Why may the same species have different elevational ranges at different sites in New Guinea?

Abstract

Species in mountainous areas have characteristic elevational ranges, but with some variation from site to site. Such variation has been studied extensively elsewhere in the world, but not yet for New Guinea bird species. Hence, we examined five sources of that variation for New Guinea birds: latitude, competition, slope, the Massenerhebung effect, and physical barriers. Decreases of species elevational ranges with latitude are illustrated by three sets of examples: 20 species confined to New Guinea's mountains, but which descend to sea level at higher latitudes in Australia (joined to New Guinea at Pleistocene times of low sea level); 13 sea-level populations of some of the same New Guinea upland species on New Guinea's Fly River bulge; and 11 populations on the Aru Islands (part of Pleistocene New Guinea and Australia). Many New Guinea species contract or expand their elevational ranges, associated with the presence or absence of competing congenerics, which segregate by elevation at sites of sympatry. The flat Karimui Basin at an elevation of 1,110 m illustrates effects of slope, because the basin supports populations of many species otherwise characteristic of the flat lowlands, and lacks populations of many hill forest species characteristic of the sloping terrain found at that elevation elsewhere in New Guinea. We provide three sets of New Guinea examples of the Massenerhebung effect described for mountains elsewhere in the world: shifts of species to higher elevations on large high mountains far from the sea than on small coastal mountains or isolated mountains. Finally, we suggest that very steep high ridges boxing in a watershed on the Foja Mts. constitute dispersal barriers that have prevented 33 species expected at that watershed's elevation from arriving or establishing themselves.