The Grassland Butterfly Index for Germany

In 2024, the European Union implemented its Nature Restoration Regulation (NRR) to restore degraded ecosystems and to reverse the loss of biodiversity. One of the objectives is to increase biodiversity in agricultural systems. To measure progress towards this goal, Member States need to report two out of three indicators, one of which is the Grassland Butterfly Index (GBI). The GBI represents a composite index which combines the trends of 17 butterfly species, some widespread and some more specialised, considered as typical for grassland habitats across Europe. The aim of the present study was to calculate a first version of the GBI for Germany using data collected within the German butterfly monitoring scheme from 2006 to 2023. For this purpose, two methodological approaches were applied: (1) the approach prescribed by the NRR (EU 2024) and (2) an alternative approach which includes other frequently applied methods to analyse time series from biodiversity monitoring schemes. Depending on the methodological approach, the GBI showed a stable development (approach 1) or a moderate decline (approach 2) over the entire study period. Both approaches, however, showed a significant decline over the last 10 years, which mainly seems to be a result of low butterfly abundances in recent years. A closer examination of the underlying species trends indicated that habitat specialists tended to suffer most from declines. These results largely correspond to patterns found in other regions and at the European scale and suggest that butterflies inhabiting agricultural systems are under threat. However, the reasons for the negative trends require further research. There is also a clear need to improve the representativity and robustness of the indicator at the national scale. Several options for expanding monitoring and widening the data basis are discussed.