There are many different Butterfly Monitoring Schemes in Europe that generate frequently, in some cases annually, reports on the data collected on their BMS. Here you can find some of the reports produced recently:
For many years and even decades, some BMS are been running and collecting data. There is really precious data from large periods of time, and that information has been processed to show the different butterfly trends and results of the multiple years collecting information on the field.
Here you can find some of these large range reports from different BMS.