20 years of Butterfly Monitoring Germany – calculating trends and indicators using citizen science data

Butterfly Monitoring Germany is a programme with a central citizen science component. It is coordinated by the Helmholtz Centre for Environmental Research – UFZ and the Society for Butterfly Conservation (GfS), and numerous butterfly experts support the programme on a voluntary basis with their expertise. Since 2005, volunteers have been counting butterflies along fixed routes (= transects) using a method established throughout Europe, known as "Pollard walks".In 2023, data was collected on a total of 620 transects. A special feature of Butterfly Monitoring Germany is that many transect walkers have been participating for many years. For example, 241 of the 620 transects evaluated in 2023 have been monitored for 10 years or more, and 73 transects have been monitored since 2005, i.e. since the start of the project.The database has improved over time, enabling us to calculate trends for 82 butterfly species. Over the years, many (but not all) species have shown a negative trend. Frequent and widespread species in particular often show a positive trend, while negative trends can be observed especially in specialised and/or rare species.Trends can also be calculated for species groups, such as grassland butterfly species. The Grassland Butterfly Index (GBI) has been calculated across Europe for many years based on data from the various butterfly monitoring programmes in the different countries. The GBI is based on trends in 17 butterfly species that are typical of grasslands. The latest trend shows a sharp decline in grassland butterfly species across Europe. In 2025, the GBI was calculated for Germany for the first time, using the same methodology as for the European Index. Unlike the European index, however, only data from 15 species of grassland butterflies can be included in the calculation for Germany. For two species, the data available is not sufficient to calculate a reliable trend.We present the results of the trend calculations for butterfly species in Germany as well as the results of the German GBI.