Recent research has found that exterminating alien species can have unpredictable, harmful effects on local ecosystems.
Alien species are animals and plants brought from another country and released into local natural environments.
Because they can harm indigenous species, efforts have been made across the nation to eliminate them.
However, the government is now reviewing its methods due to the unpredictable side effects extermination can engender.
In a western area of Ichinoseki, Iwate Prefecture, bullfrogs native to North America were spotted from the 2000s in storage reservoirs in farming areas.
They have eaten wrinkled frogs and other indigenous species in village forests, causing drastic declines in their populations. Normally hundreds of wrinkled frogs could be found in a single pond.
Prof. Tadashi Miyashita of the University of Tokyo, an expert in ecological science, said just a few American bullfrogs in the same pond can nearly eliminate the wrinkled frogs.
Kubogawa Iihatobu Shizen Saisei Kyogikai, a citizens organisation working to revive the natural environment in the area, began exterminating American bullfrogs in reservoir ponds in late June.
When members used nets to scoop out one pond, they caught more than 200 American bullfrogs in 10 minutes but only a few wrinkled frogs.
"Though the process is daunting, domestic species will go extinct unless we continue," said 27-year-old member Ryohei Sato, who participated in the work.
Carp had also been released into reservoir ponds for farming or ornamental purposes, living alongside the bullfrogs. Most of the fish came from China, however, so Sato worked to exterminate them as well.
However, research conducted on the site by a team led by Miyashita showed that the carp had eaten bullfrog tadpoles, even though both were alien species.
The carp were indirectly protecting the indigenous wrinkled frogs.
Ponds with carp had less than half as many American bullfrogs as ponds without the fish.
In experiments inside water tanks, the carp ate three times more American bullfrog tadpoles than wrinkled frog tadpoles.
The wrinkled frog tadpoles are believed to be hard to spot because they are hidden in waterweeds.
If the carp are exterminated, the number of American bullfrogs could increase.
"There are cases in which exterminating one alien species helps another alien species thrive, and thus indigenous species will, conversely, decrease," Miyashita said. "This is an important point worth paying attention to."
Sato said, "We'll use it as a reference point."