Japan's Environment Ministry has told NHK that an alien frog species is believed to be rapidly expanding its habitat on a small island designated as a UNESCO World Natural Heritage Site.
The Asian tree frog is between 5 to 7 centimeters long and native to Southeast Asia. Japan designates it as an invasive alien species that could harm local ecosystems.
The frog's existence on Tokunoshima Island in Kagoshima Prefecture was confirmed in May last year.
Since then, the species has been found at more than 40 locations and about 6,200 frogs have been killed there.
Experts are concerned that the alien frog could deprive the island's indigenous Amami green tree frog of insects they feed on and their breeding grounds.
Ministry officials have set up nets to stop the alien frog from further spreading its habitat. They are also urging residents to come forward if they hear its croak or find spawn.
The Asian tree frog was found on Iriomote Island in Okinawa Prefecture, another World Natural Heritage Site, nine years ago. The ministry got rid of the species and declared it eradicated four years later.
Ryukyu University Associate Professor Toda Mamoru warns that the Asian tree frog could expand its habitat to other islands.
He says the key to preventing this is to spot the species early, adding that residents can help by notifying authorities as soon as they spot frogs they don't usually see.