At the time of 2010, the UN Environment Programme estimated that 150-200 species of plant, insect, bird and mammal become extinct every 24 hours, and, that this is nearly 1000 times greater than "natural" rates. (source, para. 5)
My question is: How are baselines for extinction rates (regarding plant & animal species amongst the entire Earth) established, to then be able to say if an extinction rate for a given species is "normal" or not? What factors are considered most relevent?