Zoogeography of primates in insular Southeast Asia: species-area relationships and the effects of taxonomy

Given its complex zoogeography and large number of islands insular Southeast Asia makes an excellent subject for studying the interrelationships of species richness, island area and isolation. The islands are merely highpoints of an immense shallow continental shelf which during Pleistocene glacial periods was exposed periodically as dry land connecting the now isolated islands with one another. The area is home to a large number of primate taxa, including many endemic to the region (Nasalis, Presbytis, Pongo, Symphalangus, Simias, Tarsius). Worldwide, the number of described (extant) species of primates has doubled in the last two decades partially as a result of applying a different species concept (viz. Phylogenetic Species Concept PSC as opposed to the Biological Species Concept BSC). According to Isaac et al. (Trends in Ecology and Evolution 19: 464-469, 2004) this ‘taxonomic inflation’ will influence the outcome of macroecological studies. We studied the species-area relationships in Primates on 118 islands in insular Southeast Asia, and used two taxonomies (PSC and BSC). The number of primate species (PSC 37 species, BSC 23 species) is highly significantly related to surface area of the islands, and the slope of the curve is similar for both PSC and BSC species (z = 0.13). Species ‘newly’ described under the PSC are not only from large islands but also smaller ones hence affecting neither intercept nor the slope of the curve. Area alone was a much better predictor for primate species richness than models that included other macroecological variables (latitude, longitude, altitude, distance to mainland, greatest depth between island and mainland, distance to neighbouring islands). Degree of isolation has little influence on species number but both longitude and latitude are inversely correlated with the number of species per island, suggesting that species numbers decrease in a northerly and easterly direction. The low z-values suggest that for primates the islands of Southeast Asia are perhaps less isolated than previously recognised.