If environmentalists want to use their own money to purchase land and set it aside as either wildlife preserves or conservation concessions then they should do so. It is the free market at work and it is certainly worth a try because many of the other methods of conservation have not worked very well.
I don't think conservation concessions are a good idea because this would be restricting free enterprise and limiting particular land to only a small group of people. This would almost cause a monopolistic control of land that can later be used for who knows what. Land conservation itself is a great idea, but if you are only restricting it to a particular group, you are doing injustice to others.