The priority species indicator: measuring the trends in threatened species in the UK

We describe the development of two complementary priority species indicators (PSIs) to help the UK to report progress towards Aichi target 12 on the status of known threatened species. Based on species identified as national conservation priorities, the indicators present average changes in (i) 213 species for which trends in relative abundance are available from structured monitoring schemes, and (ii) 179 species for which trends in frequency of occurrence were modelled from data sets of unstructured biological records. Both indicators show substantial declines in priority species since 1970, of 67% and 40%, respectively, although the rate of decline in the relative abundance-based PSI may have lessened over the last five years (2007–2012). We discuss the biases and weaknesses of the indicators at present, and put forward suggestions as how these may be addressed, including through the development of a third PSI.