[ThisIBereave]

From Wikipedia[1], "In the past 540 million years there have been five major events when over 50% of animal species died." So if your measure for bio-diversity is the number of unique animal species, it dropped by half at each mass extinction event. The events only reflect a drop in the diversity of animal species as this is an "easily observed, biologically complex component of the biosphere", so the overall change in bio-diversity could be lesser or greater.

1: http://en.wikipedia.org/wiki/Extinction_event