In 1978, Japan released just 10 to 15 cane toads on Ishigaki Island; their population likely surpassed 20,000, and native snakes were found dead near the toads

A tiny Japanese island once tried a quick fix for its pest problem. It backfired. According to the study, ‘Methods for invasive species control are transferable across invaded areas,’ published in PLOS ONE by researchers at Kyoto University and the University of Sydney, a handful of cane toads released on Ishigaki Island in 1978 turned into a full-blown invasion. What began as a pest-control measure in sugar cane fields became a threat to native wildlife.

How a dozen toads turned into an army

Cane toads aren't from Japan; they are native to Central and South America. Humans have shuffled them around the world for decades in hopes they would eat crop-damaging insects. The same PLOS ONE research traces that path: about 150 cane toads were shipped from Puerto Rico to Hawaii in 1932. From there, descendants were moved to Taiwan in 1935 and, in the following years, from Taiwan to Japan's Daito Islands. Finally, in 1978, a small number of toads, between 10 and 15, were brought from the Daito Islands to Ishigaki to combat pests in sugar cane and rice fields.


Ironically, the same rice paddies and sugar cane fields the toads were brought in to protect turned into their paradise; that, plus a warm subtropical climate with ponds, streams, and irrigation canals to spare. Local herpetologist H. Ota estimated the island’s toad population to have probably been above 20,000, the research notes. A dozen toads becoming 20,000 in a few decades is striking, but cane toads are known for explosive breeding because one female can lay more than 30,000 eggs in a single clutch.

A poison that turns predators into victims

Cane toads carry toxins in glands on their skin and shoulders: a built-in defense system that works a little too well on animals that have never encountered it before. Native predators on Ishigaki had no idea the toad they were eating could kill them. That's exactly what happened to the Ryukyu odd-tooth snake, a species found only in this part of Japan. This snake sometimes swallows more than one toad in a single meal. This was tested directly in a study in Current Herpetology led by researchers Noriko Kidera and Hidetoshi Ota. Their experiment showed that a snake could safely swallow one small toad, but if a snake swallowed two or more toads in the same feeding bout, it risked dying in the act. The researchers also collected reports from Ishigaki residents of dead snakes discovered near toad carcasses, which suggested the danger in the wild. The Ryukyu odd-tooth snake's story shows that invasive species don't just outcompete natives for food; sometimes they poison the animals that try to eat them.

One bird cracked the toad's defense system

Not all the predators of Ishigaki have lost out in the fight against the cane toad. The crested serpent-eagle, a subspecies found only on Ishigaki and Iriomote and critically endangered in Japan even though the wider species is common across Asia, has been seen feeding on cane toads without apparent poisoning. Analyzing the eagle's genome for a 2025 BMC Ecology and Evolution study, Kyoto University scientists found a mutation in the ATP1A1 gene, the same gene linked to toxin resistance in other toad-eating animals worldwide. Tellingly, eagles on nearby Iriomote Island carry the same mutation even though Iriomote has no cane toads at all, meaning the resistance was already there before the toads ever arrived; the eagle got lucky, it didn't adapt on the fly.

Why this old story still matters

This story highlights the risks of quick fixes in nature. Cane toads were introduced into more than 40 countries as a means of controlling pests, but in many cases they became a bigger problem. The story of Ishigaki reveals both parts of this experiment: one is the native snake that could not adapt quickly enough to this change, while another is the native eagle that could. As of 2026, it's been 48 years since those toads arrived, and researchers are still studying the consequences.