Terminating Madagascar’s toxic toads: not as simple as it sounds.
In March this year, the staff of a mining company noticed some unusual amphibians lurking near a swamp in Toamasina, Madagascar’s largest seaport. Biologist Jonathan Kolby raced to the scene and confirmed conservationists’ worst fears. The amphibians – which were thought to have arrived via shipping container – were non-native, and notoriously toxic, Asian toads.
Kolby was understandably quick to issue a warning about the alien amphibians whose toxic secretions pose a grave threat to Madagascar’s unsuspecting snakes, birds and mammals. “Time is short, so we are issuing an urgent call to the conservation community and governments to prevent an ecological disaster”, he wrote in a letter to Nature (Stop Madagascar’s toad invasion now). “The toads should be hunted, their spawn should be destroyed and ponds should be drained to stop their breeding”, he (and 10 cosignatories) urged.
But this week, another team of conservationists, led by Sven Mecke, cautioned against taking “disproportionate countermeasures” to eradicate the toad (Review risks before eradicating toads). Team Mecke argue that more research is needed before the impacts of the Asian toad are fully understood, and suggest that the methods recommended for controlling the toad populations could cause more harm than good.
The arrival of the ‘toxic toads’ is the latest in a string of bad conservation news for Madagascar’s (largely endemic) wildlife, making the "act now, or wait?" decision all the more difficult. Not only do vast swathes of Madagascar’s rainforest continue to be logged causing widespread habitat loss, the pathogen responsible for a global frogpocalypse over the past 20 years (Batrochochytrium dendrobatidis (Bd)) seems to have (finally and somewhat inevitably) reared its ugly head in one of the world’s few remaining strongholds of amphibian diversity. In fact, Kolby was sampling amphibians for Bd when he got the call.
But while the potential threat posed by the Asian toad is undeniable, Team Mecke may be right to err on the side of caution.
Conservationists have been quick to compare the threat posed by the Asian toad (Duttaphrynus melanostictus) to its close relative, the cane toad (Bufo marinus). The cane toad does share some concerning characteristics with its Eastern cousin: a lack of predators, the production of lethal toxic secretions; and impressive powers of pro-creation (40,000 eggs per clutch). But the two species were introduced in two very different ways.
In a notoriously unsuccessful bid to keep cane beetles in check, around 3000 cane toads were intentionally introduced from their native South America to Queensland, Australia’s sugar cane capital. There, the large founding population of the warty warriors, combined with a lack of natural predators and a rich food supply led to a rapid and overwhelming surge in Cane toad population numbers. Australian researchers have been wrestling with their impacts (which include declines in native wildlife, poisoned pets and injured humans) ever since.
In contrast, the odds aren’t generally stacked in favour of a handful of stowaways that hitched a ride in a shipping container (though there are some notable -- and typically rodenty -- exceptions). So the likelihood of the Asian toad arriving, surviving and thriving in Madagascar isn't necessarily inevitable.
Second, it’s not altogether easy to eliminate an invasive species. Asian toads are notoriously difficult to identify (ranging in colour from grey-brown to brick-red to black), a challenge for any eradication task force, not least in a country with around 400 species of webbed-footed wonders. “Efforts by amateur conservationists and locals to destroy toad spawn and larvae could jeopardize native frog species if people do not identify tadpoles or juveniles correctly”, warns Mecke. And he's right: a study in Australia revealed that members of the public have presented authorities with several endangered species of frog for destruction in the mistaken belief that they were invasive cane toads.
Finally, the methods used to eliminate the invaders may put native species at risk. Kolby recommended draining potential breeding ponds but this is likely to be a futile control method as Asian toads are ‘opportunistic’ breeders. That is, they can produce eggs as and when a suitable water body presents itself. Their larvae don’t need pristine ponds. They’re equally likely to thrive in streams, puddles, gutters, even an abandoned cement cistern.
That said, the (super observant) mining company might just have provided a rare opportunity to stop a potential invader in its tracks. If conditions work in the toads' favour, and conservationists wait too long, the Asian toad population size could reach the point of no return, with massive repercussions in a country where 95% of the reptiles and 92% of the mammals exist nowhere else on Earth. It seems efforts should be invested in working out how, not if, the the Asian toad hops its last hop.