To piggyback on Piper's thoughts let's just say we improve Seattle's waste water/sewage by 50%. It now discharges half the nutrients and chemicals as before. We double the population and haven't changed a thing.

We each reduce our water use by 10% to save water for fish. And when population increases by 11%, the fish lose water.

What we seem to forget is that our impact, today, is excessive. Simply reducing our individual footprint is little more than warm and fuzzy unless there is a cumulative reduction in total impact.