Grassland butterfly index in Europe

The data for this indicator originate from the European Butterfly Monitoring Scheme (eBMS) (a joint initiative of Butterfly Conservation Europe and the Centre for Ecology & Hydrology) and the SPRING: Strengthening Pollinator Recovery through INdicators and monitorinG project.

The butterfly sampling method is based on butterfly transect counts. The eBMS provides methodological support to volunteers and country/regional networks, e.g. via the manual for butterfly transect counts.

The butterfly indicator is based on the fieldwork of thousands of trained professional and volunteer recorders, counting butterflies on more than 6,200 transects scattered widely across the EU under standardised conditions. National coordinators collect the data and perform the first quality control step. In 2020, more than 55,880 km of transect walks were made, more than 90% of them by volunteers, monitoring each transect an average of 15 times per year.

For each species and year, flight periods were estimated for each climate zone as defined in , with further geographical stratification to represent major geographical units covered by the eBMS database (e.g. Ireland was treated as a separate unit to continental Europe). The Flight curve model was based on the accumulation of Growing Degree Days (GDD) over the calendar days since January 1st. This model uses GDD with a base of 7°C and a maximum of 14°C. These temperatures are known to influence the development of butterflies and thereby contribute to shaping their annual flight curve. Site-level indices were produced by estimating the missing counts, based on local counts and predicted flight curve. Species’ collated indices were produced for each monitoring scheme using a Poisson generalised linear model (GLM) with site and year effects, as well as the proportion of the flight period surveyed as a weighting. A collated index for the EU was produced for each species by calculating the geometric mean of the species’ scheme-level collated indices, weighted by the area of the species’ range sampled in each Butterfly Monitoring Scheme and accounting for Butterfly Monitoring Schemes which started in later years. The EU indices for the 15 species were combined by taking the geometric mean of the indices, based on the approach in the Biological Records Centre (BRC) indicators R package. This indicator is a unified measure of biodiversity following the Common bird indicator, by averaging indices of species rather than abundances to give each species an equal weight in the resulting indicators. If positive and negative changes of indices are in balance, their mean would be expected to remain stable. If more species decline than increase, the mean should go down, and vice versa. Thus, the index mean is considered a measure of biodiversity change. A smoothed index was produced using a loess smoothing approach as in . Confidence intervals were produced using a bootstrapping approach, by resampling the site-level indices 1,000 times and applying subsequent analysis stages to each bootstrap.

The indicator is based on the following 15 species, selected by experts and considered to be characteristic of European grassland and that occurr in a large part of Europe covered by the majority of the butterfly monitoring schemes, and with grasslands as their main habitat:

• Widespread species: Anthocharis cardamines, Coenonympha pamphilus, Lasiommata megera, Lycaena phlaeas, Maniola jurtina, Ochlodes sylvanus and Polyommatus icarus;

• Specialist species: Cupido minimus, Cyaniris semiargus, Erynnis tages, Euphydryas aurinia, Lysandra bellargus, Lysandra coridon, Spialia sertorius and Thymelicus acteon.

Two further species identified as characteristic for the indicator were excluded due to insufficient data (Phengaris arion and Phengaris nausithous).