All of us rational folk here believe in evolution through selective pressures - you might have some disagreements on exact evolutionary paths, or whether in some cases lamarck was right (interuetero environmental effects, for example), but bottom line, the emergence of species depends on selective pressure.
So, when we talk about "mass extinction", which sounds really scary, let's realize that this kind of thing isn't only natural, it's *necessary* for evolution.
A more interesting measure might be this - what is the total biomass that existed before humanity, and what is the total biomass now? Changes in species distribution and existence might shuffle the deck a bit, but do we really believe that there is less life on the planet now than before humans? Maybe each large fauna was replaced by the equivalent mass in insects, or smaller animals, but does anyone actually think for a single moment that we have *less* life on the planet now?