The Bufo marinus is perhaps the most well-known invasive organism. It is seen either as a successful colonizer, or as an extreme nuisance with the potential to devastate an ecosystem. This species, more commonly known as the cane toad, has invaded many countries, most famously Australia.
The cane toad is originally native to Central and South America. Imported to other countries for the toad’s effective control of pests that devastate sugar cane, cane toads now form naturalized populations in Australia, the Philippines, Japan, Barbados, and Hawaii.
The cane toad integrated itself smoothly into ecosystems when introduced to Hawaii in 1932. Meanwhile, sugar crops in Australia were under attack by greyback and Frenchi beetles. These insects’ larvae feed on the roots of sugar cane plants, causing the plants to collapse. Because of the successful introduction of the cane toad in Hawaii, optimistic Australian naturalists brought a number of toads into Queensland, Australia as a preliminary experiment. Over 40,000 young toads were released into the wild in August 1935.
This optimism proved naïve when the toads’ introduction severely impacted the Australian ecosystem. Because the toad has no natural enemies on the continent, the population grew unchecked. Though there are few objective reports of the extent of the devastation, the effects on predators are well known. Native snakes have been virtually eliminated from toad-infested areas because of the poison that coats the toads’ skin. One study estimates that 30 per cent of Australia’s snake species will be at risk by 2030. The toads are also blamed for the decrease in waterfowl in Australian swamps.
They might not even have performed their intended task. The seasonal habits of the toads and the beetles they are supposed to consume do not match up properly. The toads usually only emerge when the beetles are old enough to fly and escape the toads. Most Australian sugar cane fields now use chemical pesticides for pest control.
Historically, cane toads in Australia were also used for a practical— if slightly curious—purpose: the detection of human pregnancy. Hormones in the urine of a pregnant woman, when injected under a toad’s skin, stimulate sperm cell release in the toad. Toad pregnancy detection was commonplace in Australia up until 1965.
So what state are the cane toads in now? One research group in Australia found that the most rapidly advancing toads are evolving longer legs. This might be good news for pest control, because the toads are also developing arthritis in their longer limbs. Some species of birds in Australia have also learned to attack the toads’ bellies instead of their heads, where the poisonous glands are. These might be reasons for the toad’s slower-than-expected spread westward across the island continent.
There is an urgent need for methods to control the cane toad population, so scientists and naturalists must be creative. The move to build a cross-national fence was widely criticized because of the cost. Also, if even a few toads crossed the barrier, they would breed easily, making the effort useless and wasteful. Male sterilization was considered, but also found to be too expensive and not effective enough. Right now, the best potential strategy seems to be biological control, such as the introduction of a virus or parasite that would naturally control the toad population. But this solution could cause further problems. After all, the introduction of a new species to control a pest is what started the whole mess in the first place.
