There are those that believe we have dominion over the earth and can do whatever we please. I believe that is rather short sighted.
We exist, therefore I believe we have a place on the planet, but I also believe we need to co-exist with other life forms. This is not a trivial statement, since it implies we can not, sustainably, totally dominate over everything else. For instance, as a trivial example: we need trees to recycle our air. Diversity in the workplace has been shown to be good, diversity in our ecosystem is not only better - its required if we hope to be able to survive environmental challenges.
Therefore, to start the discussion, I'd like to propose a "50% rule":
1) 50% of the planets surface should remain totally wild and uncontrolled by man. Oceans don't count, since what we pump into them works its way across them. Think in terms of half of every bio-region being "hands off" to humankind and allowed to proceed as nature dictates. That means forest fires are allowed to burn, wolves are allowed to roam, etc. "allowed" being a bad term, since it implies control, which is exactly what we should NOT have in these areas.
2) 50% of what is left is shared. This allows for woodland crop management, public parks, etc.
3) 50% of what is left after 2) is available for mankind to manage. eg. 25% of the landmass, water surface, etc. We don't get to pick and choose either - per 1) 50% of all coastlines must be left wild. 50% of all prairie. 50% of all forest. 50% of all wetlands... If we decide not to use our 25% (say of Antarctica) that is our choice to ignore, not space counted towards 1) or 2).
Basically this means we would need to reduce land usage dramitically. In the USA, virtually all ariable land is cultivated in crops. Here in the mid-west USA, if you see a patch of trees, you know that ground is too hilly or rocky to farm. After these rules are agreed upon, 75% of the current farmland would need to be returned to native prairie, forest, and desert.
Of course, this will have dramatic impact on the number of people the planet can support food wise. That is not a bad thing.
Implementation timing would need to be sychonized with population decrease to avoid food riots.