When photosynthesis first appeared, the oxygen it produced poisoned the existing life. Sulfur-breathers basically disappeared. In the geologic record the oxygen shows up as massive layers of iron oxide which we mine and turn into steel now. New things can radically shake up the existing environment, the degree of shakeup is the measure of how radical it is.
If you really want to see a radical shakeup that would have some very exciting effects, could I interest you in a little Total Atomic Anihilation?