Limited support for the "abundant centre" hypothesis in birds along a tropical elevational gradient: implications for the fate of lowland tropical species in a warmer future

Aim: Since Darwin, ecologists have assumed that species tend to be abundant in the centre of their geographic distributions and rare at their range edges. We test two predictions of this abundant centre hypothesis: (a) that species are common in the centre of their range and rare at the edges and (b) that empirical distributions along environmental gradients are well described by symmetric bell-shaped curves. Location: A reef-to-ridgetop elevational gradient in the ConseYopno–Urawa–Som rvation Area (YUS CA) on the Huon Peninsula in Papua New Guinea. Taxon: Birds. Methods: We use mist net capture rates to quantify species' abundances as a function of range position, then fit a series of models with different shapes, including symmetric curves, to species' empirical abundance distributions. Results: Species peak abundances were located near species' low elevation range edges, high elevation range edges and everywhere in between. Real abundance distributions were fit well by symmetric curves in fewer than half of the species we analysed. Most lowland species have high abundances at their low elevation margin near sea level. Main conclusions: We find only limited support for the abundant centre hypothesis. Most lowland species appear to currently occupy a "truncated" realized niche, suggesting that they will be able to persist at low elevations even as temperatures increase and that lowland biotic attrition is unlikely. In addition to analyses, we present (a) a full species list for the 256 species recorded thus far within the YUS CA, with elevational limits for all forest species and (b) the raw data from our extensive mist net surveys. This information provides valuable baseline data that can be used by future biodiversity scientists to evaluate the changes in the YUS avifauna and will be generally useful to biodiversity scientists studying tropical elevational gradients.