The abundance‐centre hypothesis re‐examined: body mass, blood glucose, and elevational distributions in Afrotropical forest birds

A long‐standing theory, the abundance‐centre hypothesis (ACH), predicts that species peak in abundance at the centre of their geographic range, where environmental conditions are optimal, and decline towards range limits as conditions deteriorate. Yet, tests of the ACH in birds along elevational gradients have yielded mixed results. Moreover, resource availability, rather than abiotic factors per se, may better explain species abundance patterns. Therefore, life‐history traits associated with nutritional status could be key to predicting abundance patterns. Along with the ACH, we tested two physiological traits as potential predictors of avian abundance along an Afrotropical elevational gradient: blood glucose concentrations (the abundance–glucose relationship; AGR) and body mass (the abundance–body mass relationship; ABR). We collected abundance, glucose, and body mass data from 51, 15, and 29 free‐living species, respectively, within a primary forest on Mt Cameroon (West‐Central Africa). Model fits for each species revealed that many species show a monotonic abundance pattern. Logistic models further indicated that high‐elevation birds were more likely to increase in abundance with elevation, peaking near the treeline. In contrast, lowland birds tended to decline with elevation, peaking near the lower forest edge. Bayesian phylogenetic multilevel models indicated that neither glucose nor body mass was a strong predictor of abundance. Taken together, our findings provide little support for the ACH and suggest that most lowland and highland species occupy truncated realized niches near forest elevational edges. Although our results do not link avian physiology to population density, we encourage future ecologists and ornithologists to collect larger sample sizes to effectively test the AGR and ABR hypotheses and, thus, elucidate the mechanisms behind variations in species abundances.