Contribution of citizen sciences to the production of butterfly species long‐term trends

1. Biodiversity indicators, such as temporal trends in relative species abundance, are essential to report on the status, pressures and responses of biodiversity and to guide environmental policies. These trends are often estimated from datasets collected from standardised, long-term and large-scale biodiversity monitoring schemes thanks to volunteer-based citizen science including volunteers with a broad range of naturalist skills. However, the involvement of the general public, predominantly considered as non-experts, in the observation and inventory of biodiversity raises questions about data quality and reliability. 2. Here, we focused on three nationwide butterfly monitoring schemes involving volunteers with varied naturalist skills in France and the United Kingdom. For each scheme, the temporal trends and interannual variations in abundance of 20 common species/species groups were estimated. Then a detailed analysis was conducted to ascertain which features related to butterfly biology, ecology or the protocols' characteristics best explain the differences in species trends or interannual variations between schemes involving expert naturalists and those relying on the general public. 3. Similar interannual variations between schemes were found for generalist, migratory and easily recognisable butterflies, but no variable explained differences in temporal trends. In addition, considering species groups when it is difficult to distinguish species reliably did not yield poorer results than considering individual species. 4. The considerable amount of data collected by volunteers regardless of their naturalist expertise can be used to generate reliable and robust species biodiversity indicators, including hard-to-access locations such as private gardens, something that traditional methods or expert data cannot capture.