I don't think it's the number of people that matter, but rather the behavior of those people. It appears that humanity lived in balance (Lotka-Volterra style)

I don't think it's the number of people that matter, but rather the behavior of those people. It appears that humanity lived in balance (Lotka-Volterra style) with the rest of the inhabitants of the planet until the invention of agriculture, which is still relatively recent. The transitions that led from agriculture to walled cities to city-states to modern nation-states all occurred within the past 10,000 years or so. Whereas the oldest known human fossil is something like a million years old. Divide that 10,000 by the million years our species has been on this planet, you get 1%. So for 99% of humanity's time on the planet we probably behaved a lot like most of the other land mammals. Something happened that led to the changes in our behavior, and we are now facing the consequences of our collective actions for the past thousands of years. It looks like we'll need to change even more before it's too late.