Data from: 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.

理解物种分布范围内的丰度变化，对于生态学、进化理论及应用保护科学而言至关重要。丰度中心模型基于基础生态学原理与宏观生物地理格局提出了一个通用假设：物种丰度应在其分布范围中心附近达到峰值——此处环境条件最为适宜——并向边缘逐渐降低。尽管该模型在生态学思想中具有长期影响，但对于丰度中心分布普遍性的一致支持仍难以获得；近期评估甚至对这一范式的价值提出了全面质疑。我们认为，重新审视模型背后的简化假设，通过明确预测并揭示对替代分布格局的预期，可为该领域提供富有成效的前进方向。为此，我们利用北美鸟类的标准化丰度调查数据，重新评估了地理空间与气候空间中丰度中心分布的普遍性，检验了偏差是否与假设的可预测性违背相关，并提出了更稳健的预期。在排除常见方法学缺陷后，我们发现地理中心性通常可指示气候空间中的中心性（验证了模型的一个关键假设），且71%的雀形目鸟类呈现丰度中心分布。为更好地理解例外情况，我们引入了丰度核心分布的概念——丰度中心分布是其特例。我们发现87%的物种符合丰度核心预期：丰度在分布范围内的核心区域达到峰值，并从该区域整体呈下降趋势。在地形特征导致地理与环境条件对应关系复杂化的区域（如气候异质性显著的西部），丰度核心往往偏离地理中心。此类偏差常与气候可利用性受限相关，核心区域向大陆边缘或气候极端区偏移。总体而言，我们的分析表明，丰度中心思想为理解多数物种丰度的空间变异提供了有用的概括。然而，与所有模型一样，其假设必须结合具体应用场景进行评估。