It is possible.
One of the discussions that spurred here in my cubicle was: How? How can we set aside half of the planet for anything other than ourselves? It's impossible! With almost every nation, state or person out there worrying about their piece of land it surely must be impossible.
But not quite.
Use Nuclear Leakage & Irradiation. Like the one that led to the Red Forest in Chernobyl [1].
'Radiological Reserves' are probably the only way to set aside a large area for animals/plants with a guarantee that humans will not come by. Not in the next 10,000 years!
Eat that! :)