Scale dependency of rarity, extinction risk, and conservation priority

In developing red data books of threatened species, the World Conservation Union uses measures of rarity, rates of decline, and population fragmentation to categorize species according to their risk of extinction. However, most quantitative measures of these three concepts are sensitive to the scale at which they are made. In particular, definitions of rarity based on an area-of-occupancy threshold can nearly always be met if area of occupancy is calculated from a sufficiently fine-scale (high-resolution) grid. Recommendations for dealing with scale dependency include (1) choosing a standard scale of measurement, (2) using multiple scales of measurement, and (3) developing indices that combine information from multiple scales. As an example of the second and third approach, the construction of a species' scale-area curve represents a unifying method for quantifying all three indicators of extinction risk-rarity, rate of decline, and population fragmentation-as functions of area of occupancy and measurement scale. A multiscale analysis is also of practical importance because measurements made at different scales are relevant to different extinction processes. Coarse-scale measures of rarity are most appropriate when threat is assessed on the basis of spatially autocorrelated events of a large extent, such as global climate change, whereas fine-scale measures may best predict extinction risk due to local processes such as demographic stochasticity. We illustrate our arguments with a case study of the British distributions of two related plant species that show a 200-fold reversal in their relative rarity when measured at different scales.