The currency and tempo of extinction

This study examines estimates of extinction rates for the current purported biotic crisis and from the fossil record. Studies that compare current and geological extinctions sometimes use metrics that confound different sourc es of error and reflect different features of extinction processes. The per taxon extinction rate is a standard measure in paleontology that avoids so me of the pitfalls of alternative approaches. Extinction rates reported in the conservation literature are rarely accompanied by measures of uncertain ty, despite many elements of the calculations being subject to considerable error. We quantify some of the most important sources of uncertainty and c arry them through the arithmetic of extinction rate calculations using fuzz y numbers. The results emphasize that estimates of current and future rates rely heavily on assumptions about the tempo of extinction and on extrapola tions among taxa. Available data are unlikely to be useful in measuring mag nitudes or trends in current extinction rates.