A general method for measuring relative change in range size from biological atlas data

Conservationists need information on trends in the ranges of species in order to set priorities and to assign threat statuses using IUCN criteria. For many taxonomic groups in many countries, the best data available for this purpose come from atlases that map the distributions of species in grid cells. This paper presents a method of calculating an index of relative change in range size using atlas data for two periods. The method allows for the effect of variations in the geographical coverage and intensity of recorder effort. A weighted linear regression model is fitted to the relationship between counts of grid cells (as logit-transformed proportions) in the earlier and later periods. The standardised residual for each species provides an index of its relative change in range size. The method is illustrated by analyses of three British data sets. Previous methods of assessing change from atlas data are briefly reviewed and the advantages and limitations of the new method are discussed.