Data from: Geographic ranges of genera and their constituent species: structure, evolutionary dynamics, and extinction resistance

We explore the relationships among the geographic ranges of genera, the ranges and positions of their constituent species, and the number of species they contain, considering variation among coeval genera and changes within genera over time. Measuring range size as the maximal distance, or extent, between occurrences within a taxon, we find that the range of the most widespread species is a good predictor of the range of the genus, and that the number of species is a better predictor still. This analysis is complicated by a forced correlation: the range of a genus must be at least as large as that of each of its constituent species. We therefore focus on a second measure of range, the mean squared distance, or dispersion, of occurrences from the geographic centroid, which, by analogy to the analysis of variance, allows the total dispersion of a genus to be compared to the mean within-species dispersion and the dispersion among species centroids. We find that among-species dispersion is the principal determinant of genus dispersion. Within-species dispersion also plays a major role. The role of species richness is relatively small. Our results are not artifacts of temporal variation in the geographic breadth of sampled data. The relationship between changes in genus dispersion and changes in within- and among-species dispersion shows a symmetry, being similar in cases when the genus range is expanding and when it is contracting. We also show that genera with greater dispersion have greater extinction resistance, but that within- and among-species dispersion are not demonstrable predictors of survival once the dispersion of the genus is accounted for. Thus it is the range of the genus, rather than how it is attained, that is most relevant to its fate. Species richness is also a clear predictor of survival, beyond its effects on geographic range.

本研究探讨了属的地理分布范围、其组成物种的分布范围与分布位置，以及属内含物种数量之间的关联，同时考量了同时代属间的差异以及属随时间推移发生的内部变化。以分类单元内各分布记录间的最大距离（即分布幅度）作为分布范围的衡量指标，我们发现分布最广的物种的分布范围可较好预测该属的分布范围，而属内含物种数量的预测效果更佳。但此类分析会受到强制相关性的干扰：属的分布范围必然至少不小于其任一组成物种的分布范围。为此我们转而采用第二种分布范围衡量指标：各分布记录相对于地理质心的均方距离（即离散度）。该指标可类比方差分析，将属的总离散度与物种内部平均离散度及各物种质心间的离散度进行比较。分析结果显示，物种间离散度是属离散度的主要决定因素，物种内部离散度同样发挥重要作用，而物种丰富度的影响相对较小。本研究结果并非采样数据地理范围随时间变化所产生的人为假象。属离散度变化与物种内部、物种间离散度变化之间的关系呈现对称性：无论属的分布范围处于扩张还是收缩阶段，该关联模式均保持一致。此外我们还发现，离散度更高的属具有更强的抗灭绝能力，但在纳入属的离散度这一变量后，物种内部及物种间离散度便不再能显著预测类群的存活概率。由此可见，与属的存续命运最为相关的是其自身的分布范围，而非该范围的形成方式。此外，即便排除物种丰富度对地理分布范围的影响，其本身仍是类群存活的明确预测因子。