<p>Multispecies indicators are an important source of information for quantifying changes in biodiversity. An example is the European Grassland Butterfly Indicator, which gives a trend of 17 species characteristic of European grasslands using data from different European Butterfly Monitoring Schemes. However, adapting this indicator to the regional scale is problematic for several reasons, including changes in habitat preferences in different parts of a species’ range, as well as significant loss of information in highly diverse regions where many local species associate with grasslands. Here we develop a regional indicator that uses data of 143 species present in the Catalan Butterfly Monitoring Scheme, but that weights their contribution according to their degree of association with open habitats. This indicator showed a decline of 54.6% over three decades, substantially higher than a downward trend of 39% when calculated without weighting. Although similar results were obtained using fewer species, the accuracy of the estimates increased dramatically as more species were added to the indicator. Using the same data set but giving more weight to forest species, we also developed a forest indicator. This indicator showed a decline of only 18.1%, in line with the fact that forest species are favoured by widespread land abandonment but are negatively affected by climate change in the Mediterranean region. In summary, our method represents a flexible and powerful tool for assessing biodiversity trends in grasslands and forests at a regional scale. Moreover, it is applicable to other taxa for which comprehensive monitoring data exist.</p>