|Aim Species richness patterns are generally thought to be determined by abi-otic variables at broad spatial scales, with biotic factors being only important at ?ne spatial scales. However, many organism groups depend intimately on other organisms, raising questions about this generalization. As an example, woodpeckers (Picidae) are closely associated with trees and woody habitats because of multiple morphological and ecological specializations. In this study, we test whether this strong biotic association causes woodpecker diversity to be closely linked to tree availability at a global scale.
Methods We used spatial and non-spatial regressions to test for relationships between broad-scale woodpecker species richness and predictor variables describing current and deep-time availability of trees, current climate, Quater-nary climate change, human impact, topographical heterogeneity and biogeo-graphical region. We further used structural equation models to test for direct and indirect effects ofpredictor variables.
Results There was a strong positive relationship between woodpecker species richness and current tree cover and annual precipitation, respectively. Precipi-tation also showed a strong indirect effect on woodpecker richness via the effects on tree availability. Deep-time tree availability, Quaternary climate change, human in?uence and other abiotic factors showed weaker direct effects. Human in?uence had a negative effect on tree availability, and hence a negative indirect effect on woodpecker species richness.
Main conclusions Global species richness of woodpeckers is primarily shaped by current tree cover and precipitation, re?ecting a strong biotic association between woodpeckers and trees. Human in?uence can have a negative effect on woodpecker diversity when humans reduce tree availability. Hence, woodpeckers exemplify how broad-scale diversity patterns are predominantly shaped by a bio-tic factor, and how climate and human in?uence can have indirect effects on ani-mal biodiversity via the effects on tree availability and forest cover.|