Even taking into account the difficulties of comparing the fossil and modern records, and applying conservative comparative methods that favour minimizing the differences between fossil and modern extinction metrics, there are clear indications that losing species now in the ‘critically endangered’ category would propel the world to a state of mass extinction that has previously been seen only five times in about 540 million years. Additional losses of species in the ‘endangered’ and ‘vulnerable’ categories could accomplish the sixth mass extinction in just a few centuries.
From: Barnosky AD, et al. Has the Earth’s sixth mass extinction already arrived? Nature 471: 51-57 doi:10.1038/nature09678