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Japan's declining population in agricultural regions could be having a damaging impact on biodiversity—a trend that could spread through East Asia and Europe as global fertility rates fall—according to an international study.

in the journal Nature Sustainability, the study by researchers from the University of Sheffield and Tokyo City and Kindai universities, is calling for more effective government policies that balance both rewilding and economic growth, as results show urban land use and farmland abandonment are increasing in Japan despite the population decline.

The findings add to the growing evidence that, when humans leave an area, it doesn't always lead to spontaneous rewilding—the restoration of ecosystems and nature recovering from damage caused by people.

Experts in demography from the University of Sheffield and in biodiversity from Tokyo City and Kindai say Japan is a "Depopulation Vanguard Country" (DVC) for East Asia, as the link between a declining population and loss in biodiversity is expected to be seen throughout the rest of the region.

With projections from the United Nations that 85 countries are on course to be continuously depopulating by 2050, the researchers believe the findings could also be useful for other nations outside of East Asia, particularly Southern and Eastern European countries where fertility rates are already below replacement levels and outward migration is high.

Dr. Peter Matanle, Senior Lecturer in Japanese Studies at the University of Sheffield, said, "Japan is one of many countries whose demographic trends point to long-term population loss. We call it a 'Depopulation Vanguard Country' because it presages losses in similar neighboring countries such as South Korea and China.

"We find that expected biodiversity gains from fewer people may not be occurring, and that losses are continuing. This indicates possible similar effects in other world regions, such as Southern Europe, with Italy as its DVC, and Eastern Europe, with Bulgaria and Latvia as its DVCs. In the future, we'd like to research other regions such as Europe and Latin America to find out if similar effects are being felt there."

The researchers analyzed population, land use and surface temperature in rural areas throughout Japan. They then compared this to biodiversity change among 464 species of bird, butterfly, firefly and frog spawn and 2,922 native and non-native plant species over periods of five to 17 years from 2004.

The team drew on data gathered by citizen scientists across Japan who have been counting organisms in their local area and featured more than 1.5 million data points—one of the biggest studies of its kind.

Irrespective of whether human population increases or decreases, biodiversity losses were found in most species studied. The researchers believe this was mainly due to agricultural land use change, either to urbanization, disuse and abandonment, or intensification. Only where human numbers were stable does biodiversity also appear more stable.

Kei Uchida, Associate Professor at Tokyo City University, said, "Japan's biodiversity has long been sustained by traditional rural livelihood practices such as wet rice agriculture, forest and soil management, and the maintenance of rural landscapes. These human activities have shaped and supported ecological richness for centuries.

"Even as populations decline, human interventions continue in some areas, while others are left to ecological succession. This uneven pattern of change produces a mosaic landscape which is accelerating the degradation of rural environments and undermining the biodiversity they support.

"Our research has implications for maintaining biodiversity in other depopulating countries in East Asia, and contributes to knowledge about the outcomes of human depopulation worldwide."