The problem will solve itself, unfortunately.
Even if we had cold fusion and unlimited free energy, we'd still have the problem of too many people.
If we had unlimited free energy, we'd certainly still have too many people, not to mention cooking the biosphere with the waste heat from fusion processes.
Yes!
Waste heat from thermodynamic process is something that I've never seen mentioned or discussed, only greenhouse gasses. Almost all the energy we use ends up as heat.
If we globally stopped eating meat then a large portion of the land used for agriculture to produce livestock feed would not be needed and could return to forest. And we'd all live and be healthier for it (yay less heart disease and cancer). The amount of land used to grow food to feed cows is a large portion of the total agriculture output.
Organic farming seems like it could be sustainable even if portions of our stocks are taken by insects. It'd be manageable without pesticides. And if we invest more in genetically modified vegetables we could grow them to be resistant to pests without filling the environment with pesticides.
We humans used to be pretty good at permaculture: https://en.wikipedia.org/wiki/Permaculture
All land will either be used for housing or food production, there will be none "spare" for wildlife or environmental purposes.