Rustling branches and a canopy cacophony – part howl, part screech, part snigger – proclaim the presence of black-and-white ruffed lemurs as visitors enter Ivoloina zoological park in eastern Madagascar.

The raucous primate is one of several critically endangered species in this biological refuge, which breeds and protects rare wildlife from the growing pressures on this island’s unique ecology.

But having kept the poachers, loggers and developers at bay, the park’s operators now fear the advance of a very different threat: Duttaphrynus melanostictus, widely known as the Asian common toad.

Nobody knows precisely how this toxic amphibian arrived in Madagascar. The most credible theory is that a small number were accidentally shipped inside a container from Vietnam that was unloaded at Toamasina port and opened at the giant Ambatovy nickel and cobalt processing plant. But what is certain is how quickly they have overrun the local habitat.

Villagers near the Ambatovy plant say they first noticed the creature around 2008. They had never seen a toad because Madagascar’s island evolution has only produced frogs. Locals considered the new arrival so strange and repellent they called it radaka boka (leprous toad).

Bigger and tougher than indigenous rivals, the toads quickly dominated the landscape. They also had no brightly coloured markings to warn local predators such as snakes and birds that their secretions were poisonous.

Asian toads are thought to have entered Madagascar by accident in 2008 and now number about 21 million. Photograph: Jonathan Watts for the Observer

They breed prodigiously. One female toad spawns an estimated 10,000 to 40,000 eggs – by comparison some indigenous frogs lay no more than 10. There are now said to be between 7 million and 21 million toads in Toamasina and their number and range grows every year.

“It’s biblical,” says Roderic Mahasoa, a biologist at the Madagascar Fauna and Flora Group. “If this continues for another five years, it will be a disaster.”

Madagascar is grimly accustomed to plagues and natural calamities. Five years ago, swarms of locusts devastated crops. Drought has left a famine in the south. Residents in Toamasina are rebuilding after a huge cyclone in January and deadly outbreaks of bubonic and pneumonic plagues.

But globalisation poses an arguably greater threat to the country’s biosecurity and natural wealth.

Madagascar has a lot to lose. After detaching from the African continent nearly 90 million years ago, the world’s fourth largest island has become the evolutionary laboratory for more than 8,000 plants and 900 orchids that are found nowhere else on earth. More than 80% are endangered.

Biologists at the International Union for the Conservation of Nature are worried that the toads will further degrade the ecosystem. Worldwide, invasive species are the second-biggest cause of extinction after habitat loss.

Poisonous amphibians have a particularly big impact. South American cane toads were deliberately introduced to Australia to control pests but have since wreaked havoc among local species. The Asian common toad has invaded so far into the islands of Wallacea in Indonesia that biologists fear they will soon threaten the rare Komodo dragon.

In Toamasina, counter-measures are hampered by political inertia, resource shortfalls, liability questions and the fortitude of the toads, which have thick skin and can survive for long periods without food, water or oxygen.

“They are really tough,” says Mahasoa. “There have been eradication trials with citric acid, hand captures, tadpole traps and pits.” The most humane solution found so far involves partly immersing the toads in high concentrations of ethanol to anaesthetise them followed by decapitation.

Even for a rich country, biosecurity is challenging. For Madagascar, one of the world’s poorest nations, it is extremely hard to find the resources to kill toads when 80% of the population live on less than $2 a day and 4 million people go hungry.

The endemic white frog is among the species that are threatened by the arrival of the Asian rival. Photograph: Jonathan Watts for the Observer

With every day that passes, the cost rises. Calculations by the James Reardon, an eradication expert with New Zealand’s Department of Conservation, who co-authored a study on the subject, show the invasion could have been eradicated for $5,000 (£3,600) if the toads had been detected at the border a decade ago. Even a year or two later, they could have been destroyed for $500,000. Today, elimination is considered impossible and it would cost several million dollars even to control the spread.

“It’s easy at this point to ask why we would even be so foolish to consider eradication at such a high cost and risk. However, I would argue that had we had the knowledge we have now when cane toads in Australia were restricted to a few square miles, we would try and eradicate them. It’s also critical to remember that once established these toads will irrevocably alter the species assemblies and ecology of Madagascar,” Reardon said.

It is hard to imagine anywhere further from the tourist image of biodiverse Madagascar than the roadside near the Ambatovy plant. On one side are smoke stacks. On the other is a tip for rubbish from the city and debris from the cyclone. This is ground zero for the toads, which can be seen hopping in front of car headlights.

Villagers say government experts have advised them to kill toads with hand-crafted spikes. “Someone came and told us how to do it,” said Laikodahy Eloi, the chief of Antetezantona village. “It’s a big fight against the toads. We kill them because we hear they are toxic, though no one has been hurt so far.”

He said he had killed more than 30 and his sons even more. Once dead, they were told to bury the toads away from a water source to prevent contamination. But the villagers have more pressing concerns. Many are rebuilding wooden shacks flattened by the recent cyclone.

There has been limited corporate support from Ambatovy, which has participated alongside government officials in eradication campaigns. The giant factory – a joint venture between Sumitomo of Japan, Sherritt of Canada and the Korean Resources Corporation – processes ore from one of Africa’s biggest mines. The nickel – used across the world in batteries, laptops. mobile phones, electric cars and stainless steel – is now, along with vanilla, the main source of export earnings for Madagascar. The plant has been the focus of several environmental complaints, including the use of insecticides that kill local pollinators, the erosion of paddyfields and excessive use of water, causing drought.

The company has downplayed responsibility for the toads but says it will help to deal with the problem.

