows to a large size. Once established, the abundance of its food never seems to reach the pre-toad level. Both food and cane toad growth rates level off with toads usually reaching about 11 cm (4 inches) in length.