DOI: 10.5061/dryad.dz08kps1b
title: Data from: Abundant-core thinking clarifies exceptions to the abundant-center distribution pattern

description: 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.
description: AbundantCenter R Code.txt: R code used for
 processing data and performing analyses.
 SppList.csv: List of species in Breeding Bird
 Survey data. ‘SppID’ is the identification number
 assigned for our analysis; we combined infraspecific taxa (e.g. great blue
 heron and the white morph known as great white heron) and did not include
 taxa that could not be identified to species (these are given ‘NA’ for
 SppID). ‘Aou’ are the original BBS species
 indentification numbers. 'Common.Name’ provide the
 English common name for species without infraspecific taxa
 names. ‘Common.Name.1’ includes infraspecific taxa
 names. ‘scinamemap’ is the file name for the BirdLife
 range map shapefile. SppRelationships.zip:
 Contains image files (.png) for each species that show geographic
 and climate abundance maps, and abundance-range position
 relationships In maps, cell aggregated abundances are
 depicted from low values in blue to high values in orange; the geographic
 or climatic area covered by North America is depicted in black; cells with
 Breeding Bird Survey data but where the species was not observed are in
 grey. The estimated center of the range is depicted with a green ‘X’ and
 the estimated core of abundance with a pink circle; the cells used for
 estimating the core (10% highest abundances, see methods) are indicated
 with red points. Spearman’s rank correlation coefficient was used to
 assess support for abundant-center and abundant-core predictions.
 Significant positive relationship for distance from edge (ρGeoEdge and
 ρClimEdge) and negative relationships for distance from center (ρGeoCent
 and ρClimCent) are consistent with abundant-center distribution patterns;
 significant negative relationships for distance from the abundance core
 (ρGeoCore and ρClimCore) are consistent with abundant-core distribution
 patterns.