Although forests are among the most important habitats for terrestrial biological diversity, there are difficulties in quantifying this contribution. Similarly, much of the biodiversity of mountain areas is found in their forests, but SDG Indicator 15.4.1 (on PAs for mountain biodiversity) does not yet have information by ecosystem type. As at least 50 percent of the world's species are thought to be hosted by tropical forests, it has been proposed that SDG Indicator 15.5.1 (the Red List index) should separately identify the extinction risk for forest-dependent species.