Estimating rates of butterfly decline from distribution maps: The effect of scale

摘要: Analysis of rates decline for butterflies in the British county Hertfordshire, from presence/absence data grid squares before and after 1970, showed that complete extinction took place 66·9% 2 km occupied 1970 (average 18 species). For 12 species intermediate rarity, a resulted estimates were on average 35% higher than based 10 grid, scale at which have been mapped nationally. Even are likely to be underestimates because pre-1970 records incomplete still conceal declines within squares. In Plebejus argus, exact location every local population is known North Wales, would seriously underestimate declines, example giving only 56% loss if 90% populations become extinct. Our results analysis simulated distributions indicate that: few very rarest species, maps may closely reflect losses; identify but common losses fail detected maps. Per-population even some relatively or as high rarest. Because most commoner initially had many per square, their underestimated not by existing mapping schemes. We propose scheme monitoring changes status well rare network intensively different scales.