We probably are.
Previous mass-extinctions have (we believe) happened as a consequence of some natural disaster usually causing catastrophic climate change.
On this occasion, it is the work of man.
We cannot return the Earth to the way it was when we had many more species - we can't replant the jungles and rainforests. We cannot take land out of food production - our increasing population needs feeding.
There are conservation actions we can take, but these are really only "delaying tactics" before extinction occurs. Your grandchildren may still see Tigers, Hippos and other threatened species in a zoo. In the wild, they'll be gone.
Mass extinctions in the past have led to new species prospering (The so-called "Cambrian Explosion", for instance). The KT extinction event, that brought about the end of the dinosaurs led to a leap forward for the mammals.
It is thought unlikely that this will happen in this instance, until mankind is itself extinct. A sobering thought.
