305 USING DATA FROM LOCAL FLORAS TO ASSESS FLORISTIC CHANGE Using data from local floras to assess floristic change

Local floras provide valuable baselines which have seldom been used to measure floristic change. This is largely due to the paucity of historical data for many counties but also because of changes in recording behaviour over the past 350 years. One of the simplest ways to assess change however, is to calculate extinction rates: in English counties at least, this method has shown an average loss of around one species every two years during the last century. In contrast, quantitative assessments of increase/decline have proven more difficult because of the nature (or lack) of historical data. In order to overcome this problem regression analyses have been used to calculate relative changes in species distributions. This approach is particularly useful because it takes into account differences in recorder behaviour between surveys. In contrast, absolute floristic changes have only been recorded from sample sites (“habitat studies”) in two counties (Bedfordshire and Dorset)