Invasive red-ear slider turtles now vastly outnumber endemic Japanese turtles and are causing significant stress to the ecosystem, the Environment Ministry said Friday.

A study has put the number of red-ear sliders at 8 million, eight times the total population of endemic species.

Originally from the United States, the animals are widely kept as pets. However, they can grow to a considerable size and are often dumped in ponds and rivers when they outgrow their lodgings.

The species is known in Japan as midorigame and in some other countries as the red-ear terrapin.

"The growing population of red-ear slider turtles would mean the depopulation of insects, fish and other turtles that live on water weeds," Masato Morikawa, an official in charge of monitoring alien species, told The Japan Times on Monday. "The population has gradually but continuously been increasing over the years."

The species is believed to have been introduced after World War II. From the 1970s, the animals were widely sold at matsuri (festivals) and pet shops.

It is only one of several invasive turtle species now displacing local species.

The ministry said red-ear slider turtles have mainly colonized waterways in Kanto, Chubu and the Inland Sea areas. The entire population is estimated to consume up to 320 tons of water weeds each week.

Morikawa conceded that the need to control other alien species is more pressing as they can cause harm to humans: the poisonous red-back spider and the snapping turtle, which can bite off a finger, are among the priorities.

"At this point, the red-ear slider turtles are exterminated only in areas that are extremely overpopulated, but we are strengthening measures against them," he said.

The ministry plans to restrict imports, crack down on the abandoning of pets and step up culls.