Population abundance data and species range maps

Aim – The abundant-center hypothesis (ACH) predicts a negative relationship between species abundance and the distance to geographic range center. Since its formulation, empirical tests of the ACH have involved different settings (e.g. the distance to the ecological niche or to the geographic range center), but studies found contrasting support for this hypothesis. Here, we evaluate whether these discrepancies might stem from differences regarding the context in which the ACH is tested (geographical or environmental), how distances are measured, how species envelopes are delineated, how the relationship is evaluated and which data are used.

Location – Americas.

Time Period – 1800-2017.

Major taxa studied – mammal, bird, fish and tree seedlings.

Methods – Using published abundance data for 801 species, together with species range maps, we tested the ACH using three distance metrics in both environmental and geographical spaces with range and niche envelopes delineated using two different algorithms, totaling 12 different settings. We then evaluated the distance-abundance relationship using correlation coefficients (traditional approach) and mixed-effect models to reduce the effect of sampling noise on parameter estimates.

Results – Similar to previous studies, correlation coefficients indicated an absence of effect of distance on abundance for all taxonomic groups and settings. In contrast, mixed-effect models highlighted relationships of various strengths and shapes, with a tendency for more theoretically-supported settings to provide stronger support for the ACH. The relationships were however not consistent across taxonomic groups and settings, and were sometimes even opposite to ACH expectations.

Main conclusions – We found mixed and inconclusive results regarding the ACH. These results corroborate recent findings, and suggest either that our ability to predict abundances from the location of populations within geographical or environmental spaces is low, or that the data used here have a poor signal-to-noise-ratio. The latter calls for further testing on other datasets using the same range of settings and methodological framework.

Methods
All the data used in this paper have either already been published (see Dallas et al., 2017 and Osorio-Olvera et al., 2020) or are freely available (range maps can be dowloaded here https://www.fs.fed.us/nrs/atlas/littlefia/ for trees and here https://www.iucnredlist.org/resources/spatial-data-download for mammals, fish and birds; climatic data can be downloaded here https://www.worldclim.org/data/worldclim21.html).

一、研究目的
丰度中心假说（abundant-center hypothesis, ACH）预测物种丰度与该物种地理分布范围中心的距离呈负相关关系。自该假说提出以来，针对其开展的实证检验涉及多种不同情境（例如与生态位中心或地理分布范围中心的距离），但现有研究对该假说的支持结论并不一致。本研究旨在探究上述分歧可能源于以下方面的差异：丰度中心假说的检验情境（地理空间或环境空间）、距离测算方式、物种分布包络线的划定方法、相关关系的评估手段，以及所使用的数据类型。

二、研究区域
美洲。

三、研究时段
1800年—2017年。

四、研究类群
哺乳类、鸟类、鱼类以及林木幼苗。

五、研究方法
本研究整合了801个物种的已发表丰度数据，结合物种分布范围地图，分别在地理空间与环境空间中采用3种距离测度指标，并通过2种不同算法划定物种分布包络线与生态位包络线，共计设置12种不同的检验情境。随后，分别采用相关系数法（传统分析方法）与混合效应模型对距离-丰度的相关关系进行评估，以降低抽样噪声对参数估计结果的影响。

六、研究结果
与既往研究一致，相关系数分析显示，在所有类群与检验情境下，距离对物种丰度均无显著影响。与之相对，混合效应模型则揭示出强度与形式各异的相关关系，且理论上更合理的检验情境往往能为丰度中心假说提供更强的支持证据。但上述相关关系在不同类群与检验情境间并不一致，甚至有时与丰度中心假说的预期结果相悖。

七、主要结论
本研究关于丰度中心假说的检验结果混杂且未得出明确结论。这一结果与近期相关研究结论一致，提示存在两种可能性：其一，基于种群在地理空间或环境空间中的位置预测物种丰度的能力仍较为有限；其二，本研究所使用的数据信噪比偏低。针对后一种可能性，后续研究可采用相同的检验情境与方法学框架，基于其他数据集开展进一步的验证工作。

方法补充说明
本研究使用的全部数据均已公开发表（详见Dallas等，2017与Osorio-Olvera等，2020）或可免费获取：林木的分布范围地图可从https://www.fs.fed.us/nrs/atlas/littlefia/下载，哺乳类、鱼类与鸟类的分布范围地图可从https://www.iucnredlist.org/resources/spatial-data-download下载；气候数据可从https://www.worldclim.org/data/worldclim21.html下载。