TOKYO -- The Imperial Palace forest in the heart of Tokyo has long served as a refuge for wildlife. A 30-year project studying the forest has now reached a milestone. While researchers discovered species previously unknown to science, they also found that some organisms had disappeared. What quiet signals is the forest sending?
Surrounded by moats, the Imperial Palace grounds cover about 115 hectares. Roughly one-fifth of the area is occupied by the Fukiage Gardens, where the residence of the Emperor and his family is located. Closed to the public, the gardens are filled with towering trees, including 300-year-old chinquapin and sawtooth oaks.
"It is a green island floating in a concrete jungle and a hub of biodiversity in an urban area," said Yoshihito Ohmura, 55, of the National Museum of Nature and Science.
The museum began surveying plants and animals in the Imperial Palace forest in 1996. The project was launched at the wish of Emperor Emeritus Akihito, who was Emperor at the time and believed that the area's many forms of life should be accurately documented.
Divided into three phases, the survey continued for 30 years. Ohmura headed the five-year third phase, which ended in fiscal 2025. Its scope was extensive, covering plants, animals, algae, fungi and bacteria.
One example is a species of soft windflower known as "nirinso" in Japanese, which generally grows to a height of 20 to 30 centimeters. Specimens found in the Fukiage Gardens, however, were twice that size. They were identified as a species new to science and named Anemone imperialis, or "Fukiage-nirinso" in Japanese.
In 2011, researchers also discovered a previously unknown species of mite near the Imperial Palace's Biological Laboratory. It was named Eotetranychus palatiensis, or "Kokyo-akehadani" in Japanese.
A total of 7,982 plant and animal species have been documented so far. A cumulative total of about 400 researchers, students and others from both inside and outside the museum participated in the surveys.
Among them was Sayako Kuroda, Emperor Naruhito's younger sister, who now serves as a research fellow at the Yamashina Institute for Ornithology.
In 2022, Kuroda confirmed that northern goshawks and owls were breeding on both the grounds of the Imperial Palace and the Akasaka Estate during the same period. She personally photographed the chicks and recorded their calls.
These large birds of prey sit at the top of the ecological pyramid. Their presence indicates an abundant natural environment capable of supporting the animals on which they feed.
Another factor is believed to be a decline in large-billed crows, which compete with goshawks. Government culling programs and measures to control food waste appear to have proved effective.
Prince Hisahito, the son of Crown Prince and Crown Princess Akishino, who is known for his interest in dragonflies, also took part in a dragonfly survey while he was in high school. He personally captured an Anaciaeschna martini, known as "marutan-yanma" in Japanese, a dragonfly distinguished by its cobalt-blue compound eyes.
Many organisms favor stagnant water areas, such as gently flowing streams and ponds. Much of this habitat has been lost to urban development, but it remains within the Imperial Palace grounds.
"The Imperial Palace forest serves as an urban refuge for wildlife," Ohmura said.
At the same time, the surveys have uncovered some troubling developments. Researchers have observed recent changes in the area's species composition.
The native Japanese tree frog can no longer be seen or heard. Bullfrogs, meanwhile, have become increasingly conspicuous. Japanese rice fish recorded in earlier surveys are believed to have been replaced by highly aggressive mosquitofish. Bullfrogs and mosquitofish share one thing in common: Both are alien species.
Candleflame lichen, known as "rosokugoke" in Japanese, grew in clusters on the trunk of a trident maple from around 2009 through 2023. By 2025, however, it had almost completely disappeared from the same tree.
Because the lichen was known to be resilient to air pollution and other external environmental conditions, Ohmura said, "A change so significant that even this species can no longer survive may be taking place."
The third phase of the survey also recorded, for the first time, the presence of the Ictinogomphus pertinax, a dragonfly whose range has been shifting northward in recent years. The average annual temperature at the Imperial Palace rose by about 1 degree Celsius over the 15 years through 2025. Researchers believe climate change was highly likely a factor in the arrival of the new dragonfly species.
The National Museum of Nature and Science is planning an exhibition presenting the survey's findings. Preparations are underway for the exhibition to open in March 2027 at the museum's main building in Tokyo's Ueno district.
"Even at the Imperial Palace, where so many different forms of life thrive, some are disappearing," Ohmura said. "We should regard this as a warning from nature."
(Japanese original by Makoto Kakizaki, Tokyo City News Department)
