Frontier forests relatively undisturbed and big enough to maintain all of their biodiversity, including viable populations of the wide-ranging species associated with each forest type. A frontier forest must meet seven criteria:
1. It is primarily forested.
2. It is big enough to support viable populations of all indigenous species associated with that forest type, especially wide-ranging animal species such as elephants, harpy eagles, or brown bears.
3. It is large enough to keep these species' populations viable even in the face of natural disasters such as hurricanes, fires, and pest or disease outbreaks.
4. Its structure and composition are determined mainly by natural events, though limited human disturbance by traditional activities of the sort that have shaped forests for thousands of years—such as low-density shifting cultivation—is acceptable. It remains relatively unmanaged by humans, and natural disturbances (such as fire) shape much of the forest.
5. The forest has a mix of trees of different ages.
6. It is dominated by indigenous tree species.
7. It is home to most, if not all, of the other plant and animal species that typically live in this type of forest.
A non frontier forest would include forests that do not meet those criteria. Primarily they would include forests that lack biodiversity or have been disturbed by human activity.