The State of Nature report, compiled by 25 wildlife organisations - from the RSPB to the British Lichen Society - collates assessments of 3,148 species.
Conservationists hope it will offer clues to the fate of the UK's 59,000 species.
Beetles and wildlfowers are among the most vulnerable species.
According to the document, reasons for the decline are "many and varied" but include rising temperatures and habitat degradation.
Species requiring specific habitats have fared particularly poorly compared to the generalists able to adapt to the country's changing environment.