Nobody knows for sure how or when they first arrived, but for the last few years, millions of toxic toads have plagued eastern Madagascar.

The Asian toads (Duttaphrynus melanostictus) first appeared between 2007 and 2010, most likely in shipping containers, and have rapidly increased in number to about four million.

The toads are threatening much of Madagascar's unique wildlife in the area, including hundreds of its endemic frog species.

They also pose a threat to humans. If anyone eats the toads, their toxins could kill. 

A new report is calling for action to prevent an "environmental catastrophe". It states that the priority is to control the toads' population growth while eradicating those that remain. 

Separated from mainland Africa, Madagascar is home to a range of creatures found nowhere else in the world.

These include the 106 species of lemur, many of which are already dangerously rare. It is also home to hundreds of unique plants.

It's got the capacity to ultimately occupy most areas of Madagascar

As the toad population continues to spread, much of Madagascar's biodiversity could be affected.

"It could disrupt food chains and cause native predators, prey, and competitors to decline or even go extinct," says report co-author Christian Randrianantoandro of Madagasikara Voakajy, a Malagasy biodiversity organisation.

Predators that feed on amphibians could be poisoned by eating the toads. These include snakes, hawks and fossa, a cat-like predator also unique to Madagascar.

"We expect population clashes. It pretty much disrupts everything," says Christopher Raxworthy of the American Museum of Natural History, another co-author of the report. "It's got the capacity to ultimately occupy most areas of Madagascar. It's a very serious threat."

Fortunately the toads have not yet spread into all areas of Madagascar. They currently occupy 110 sq km, slightly less than a fifth of the island.

This might sound crazy, but you can train dogs to sniff for animals including toads

If eradication starts now, the toads could still be contained. But if no action is taken, the report says they will have a lasting and irreversible impact on Madagascar's rich biodiversity. 

If the toads find their way into a long stretch of rivers and canals – the Pangalanes Canal system – eradication may no longer be an option as they would be able to quickly spread to other parts of the island.

There are several ways to get rid of the toads, including collecting them by hand, a process already undergoing trials. However, they are not always easy to spot, so another option is to spray a mildly acidic solution onto areas where they live. This kills the toads within 24 hours.

"It's available locally and is cheap," Raxworthy says. "It's the same concentration as lemon juice, so it's not toxic."

Other methods include tadpole traps and trained sniffer dogs. "This might sound crazy, but you can train dogs to sniff for animals including toads, to collect the last populations," Raxworthy says.

Future generations will be furious, should we not make an eradication effort now

There is hope that efforts like these could provide a solution. But eradication needs to continue on a larger scale, and quickly.

Conservationists recently sealed off an area to search for toads, and collected 1,200 in two weeks. An adult female can produce about 40,000 eggs a year. That means the population can expand really rapidly. 

There is a lot at stake. Over the past few decades over $1bn has been invested into protecting Madagascar's wildlife.

"If we fail over a question of a few million dollars, the impact on Madagascar's biodiversity could be dramatic," says Raxworthy. "It would be heart-breaking."

"Future generations will be furious, should we not make an eradication effort now, while there is still a chance of success."

The authors of the report are appealing for money to fund the eradication programme from the owners of the Ambatovy mine, one of the largest nickel mines of its type in the world.

Eradication needs to continue on a larger scale, and quickly

The amphibians were first observed on the island at roughly the same time that construction began on a coastal processing plant linked to the mine. Toads also became increasingly common on the plant site from 2011 onwards. However, it is not clear whether the mine had any involvement in bringing the toads to Madagascar.

"[The toads'] genetic profile in Madagascar resembles populations known in Vietnam, Cambodia and Thailand," says Raxworthy. "One of the plant contractors is a company based in Thailand."

"We can't prove it but the body of evidence suggests that they [introduced] them by accident," he says. "We want to put them under public pressure to contribute in a meaningful way to try to eradicate the toads."

In a statement provided to BBC Earth, the plant's co-owners Sherritt International Corporation said that they are working "to support efforts to contain and or eradicate the toads" and that they are carrying out an eradication programme at their plant site. 

However, they do not acknowledge that the toads arrived on their containers, which they point out represent a small percentage of the total number of containers that come into Madagascar's port of Toamasina (also known as Tamatave) each year. "During the years that the toad is speculated to have arrived, Ambatovy accounted for less than 5% of the Port's total traffic."

But the report authors say that the way the toads are distributed suggests that they did not escape into the wild at the port. The centre of the area in which toads are now found is a region just outside Toamasina with little economic activity – except for the mine. 

"Of course, other importers could also have introduced the toad to this area south of Tamatave," says Raxworthy. "But the mine plant is massive in scale compared to anything else going on in this area."

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