Digital Distribution Maps of the IUCN Red List of Threatened Species
We began with the datasets of terrestrial mammal species as defined by the IUCN Red List database (IUCN 2013). Then we focused on terrestrial mammal species living only on islands, and excluded all species that did not have distributions confined to islands only. We defined islands as landmasses smaller than Greenland (2,130,800 km2), with New Guinea (785,753 km2) as the largest island. IUCN's terrestrial mammal spatial data had 1728 unique species identified as residing on an island. When we intersected this with the GSHHS shoreline data, which fulfilled our definition for island, there were 1501 species.
Finally, we restricted this to obligate islanders only, i.e. species not found on any continental mainland, and had 389 species with island-only distributions. We excluded those species with distributions that also encompassed continental mainland because we expected that they would not experience the same level of fragmentation threat as species with an island-confined existence. The mainland can be a potential population source that would not compare evenly in the calculations, particularly as our GSHHS data would not be able to define the species distribution extent on mainland.
Fig. 1 (a) Map of GSHHS-defined islands, highlighting all those containing mammals for which we have ED scores. (b) The highest λM score (1.0) of IUCN-defined island mammals ranges, where endemics confined to one island are automatically assigned an λM score of 1.0. This indicates where the most valuable patches are within a species distribution, and consequently what would be most worth saving
After finding those islands where both GSHHS and IUCN datasets intersected, we calculated the relative λM of every patch within a species' distribution and scaled their values from 0 to 1.0, with the highest value indicating the island/patch that contributed most to the overall long-term persistence (see Fig. 2). We also designated any species with only one island/patch in their distribution automatically with a λM score of 1 (see Fig. 1b), because of its significant importance for that species. We then took these scores and for each, multiplied by the species' ED score. To
Fig. 2 Example map showing how the relative log-scaled λM scores rank within a species' distribution. Here is the distribution of the Wallace's three-striped dasyure (Myoictis wallacei), which occurs in the Aru Islands (Indonesia), and in the southern lowlands on the island of New Guinea (Indonesia and Papua New Guinea) from Merauke in the west to Avera on the Aroa River in the east (Leary et al. 2008)
further give an average λM-ED score per island, we took the sum of species' scores and divided this by the number of island mammal species (in our dataset) residing on that island.