Emergent neutrality (EN) suggests that species must be sufficiently similar or sufficiently different in their niches to avoid interspecific competition. Such a scenario results in a transient pattern with clumps and gaps of species abundance along the niche axis (e.g. represented by body size). From this perspective, clumps are groups of coexisting species with negligible fitness differences and stochastic abundance fluctuations. Plankton is an excellent model system for developing and testing ecological theories, especially those related to size structure and species coexistence. We tested EN predictions using the phytoplankton community along the course of a tropical river considering 1) body size structure, 2) functional clustering of species in terms of morphology-based functional groups (MBFG) and 3) the functional similarity among species concerning their functional traits. Two main clumps in the body size axis (clump I and II) were conspicuous through time and were detected in different stretches of the river. Clump I comprised medium-sized species from the MBFGs IV, V and VI while clump II included large-bodied species from the MBFGs V and VI. Pairwise differences in species biovolume correlated with species functional similarity when the whole species pool was considered, but not among species within the same clump. Although clumps comprised multiple MBFGs, the dominant species within the clump belonged always to the same MBFG. Also, within-clump species biovolume increased with functional distinctiveness considering both seasons and stretches, except the lower course. These results suggest that species within clumps behave in a quasi-neutral state, but even minor shifts in trait composition may affect species biovolume. Our findings point that EN belongs to the plausible mechanisms explaining community assembly in river ecosystems.

中文翻译：

---

浮游植物中的块状共存：功能相似性在群落组装中的作用

紧急中立性 (EN) 表明物种必须在其生态位上足够相似或足够不同，以避免种间竞争。这种情况会导致沿生态位轴（例如，由体型表示）的物种丰度存在丛生和缺口的瞬态模式。从这个角度来看，团块是一组共存的物种，其适应性差异和随机丰度波动可以忽略不计。浮游生物是开发和测试生态学理论的优秀模型系统，尤其是与大小结构和物种共存相关的理论。我们使用热带河流沿线的浮游植物群落测试了 EN 预测，考虑 1) 体型结构，2) 物种在基于形态的功能组 (MBFG) 方面的功能聚类和 3) 物种之间关于其功能特征的功能相似性。身体大小轴上的两个主要团块（团块 I 和 II）随着时间的推移很显眼，并在河流的不同河段中检测到。丛 I 包括来自 MBFG IV、V 和 VI 的中型物种，而丛 II 包括来自 MBFG V 和 VI 的大型物种。当考虑整个物种库时，物种生物量的成对差异与物种功能相似性相关，但在同一丛中的物种之间则不然。尽管团块包含多个 MBFG，但团块内的优势物种始终属于同一个 MBFG。此外，考虑到季节和延伸，丛内物种生物量随着功能的独特性而增加，除了较低的课程。这些结果表明丛中的物种在准中性状态下表现，但即使性状组成的微小变化也可能影响物种生物量。我们的研究结果表明，EN 属于解释河流生态系统中群落组装的合理机制。