Recent global changes have decoupled species richness from specialization patterns in North American birds

Aim: Theory suggests that increasing productivity and climate stability toward the tropics can explain the latitudinal richness gradient by favouring specialization. A positive relationship between species richness and specialization should thus emerge as a fundamental biogeographic pattern. However, land use and climate change disproportionally increase the local extirpation risk for specialists, potentially impacting this pattern. Here, we empirically quantify the richness-specialization prediction and test how 50 years of climate and land use change has affected the richness-specialization relationship. Location: USA Time period: 1966-2015 Major taxa studied: Birds Methods: We used the North American breeding bird survey to quantify bird community richness and specialization to habitat and climate. We assess i) temporal change in the slope of the richness-specialization relationship, using a Generalized Mixed Model; ii) temporal change in spatial covariation of richness and specialization as driven by local environmental conditions, using Generalized Additive Models; and iii) land use, climate and topographic drivers of the spatio-temporal changes in the relationship, using a multivariate method. Results: We found evidence for a positive richness-specialization relationship in bird communities. However, the slope of the relationship declined strongly over time. Richness spatially covaried with specialization following a unimodal pattern. The peak of the unimodal pattern shifted toward less specialized communities over time. These temporal changes were associated with precipitation change, decreasing temperature stability and land use. Main conclusions: Recent climate and land use changes induced two antagonist types of community responses. In human-dominated areas, the decoupling of richness and specialization drove a general biotic homogenization trend. In human-preserved areas under increasing climate harshness, specialization increased while richness decreased in a specialization trend. Our results offer new support for specialization as a key driver of macroecological diversity patterns, and show that global changes are erasing this fundamental macroecological pattern.