No evidence for a positive correlation between abundance and range size in birds along a New Guinean elevational gradient
A general pattern in biogeography is that species with high local abundances tend to have large geographic ranges, while species with low local abundances have small ranges. However, many tropical biotas do not show positive abundance–range–size correlations, potentially because eco-climatic stability in the tropics promotes specialisation or because density compensation permits some higher-elevation species to be both abundant and small-ranged. I explored these ideas by studying the abundance–range-size correlation for 37 species of small-bodied understorey New Guinean birds that live along a reef-to-ridgetop elevational gradient. Abundance (capture rates) is not related to range size (elevational breadth) in this dataset. In fact, when conducting phylogenetic regressions, abundance is significantly negatively related to range size. Because species’ abundances do not systematically vary as a function of elevational zone, this pattern is not due to density compensation. Instead, elevational specialisation appears to explain the abundance–range-size correlation, interspecific competition being an important driver of elevational specialisation. If specialised taxa are sometimes able to achieve high local abundances compared to broader-ranged taxa, specialisation may break any consistent association between abundance and range size. Further studies are necessary to test the generality of this explanation.