It has become a classical example of how man’s interference with nature might have disastrous consequences: the introduction of the cane toad to Australia. It was introduced in 1935 to control the numbers of the native grey-backed cane beetle, whose larvae feed on the roots of the sugar cane. These toads turned out to be very ineffective beetle predators (as the beetles resided in the top of the plant where the toad couldn’t reach them) but very effective at finding other food sources and reproducing themselves. This turned out to be an ecological experiment with dire consequences, as native Australian carnivores have been decimated due to their appetite for this poisonous toad. A letter that has been published in Nature this week, warns about a similar ecological disaster that could be happening soon in Madagascar. It is a close relative of the cane toad, the Asian common toad, that has set foot in Madagascar and potentially poses a threat to the endemic wildlife of this great and biodiverse island.
One of the authors of the letter, the Australian scientist Kolby, reports that the Asian common toad has been first sighted in March around Toamasina, the main port of Madagascar. It is therefore highly likely that the toad has hitched a ride in container shippings to finally end up on the island. In the meantime, more toads have been spotted with some individuals coming as close as 25 kilometers from the Betampona nature reserve and other individuals not too far from other internationally important biodiversity hotspots. From the cane toad in Australia it is known that it can colonize new areas at a rate of around 50 kilometers a year. 25 Kilometers might therefore be a distance that its close relative, the Asian common toad, might traverse quickly too. The Asian common toad (Duttaphrynus melanostictus), as its name suggests, occurs throughout a large part of Asia, can reach a good size of 20 centimeters and carries poison in black warty bumps along their bodies, meant to ward off predators. It is this poison, a bufotoxin, that has had such a big impact on Australian wildlife and might prove to be fatal to many Madagascaran predators too.
Just like the Australian wildlife, the Madagascaran wildlife has been separated from the mainland flora and fauna a long time ago and has evolved into many different species that are unique to this island. This high degree of endemism means Madagascar is a unique biodiversity hotspot. Species like lemurs and the carnivorous fossa, as well as many amphibians are found nowhere else on earth. Incredibly, of the more than 230 frogs that have been described in Madagascar until now, only two species are not endemic to the island. Examples of these wonderful amphibians are the frogs from the Mantellidae family, Madagascar’s very own poison frogs. Because they have evolved in such an isolated place, these animals have not been able to learn to avoid eating these toads that can prove to be fatal with their toxin glands. There are for instance more than 50 endemic snakes on Madagascar that are likely to eat them. Although the experience has shown that some Australian animals in due time have been able to adapt to their amphibian prey (for instance Australian crows have learned to flip toads over and eat only their bellies, as the toxin glands are in the toad’s neck and back), with numbers of more than 200 million individuals, the cane toad is considered a pest that has a disastrous effect on the numbers of native Australian wildlife. And as Madagascaran wildlife is already under immense human pressure, these toxic Asian toads might push some populations over the edge. And Kolby and colleagues warn as well for effects these toads may have on human health, as they might contaminate drinking water and transmit parasites to humans.
Because of these potential enormous threats, the scientist urge to take immediate action. “Time is short, so we are issuing an urgent call to the conservation community and governments to prevent an ecological disaster”. Already toads are being collected and removed and the Madagasikara Voakajy, a non-governmental organization in Antananarivo devoted to biodiversity, is tracking the spread of the amphibians. But he scientists advice to start an eradication programme, consisting of hunting mature individuals, destroying the spawn and draining breeding ponds to prevent the establishment of a large reproductive population. Because once they have taken hold of this island with considerable numbers, eradicating them will be impossible. As well, they might spread out to other neighboring islands in the Indian Ocean, like the Seychelles and the Comores with equally devastating effects. Although I normally would not support eradication programmes, the potential downfall of Madagascaran biodiversity surely has priority here.