Language extinction triggers the loss of unique medicinal knowledge

Significance The United Nations proclamation of 2022–2032 as the International Decade of Indigenous Languages aims to raise global awareness about their endangerment and importance for sustainable development. Indigenous languages contain the knowledge that communities have about their surrounding plants and the services they provide. The use of plants in medicine is a particularly relevant example of such ecosystem services. Here, we find that most medicinal knowledge is linguistically unique—i.e., known by a single language—and more strongly associated with threatened languages than with threatened plants. Each indigenous language is therefore a unique reservoir of medicinal knowledge—a Rosetta stone for unraveling and conserving nature’s contributions to people. Over 30% of the 7,400 languages in the world will no longer be spoken by the end of the century. So far, however, our understanding of whether language extinction may result in the loss of linguistically unique knowledge remains limited. Here, we ask to what degree indigenous knowledge of medicinal plants is associated with individual languages and quantify how much indigenous knowledge may vanish as languages and plants go extinct. Focusing on three regions that have a high biocultural diversity, we show that over 75% of all 12,495 medicinal plant services are linguistically unique—i.e., only known to one language. Whereas most plant species associated with linguistically unique knowledge are not threatened, most languages that report linguistically unique knowledge are. Our finding of high uniqueness in indigenous knowledge and strong coupling with threatened languages suggests that language loss will be even more critical to the extinction of medicinal knowledge than biodiversity loss.