Herbarium digitisation sheds light on historical distribution and drivers of population extinction of a peat bog specialist

Societal Impact Statement Biodiversity loss threatens ecosystem services and human well‐being. Understanding the extent and causes of changes in biodiversity over time can help protect species and their habitats. Herbaria house carefully documented and curated specimens collected by generations of botanists. We show how historical herbarium specimens can be made usable for the conservation of species through digitisation and georeferencing. Revisiting localities where a species occurred in the past can reveal population extinctions, identify the causes of biodiversity change and contribute to the development of effective conservation measures. Summary Habitat loss and degradation threaten biodiversity globally. The habitat of the peat bog specialist Eriophorum vaginatum has declined dramatically over the past two centuries, and contemporary habitat degradation and climate change may threaten remaining populations. We aimed to estimate population extinction frequencies, identify drivers of extinctions, and assess the extinction risk of remaining E. vaginatum populations in Switzerland. We searched, digitised and georeferenced historical E. vaginatum specimens from nine Swiss herbaria, and revisited 197 localities where the species was collected between 1803 and 1949, to assess population extinctions. To identify environmental predictors of local extinctions, we used species distribution modelling (SDM). With the resulting model, we predicted extinction probabilities for contemporary E. vaginatum populations. Our revisitation study revealed that extinctions were most common on the Swiss Plateau, a densely populated and intensively farmed region, where 68% of populations became extinct. Extinctions were less common in the other biogeographic areas of Switzerland. The strongest predictors of extinction were soil nutrient content, pH and nearby plantations, whereas the presence of large wetland and wetland‐like areas and nearby shrubland were the strongest predictors of persistence. We predicted a high risk of extinction for 3.2% of the remaining populations. This study highlights the value of historical herbarium records in assessing long‐term environmental impacts on species. We emphasise the ongoing threats to E. vaginatum from agriculture and climate change and underscore the importance of conserving and restoring large habitats with protective buffer zones.