“During the timeframe that the toad is speculated to have arrived in Madagascar, Ambatovy accounted for less than 5% of the Port of Toamasina’s total traffic – and that percentage is even lower for shipments coming from the toad’s zone of origin, according to our records and those we obtained from Madagascar’s customs office. Research has not found evidence of the real carrier of this invasive species in Madagascar,” the company’s media office in Toronto told the Observer via email.

“We recognise that the spread of invasive toads can have a serious impacts on local flora and fauna. Ambatovy has joined the government’s national committee, which is comprised of several stakeholders, to mitigate the impacts of the Asian toad. We continue to bring our contribution in its eradication that we hope will lead to lasting results.”

The issue touches on a broader global debate about invasive species and the ethics and efficacy of keeping ecological systems intact and distinct while the global economy grows, homogenises and sweeps across borders.

Adolphe Martin, from Antetezantona, shows off the metal spike that he uses to kill the poisonous toads. Photograph: Jonathan Watts for the Observer

Revisionists in the UK and US, such as Chris Thomas and R Alexander Pyron, have recently argued that globalisation is biologically creative rather than destructive. Trade and tourism, they argue, have created a global supercontinent where species interact more widely than ever, leading to new forms of life even as less-adaptable creatures become extinct.

Conservationists and the vast majority of biologists say it is dangerous folly to assume the natural world will evolve through pesticides, pollution, invasive species and loss of habitat. Diversity, they argue, is being destroyed by the super-accelerated change wrought by humans and the species they are spreading across the world. Animals and plants that have evolved over millennia are being decimated, leading to a loss of natural complexity, possibility and resilience.

In the case of the unwittingly globalised toad, it is a question of whether it adapts like a refugee – discriminated against at first, but slowly adapting and contributing to its new environment – or like a multinational corporation that inexorably overpowers local competition and reduces choice.

The toads’ human impact is not yet completely clear. Studies in Asia suggest the toads can cause heart failure if eaten, although there has only been one known death. Of more concern is the indirect impact. Villagers in Madagascar say the toads are poisoning snakes, which has prompted concerns of a rise in rat populations, which could be a factor in the recent bubonic plague outbreak.

In the short term the concern for conservationists is that the toads will spread to the Ivoloina zoological park and the important Betampona nature reserve. The primates who are the star attractions of the park will not be directly affected but the forest they depend on will erode.

“We are very worried because no action on the field has been taken in the last few months and the toads are approaching,” says Virginia Rodriguez Ponga, a vet and ecologist who is programme manager for the Madagascar Fauna and Flora Group. “Tourists come to see the lemurs but you need to protect even the smallest elements in an ecosystem. They are tiny but important.”

These protected areas are holdouts for species that have been lost elsewhere as a result of the deforestation of Madagascar’s forests, initially by British and French colonialists and more recently by illegal rosewood loggers serving the Chinese market and locals who use wood for house building and fuel.

Thanks to this habitat loss most of Madagascar’s 113 lemur species are endangered. The latest discovery – the Groves’ dwarf lemur from the south of the island – was only confirmed last month, but it may well follow rapidly on to the red list of threatened species.

The first sightings of the invasive toad were close to the Ambatovy processing plant in Toamasina, one of the biggest industrial projects in Madagascar. Photograph: Jonathan Watts for the Observer

Cultural globalisation is also having a negative impact. “It used to be taboo for people here to eat tortoise and some types of lemur, but the Chinese want to try everything. They pay locals to cook these dishes and then it changes our culture,” says Ivoloina guide Jean Rodrigue Wilfred. “I’ve had Chinese tourists come to this park and ask me for recommendations of hotels that serve lemur.”

More broadly, as more Malagasay swap traditional beliefs for Christianity, he says they no longer treat lemurs as the sacred souls of their ancestors and some now sell the animals on the street as pets or food.

To try to counter such trends, the park hosts weekend ecology classes for local children. Displays at the education centre stress the message that conservation is not just about protecting cute lemurs.

“There is no economy without nature,” reads one display beside samples of honey, berries, writing materials and other products from the forest. This is even more apparent outside, where clove and cinnamon trees tower over plants that guides say are vital for the global pharmaceutical industry, such as Catharantus roseus, the Madagascar periwinkle, which is used in diabetes and cancer treatments, and Mimosa pudica, which is being studied by researchers working on drugs to slow the onset of Alzheimer’s disease.

Here and elsewhere across the country, many more species are still to be discovered. At least five new types of lemur are likely to be confirmed in the coming year or two, according to specialists in the field. Although the primates dominate the headlines, just as important is the interaction of plants, insects, amphibians and other forms of life that make Madagascar such a unique environment.

Unless a major new eradication campaign is launched soon, the concern is that the toads will spread from Toamasina – the island’s biggest port – across the country via container lorries and cars.

It was also revealed earlier this month that Madagascar is suffering at the claws of a strain of mutant, all-female crayfish that can clone itself.

The crayfish, now banned in the European Union, has been distributed by the aquarium trade and has rapidly spread across Madagascar in less than a decade because of its popularity as a cheap source of protein for humans. Experts say the creature could out-compete seven native crayfish.

On the nine-hour drive back to the capital, Antananarivo, it is evident that this exceptional landscape could do without more environmental threats. Most of the hillsides are cleared of forest.

Villagers sell part of what they have cut as charcoal. There are a growing number of mines. More wetlands are being converted into rice paddies. None of these developments appears to have made much difference in reducing the country’s poverty.

Globalisation and development, it seems, have taken more than they have given to Madagascar’s people, as well as leaving the country with far fewer lemurs and a lot more toads.