Biomass, abundances, and abundance and range size relationship of birds along a rainforest elevational gradient in Papua New Guinea

The usual positive interspecific relationship between range size and abundance of local populations was shown to have notable exceptions in Afrotropical montane areas, where range restricted bird species are unusually abundant. We tested how the local abundances of passerines and nonpasserine of Mt Wilhelm elevational gradient in Papua New Guinea relate to their geographic range size. We collected the data on bird assemblages at eight elevations (from 200 to 3,700 m, 500 m elevational increment) using a standardised point count at 16 locations per elevation. We partitioned the birds into feeding guilds and we obtained data on range sizes from Bird Life International data zone. We observed positive trends of the abundance and range size relationship in lowland changing to negative trends towards higher elevations. The total assemblage abundances showed hump shaped pattern, with passerine birds, namely then passerine insectivores, driving the observed pattern. In contrast to abundances, mean biomass of the bird assemblages decreased with increasing elevation (i.e. showed opposite pattern than mean abundances). Our data suggest that montane bird species have abilities to maintain dense populations which compensate for lower area available near mountain tops.