Nonlinear relationship between biodiversity and human population density: evidence from Southeast Asia
The geographical distribution of terrestrial vertebrate biodiversity was assessed in Southeast Asia, an area characterized by highly heterogeneous human population densities (HPDs). We tested the hypothesis that recent (between 1990 and 2000) population increases may result in increased threats to biodiversity. GIS data were compiled from the International Union for the Conservation of Nature, BirdLife, and the Socioeconomic Data and Applications Center. Polygon data sets were transformed into rasters with different resolutions from 2.5$$'$$′ to 1$$^{\circ }.$$∘. Relationships between species diversity were assessed globally and separately for terrestrial vertebrates (mammals, birds, reptiles, and amphibians) and for five countries (Cambodia, Laos, Myanmar, Thailand, and Vietnam). The number of species within a 20 km$$^{2}$$2-cell varied greatly and reached a maximum of 801. The species-rich cells ($$>700$$>700 species) were located in a single, almost continuous area across Myanmar, Thailand, Vietnam, and South China. The relationship between vertebrate biodiversity and population density showed that biodiversity increased with HPD until around 10 persons km$$^{-2},$$-2, then decreased until around 200 persons km$$^{-2}$$-2 where it stabilized. This relationship was mediated by land cover and protection status with significant interactions between these variables and with population density. The areas with moderately increasing populations between 1990 and 2000 were those with the highest overall diversity (median 607) whereas those with decreasing populations were those with the lowest diversity (median 479). Our results suggest that population pressure on biodiversity has increased between 1990 and 2000 and this is a relatively widespread phenomenon. The presence of a continuous area of high vertebrate biodiversity calls for international action at a regional scale involving China, Laos, Myanmar, Thailand, and Vietnam.