Mass effect, allowing species to persist in unfavourable habitats, and dispersal limitation, preventing species from reaching favourable habitats, are the two major dispersal processes. While dispersal limitation can be detected by experimental or modelling techniques, mass effect is more challenging to evaluate, which hampers our ability to disentangle the influence of the environment versus dispersal on species distribution. This is undesirable for biomonitoring programs built on known species-environment relationships. We developed an approach for detection of species influenced by mass effect. We tested it on stream diatoms, a widely used taxonomic group for stream biomonitoring, from four French watersheds. This approach combined (a) an appropriate spatial framework, the asymmetric eigenvector map (AEM), used in species distribution modelling to measure the relative influence of dispersal versus niche processes, (b) an analysis of negative co-occurrence patterns to separate mass effect from dispersal limitation and (c) a measurement of niche breadths to distinguish between non-spatially structured generalists and species influenced by mass effect. We propose that species characterized by low negative co-occurrence values, a high correlation to spatial factors and average to low niche breadths are sensitive to mass effect. Synthesis and applications. We suggest that the sensitivity of species towards mass effect should represent a new ecological trait to be considered for fundamental and applied issues concerning ecology and water quality assessment. Almost all of the species identified here as influenced by mass effect are contributing to the calculation of different diatom-based indices (e.g. Biological Diatom Index or Specific Pollution-sensitivity Index) and should be treated with caution when assigning ecological status classes to water bodies.

中文翻译：

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一种用于检测受质量效应影响的物种的元社区方法

质量效应使物种能够在不利的栖息地中生存，而扩散限制则阻止物种到达有利的栖息地，这是两个主要的扩散过程。虽然可以通过实验或建模技术检测到扩散限制，但评估质量效应更具挑战性，这阻碍了我们区分环境与扩散对物种分布的影响的能力。这对于建立在已知物种-环境关系上的生物监测程序来说是不可取的。我们开发了一种检测受质量效应影响的物种的方法。我们在来自四个法国流域的河流硅藻上对其进行了测试，这是一种广泛用于河流生物监测的分类群。这种方法结合了 (a) 一个合适的空间框架，非对称特征向量图 (AEM)，用于物种分布建模以测量扩散与生态位过程的相对影响，(b) 分析负共现模式以将质量效应与扩散限制分开，以及 (c) 测量生态位宽度以区分非空间结构受质量效应影响的通才和物种。我们提出以低负共现值、与空间因素高度相关以及平均到低生态位宽度为特征的物种对质量效应敏感。合成与应用。我们建议物种对质量效应的敏感性应该代表一种新的生态特性，在生态和水质评估的基础和应用问题中应予以考虑。