Arguably, Clovis people at the end of the last Ice Age killed off the majority of large mammals in North and South America.  Living at a subsistence level is no real remedy against impact on the environment.<p>What I believe will happen, and really, we are already starting to see this happen in the United States, is that the concentration of humanity into denser populations in the urban and suburban zones means that the more rural zones become abandoned and grow back up.  New England is more densely forested now than it has been in 200 years, because the population has concentrated in the cities, and land that was formerly cleared for intensive agriculture has fallen into disuse.