Collated indices are a relative measure of butterfly abundance across monitored sites in the UK, calculated from data collected by the UK Butterfly Monitoring Scheme (UKBMS). Collated indices are calculated annually for each individual butterfly species that has been recorded on five or more sites in that year. Based on this criterion collated indices have been calculated for the entire UKBMS time series from 1976 to the current year for the majority of species. For some rarer species the time series starts in a later year due to lack of data. Collated indices are calculated using a statistical model that accounts for missing data. The number of sites for each species ranges from 5 to several hundred and varies from year to year. Since 2008 more than 1,000 sites have been monitored across the UK each year. Collated indices are calculated so that we can determine how butterfly populations are changing over time across the UK. This data can be used, for example, to determine where to target conservation efforts and to measure the condition of the UK countryside. Butterflies are recognised as important indicators of biodiversity and environmental change (e.g. as official UK Biodiversity Indicators), and have been used in numerous research studies to understand the impacts of changes in climate and the extent and condition of habitats.
The UK Butterfly Monitoring Scheme is organized and funded by Butterfly Conservation (BC), the Centre for Ecology & Hydrology (CEH), the British Trust for Ornithology (BTO), and the Joint Nature Conservation Committee (JNCC). The UKBMS is indebted to all volunteers who contribute data to the scheme.
Collated indices are produced using a series of statistical models on butterfly transect counts recorded by a volunteer network. These indices are a relative measure of each species' abundance across the UK compared to the baseline in 1976. The statistics are presented as log10 values. These values are centred around an arbitrary value of 2 as a mean for the time series in order to help show which years are below or above average (a technique used in other biodiversity indicators).