Frontier forests are the world's remaining large intact natural forest ecosystems. These forests are -- on the whole -- relatively undisturbed and big enough to maintain all of their biodiversity, including viable populations of the wide-ranging species associated with each forest type. As defined in this assessment, a frontier forest must meet seven criteria:
- It is primarily forested.
- It is big enough to support viable populations of all indigenous species associated with that forest type -- measured by the forest's ability to support wide-ranging animal species (such as elephants, harpy eagles, or brown bears).
- It is large enough to keep these species' populations viable even in the face of the natural disasters -- such as hurricanes, fires, and pest or disease outbreaks -- that might occur there in a century.
- 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. As such, it remains relatively unmanaged by humans, and natural disturbances (such as fire) are permitted to shape much of the forest.
- In forests where patches of trees of different ages would naturally occur, the landscape exhibits this type of heterogeneity.
- It is dominated by indigenous tree species.
- It is home to most, if not all, of the other plant and animal species that typically live in this type of forest.