In 1979, Japan released 30 mongooses onto Amami Oshima to kill venomous snakes, and it took 45 years to fix what happened next

There is a particular kind of chaos that comes from a good idea going really wrong. In 1979, Japan imported 30 small Indian mongooses to Amami Ōshima, a lush subtropical island in Kagoshima Prefecture, to control the island’s venomous habu snakes. It felt like a nice, natural fix. It was far from that.

Japan is still trying to clean up the mess nearly 50 years later, and the tale has become one of the most studied conservation failures in modern ecological history.

The plan that looked perfect on paper
The habu (Protobothrops flavoviridis) is a pit viper native to the Ryukyu Islands and a real threat to people living and working outdoors. When Japan saw an opportunity to control the population with a natural predator, the obvious choice was the mongoose. Mongooses are known for their aggressive snake-hunting, fast, fearless and effective.

What the plan didn't take into account was timing. Mongooses are daytime animals. Habu snakes are active at night. Basically, the two animals were on opposite schedules and the snakes carried on largely unbothered.

The mongooses, however, had an entire island of prey to themselves and no natural predators to keep them in check.

Wrong predator, wrong ecosystem
The mongooses began hunting whatever they could find during the daytime instead of going after habu. The consequences were severe. Research published in the journal Biological Invasions by Watari, Takatsuki and Miyashita found that seven native species with larger body sizes, including mammals, birds, reptiles and amphibians, were rarely seen in mongoose-infested areas of the island. One of the hardest hit was the Amami rabbit (Pentalagus furnessi), a species so evolutionarily ancient that it is often referred to as a “living fossil.” It was only recently rediscovered, and is already on the endangered species list.

The damage cascaded through the food web. Native frogs, endemic snakes and ground-nesting birds all suffered population crashes. The mongoose had not solved an ecological problem; it had created a far worse one.

From 30 to 10,000
By the turn of the century, the magnitude of the mistake was undeniable. Research on the impacts and control of the introduced small Indian mongoose on Amami Island showed that the mongoose population peaked at an estimated 10,000 individuals with an annual growth rate of around 30% before eradication efforts began, and that the mongoose had a major negative impact on agriculture and native animals in mountainous areas, rather than controlling snakes.

In 1993, 14 years after the introduction, Japan launched a local government control project. By 2000, the national Ministry of the Environment had a full eradication program underway. The government set out about 30,000 traps around the island, set up sensor cameras and enlisted residents to form a citizen task force, the Amami Mongoose Busters. Almost 32,600 mongooses had been caught by the end of the program.

A historic declaration and a rare win
The last record of a mongoose being caught on the island was in April, 2018. The Japanese Ministry of Environment announced the eradication of the mongoose from Amami Ōshima on September 3, 2024, after being undetected for 6 years. The island covers 712 km² and is a UNESCO World Heritage Site; this was the first time that an established invasive predator had been successfully eradicated from such a large area, something thought impossible without widespread poisoning. Ministry officials called it a “miracle.”

What this means beyond Japan
The Amami case is now required reading for ecologists and wildlife managers everywhere. The core failure wasn’t evil; it was a mismatch of what made sense and what the ecosystem needed. The most important error was the release of a predator without a full understanding of the behavioral, temporal and ecological dynamics of both species.

The lesson for conservation work in the U.S., where similar debates still rage over predator reintroduction and invasive species management, is stark: ecosystems are not simple machines. Sometimes pulling a lever doesn’t push the right button. Sometimes it just throws it all off a cliff.

It took almost 50 years and a lot of public effort to solve Japan’s mongoose problem. Meanwhile, the habu snakes are still out there.