Abundance variations within feeding guilds reveal ecological mechanisms behind avian species richness pattern along the elevational gradient of Mount Cameroon

Two distinct diversity patterns are observed along tropical elevations: (a) decreasing number of species toward high elevations and (b) a hump-shaped pattern with the peak at mid-elevations. As diversity is likely supported by ecological capacity of the environment, decomposition of the overall richness into ecological facets and considering number of individuals within them is crucial for the proper understanding of richness patterns. We examined abundances of different avian guilds along the forested part of the elevational gradient on Mt. Cameroon. We (a) compared richness and abundance elevational patterns, (b) assessed the effective contribution of multiple guilds to richness and abundance patterns, and (c) assessed to what extent observed abundances of guilds differed from those expected by chance. We sampled birds in 2011–2015 during the dry season at seven elevations (30 m, 350 m, 650 m, 1100 m, 1500 m, 1850 m, 2200 m a.s.l.). For each assemblage, we estimated proportions of species and individuals that use particular diets, foraging modes, and feeding strata. We found that a rather decreasing pattern of species richness turns into a hump-shaped one if we look at the total abundances, implying different mechanisms behind these patterns. The number of species and individuals thus do not seem to be directly related, contrary to “the more-individuals hypothesis.” Abundances of foliage gleaners at mid-elevations, nectarivores at high elevations, and frugivores at low elevations deviated from random expectations. Our results imply that parts of ecological space are filled separately by bird species and individuals along elevation of Mt. Cameroon.