Abundant-core thinking clarifies exceptions to the abundant-center distribution pattern

Understanding variation in abundance within species' ranges is fundamental for ecological and evolutionary theory and applied conservation science. The abundant-center model provides a general hypothesis based on basic ecological principles and macroscale biogeographic patterns: abundance should peak near the center of a species' range, where environmental conditions are most favorable, and decline towards the periphery. Despite longstanding influence in ecological thinking, consistent support for the ubiquity of abundant-center distributions remains elusive, and recent assessments have questioned the value of this paradigm altogether. We suggest that revisiting the simplifying assumptions that underly the model provides a productive path forward by clarifying predictions and revealing expectations for alternative distribution patterns. Towards this end, we use standardized abundance surveys of North American birds to reassess the prevalence of abundant-center distributions in geographic and climate space, test whether deviations are associated with predictable violations of assumptions, and provide more robust expectations. After accounting for common methodological pitfalls, we find that geographic centrality is generally indicative of centrality in climate space (confirming a key model assumption) and that abundant-center distributions occurred in 71% of passerines. To better understand exceptions, we introduce the concept of abundant-core distributions, of which the abundant-center is a special case. We find that 87% of species fit abundant-core expectations, with abundances peaked and generally declining from a core region within the range. Abundance cores tended to deviate from geographic center where topographic features complicate correspondence between geography and environmental conditions (e.g. the climatically heterogenous west). Such deviations were often associated with truncated climatic availability, with core regions offset towards the continental edge or climate extremes. Overall, our analyses suggest that abundant-center thinking provides a useful generalization for understanding spatial variation in abundance for many species. However, as with any model, its assumptions must be assessed within the context of given applications.