1. A reduction in forest area should result in a reduction of its number of species and, moreover, do so in a characteristic way according to the familiar species-area relationship. Brooks, Pimm & Collar (1997) applied this formula to the losses in forest area in the Philippines and Indonesia. Independently derived totals of the number of endemic bird species that are threatened with extinction broadly agree with these predicted losses. In some cases, however, predicted losses overestimate or underestimate the actual numbers of threatened species.
2. Within an island, the proportionate deforestation to date might be most extensive where there are many endemic species, or where there are few. To test this possibility, we obtained recent forest cover data for the region. We separated lowland (< 1000 m a.s.l.) from montane (> 1000 m a.s.l.) forest cover by overlaying topographic maps. From these data, we predict separately the numbers of montane and lowland endemic bird species likely to become extinct as a result of deforestation. We then compared these totals with the numbers considered threatened in the latest Red List.
3. Our predictions based on deforestation closely match the numbers of threatened endemic birds in the lowlands, but underestimate them in montane regions.
4. Our predictions based on deforestation underestimate the number of threatened montane mammal species even more seriously.
5. Lowland faunas of insular South-east Asia are under extreme threat because of massive deforestation. The region's montane faunas appear seriously threatened even by low levels of deforestation.