This week in the New Yorker, Elizabeth Kolbert tells us that up until modern times, there have been five big mass extinctions of life on Earth. Most biologists now agree that we are in the midst of the sixth big mass extinction, one caused by humans.
Currently, a third of all amphibian speicies, nearly a third of all reef-building corals, a quarter of all mammals, and an eighth of all birds are classified as "threatened with extinction." These estimates do not include the species that humans have already wiped out or the species for which there are insignificant data. Nor do the figures take into account the projected effects of global warming or or ocean acidification. Nor, of course, can they anticipate the kinds of sudden, terrible collapses that are becoming almost routine.
See also a related audio clip by Kolbert, the Holocene extinction event, and colony collapse disorder.