Abundance leads to more species, particularly in complex habitats: a test of the increased population size hypotheses in bird communities

Abstract Aim To examine the relationship between abundance and species richness in a bird community in order to (a) test the effects of sampling and three increased population size hypotheses and (b) assess the effects of energetic constraints (climate and habitat) on the relationship. Location The Cantabrian Mountains ( c . 16,000 km 2 ), Spain, within the temperate broadleaf and mixed forests biogeographical region. Methods Species richness and individual abundances from a survey of 1106 10‐min 100‐m radius bird point counts spanning 5 years were modelled with generalized additive models, controlling for habitat and climate. Individual‐based rarefaction curves were built to facilitate the comparison among habitats. Results Species richness depended more on the number of individuals than on habitat or climate and increased with abundance along a nonlinear, strictly monotonic curve. The nature of this relationship varied with habitat type (contradicting sampling effect) and was not stronger for rarer species (contradicting the ‘more‐individuals hypothesis’). Community specialization and nestedness were related to climate constraints (supporting the niche‐specialization hypotheses). Main conclusions Our analyses strongly suggest that niche packing, particularly through niche breadth, determines the way that energy inputs translate into species richness, arguably through the abundance of individuals.