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Frontiers of Biogeography

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Doubling diversity: a cautionary tale of previously unsuspected mammalian diversity on a tropical oceanic island


The development of meaningful models of species richness dynamics in island ecosystems requires accurate measurement of existing biodiversity. To test the assumption that mammalian diversity on tropical oceanic islands is well documented, we conducted a 12-year intensive survey of the native mammal fauna on Luzon Island, a large (ca. 103,000 km2), mostly volcanic island in the Philippines, which was thought to be well known. Prior to the start of our study in 2000, 28 native, non-flying mammals had been documented, and extrapolation from prior discoveries indicated that the rate of discovery of new species was steady but low. From 2000 to 2012, we surveyed non-flying mammals at 17 locations and discovered at least 28 additional species, doubling the number known. Nearly all of the new species are restricted to a single mountain or mountain range, most of which had not been sampled previously, thus also doubling the number of local centers of endemism within Luzon from four to eight. The number of species on a mountain is strongly correlated with the elevation of the peak, and the number of endemic species on a mountain range is strongly correlated with the maximum elevation of the range. All 28 of the new species, and 20 of the species discovered prior to 2000, are members of two morphologically and ecologically diverse endemic clades (“cloud rats” and “earthworm mice”), which strongly implies that species richness has primarily been the product of speciation within the island. We reject the general assumption that mammals on tropical oceanic islands are sufficiently well known that analysis and modeling of the dynamics of species richness may be conducted with precision. In the development of conceptual biogeographic models and implementation of effective conservation strategies, existing estimates of species richness, levels of endemism, and the number of subcenters of endemism should be actively reassessed and verified through robust field, museum, and laboratory studies.

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