The Toad Crisis in Tokyo: What Urban Communities Can Do

Many people imagine Tokyo as a landscape of concrete and skyscrapers. Yet every spring, Japanese common toads emerge from hidden corners of the city to breed in ponds, gardens, and remnant green spaces. Today, however, many of these populations are in decline.

I recently published a review paper in Frontiers in Ecology and Evolution titled "Amphibian conservation in urban areas: a review of green infrastructure for achieving One Health" (1). Through it, I want to emphasize that local amphibian conservation activities at the individual and community levels, including the management of private gardens, can play a crucial role in achieving One Health on a global scale through the concept of Green Infrastructure (GI). In this article, I would like to share my experiences with amphibian conservation in Tokyo, and particularly the current situation of its toads.

Protecting Microhabitats, Starting from Private Gardens

Tokyo is a densely populated city of more than 14 million inhabitants, yet scattered green spaces remain where you can still observe the reproductive behavior of Japanese common toads (Bufo formosus) — what we in Japan call kaeru gassen, a term that refers to the intense competition among males and evokes images of historic samurai battles. I have watched nearly 100 toads breeding in the pond on the grounds of the temple where I grew up. Tokyo's toads also live quietly near office buildings in the city center and along the side streets of commercial districts. This is largely unknown even among Tokyo residents, so amphibian enthusiasts should certainly try to find them when visiting. The breeding season runs through early spring, from mid-February to early March, and juveniles and mature toads can be found on damp nights from May to October.

A Rapid Decline in Population, and Genetic Disturbance

In recent years, however, Tokyo's toad populations have declined rapidly. In a pond on the University of Tokyo campus where I studied breeding behavior, more than 500 toads came to lay eggs ten years ago (2); as of 2026, not a single egg mass has been found. The same trend is seen across central Tokyo's green spaces, with some local populations of around 1,000 individuals disappearing over the last 30 years. Based on repeated breeding surveys across 24 urban green spaces in Tokyo during 2015–2023, six sites appear to have already lost their local breeding populations (3).

In addition, the introduced western Japanese common toad (B. japonicus) continues to spread. Hybridization between the native B. formosus and the non-native B. japonicus is progressing from the urban core toward the suburbs (3, 4). Maintaining the regional character of the Japanese common toad's genetic diversity is a real challenge for Tokyo.

From Private to Public

Conservation efforts are already taking place at many levels, from managing private ponds to volunteer groups patrolling neighborhoods. Road signs, for example, have been installed to reduce road mortality.

Urban planning is also crucial. Town development built around water and greenery — and Blue-Green Infrastructure (BGI), which promotes habitat connectivity — is an important concept for amphibian conservation. By spreading the principles of BGI, conservation at the habitat level becomes possible, which would be highly effective for ensuring the survival of local populations.

Intergovernmental cooperation is necessary to make habitat connectivity function effectively, and feedback from citizens' conservation efforts back to local governments is crucial to sustaining long-term work.

How Amphibian Conservation Benefits Human Health

Amphibians connect aquatic and terrestrial ecosystems and play important roles in environmental health. They may also contribute to future advances in medicine and public health. Many people may never become amphibian enthusiasts. Nevertheless, discoveries arising from amphibian research — including studies of microbiomes, bioactive compounds, and ecosystem health — show that amphibian conservation can benefit human well-being in unexpected ways. Recent studies, for instance, have shown that the bioactive peptide cathelicidin-DM (C-DM), secreted from the skin of toads, can inhibit the growth of liver cancer cells (5), and that the bacterium Ewingella americana, isolated from the gut of the Japanese tree frog, possesses extremely potent anti-cancer properties (6). Supporting toads and frogs means supporting our own lives.

Connecting People Through Toad Conservation

Many of the challenges facing Tokyo's toads are shared by amphibian conservationists around the world. Habitat fragmentation, road mortality, invasive species, and declining local populations are global concerns. By strengthening connections among citizen scientists, volunteer groups, researchers, and conservation practitioners across countries, we can learn from one another and build more effective strategies together. I would like to propose creating a network of volunteers from various countries to exchange information. Every garden pond, every wetland, and every citizen observation can contribute to amphibian conservation. Urban conservation begins locally, but its impact extends far beyond city boundaries. Perhaps a shared love of toads can help connect people around the world — and bring a little more happiness along the way.

References

1. Hase, K. Amphibian conservation in urban areas: a review of green infrastructure for achieving One Health. Frontiers in Ecology and Evolution 14, 1740190 (2026). doi:10.3389/fevo.2026.1740190
2. Hase, K. & Shimada, M. Female polyandry and size-assortative mating in isolated local populations of the Japanese common toad Bufo japonicus. Biological Journal of the Linnean Society 113(1), 236–242 (2014). doi:10.1111/bij.12339
3. Hase, K. et al. The introduction of the Japanese common toad in Tokyo and the current state of their breeding behavior (in Japanese). Bulletin of the Herpetological Society of Japan No. 2, 215–223 (2022). Available at: https://herpetology.jp/publications/bhsj/toc.php?issue=2022.2.ja
4. Hase, K. et al. Population admixture and high larval viability among urban toads. Ecology and Evolution 3(6), 1677–1691 (2013). doi:10.1002/ece3.578
5. Hu, H. et al. Study on the anticancer function and mechanism of cathelicidin-DM in liver cancer. Genomics 117(3), 111049 (2025). doi:10.1016/j.ygeno.2025.111049
6. Iwata, S. et al. Discovery and characterization of antitumor gut microbiota from amphibians and reptiles: Ewingella americana as a novel therapeutic agent with dual cytotoxic and immunomodulatory properties. Gut Microbes 17(1) (2025). doi:10.1080/19490976.2025.2599562