Data from: Abundance inequality in freshwater communities has an ecological origin

The hollow-shaped species abundance distribution (SAD) and its allied rank abundance distribution (RAD)—showing that abundance is unevenly distributed among species—are some of the most studied patterns in ecology. To explain the nature of abundance inequality, I developed a novel framework identifying environmental favorability, which controls the balance between reproduction and immigration, as the ultimate source and species stress tolerance as a proximate factor. Thus, under harsh conditions, only a few tolerant species can reproduce, while some sensitive species can be present in low numbers due to chance immigration. This would lead to high abundance inequality between the two groups of species. Under benign conditions, both groups can reproduce and give rise to higher abundance equality. To test these ideas, I examined the variability in the parameters of a Poisson lognormal fit of the SAD and a square root fit of the RAD in diatom and fish communities across US streams. Indeed, as environmental favorability increased, more sensitive forms were able to establish large populations, diminishing the abundance disparity between locally common and rare species. Finally, it was demonstrated that in diatoms, the RAD belonged to the same family of relationships as those of population density with body size and regional distribution.中空型物种多度分布（SAD）及其关联的等级多度分布（RAD）——反映了物种间多度分布不均的现象——是生态学中研究最为广泛的模式之一。为阐释多度不平等的本质，本文提出了一套全新的理论框架：将调控繁殖与迁入平衡的环境适宜度作为多度不平等的终极驱动因素，将物种胁迫耐受性作为其近因因素。据此，在严苛环境下，仅少数耐受物种能够完成繁殖，而部分敏感物种仅能通过偶然迁入以极低丰度存在，这将导致两类物种间的多度差异显著增大；在适宜环境中，两类物种均可完成繁殖，进而提升了多度分布的均一性。为验证上述假说，本文针对美国溪流中的硅藻与鱼类群落，分别采用泊松对数正态拟合物种多度分布模型与平方根拟合等级多度分布模型，对其参数变异性展开了分析。结果显示，随着环境适宜度提升，更多敏感物种得以建立大规模种群，削弱了本地常见种与稀有种间的多度差距。研究还证实，在硅藻群落中，等级多度分布所呈现的关系类型，与种群密度同体型、区域分布的关系属于同一范畴。