Aim Increased specialization has been hypothesized to facilitate local coexistence and thus high species richness, but empirical evaluations of the richness–specialization relationships have been relatively scant. Here, we provide a first assessment of this relationship for terrestrial bird assemblages at global extent and from fine to coarse grains.
Methods We use two indices of specialization that describe species-level resource use: diet and habitat specialization. The relationship between richness and mean assemblage-level specialization was independently assessed at realm, biome-realm, 12,100 km2 equal-area grid cells and fine-grained scales. To identify assemblages that are diverse relative to environmental conditions we: (1) applied quantile regressions, (2) statistically accounted for other environmental variables which may constrain richness, and (3) parsed the data according to the residuals of a model relating species richness to the environmental variables.
Results Assemblage species richness increases with both measures of specialization at all scales. Statistically, richness appears constrained by levels of specialization, with the highest richness values only found in specialized assemblages. Richness is positively associated with specialization even after accounting for gradients in resource availability. Net primary productivity and assemblage specialization have complementary statistical effects on assemblage species richness. Contrary to expectations based on niche partitioning of local resources, the relationship between specialization and richness is steep even at coarse scales.
Main conclusions The results demonstrate that for an entire clade, totalling > 9000 species, specialization and species richness are related, at least for diverse assemblages. The strong patterns observed across scales suggest that this relationship does not solely originate from (1) limits on coexistence in present-day assemblages, or (2) increased specialization in richer assemblages imposed by species’ abilities to partition ecological space. Instead, regional-scale influences on the species pool may determine much of the observed relationship between richness and specialization. Although causal attribution is not straightforward, these findings support the idea that, for the scale of our analysis, specialization may be related to the past origination of high-diversity assemblages, rather than their contemporary assembly.