Data from: Patterns of species relatedness created by competitive exclusion depend on species niche differences: evidence from Iberian Atlantic grasslands

It is commonly assumed that closely related species share more similar niches than do distantly related species, thus limiting their ability to coexist and leading to patterns of phylogenetic over-dispersion. On the contrary, recent theoretical developments argue that competitive exclusion may lead to patterns of either over-dispersion, clustering or randomness, depending on the relative importance of niche differences and interspecific competitive ability differences. In this study, we utilized semi-natural grassland communities to test the hypothesis that the pattern of species relatedness generated by competitive exclusion depends on species niche differences. Instead of inferring processes from observed patterns, we experimentally manipulated grassland plots to test the effects of competitive exclusion. We compared grazed plots (in which grazing functioned as an equalizing mechanism and suppressed aboveground competition) with neighbouring plots experimentally excluded from grazing over the course of nine years (in which aboveground competition was not prevented). Weak niche differences between species allowed competitive exclusion to generate phylogenetic clustering, because phylogenetically structured plant canopy height and capacity for lateral spread conferred superior competitive ability to grasses that outcompeted species in all dicot branches. By contrast, moderate niche differences allowed competitive exclusion to result in a random pattern of phylogenetic species assembly, because species niche differences partially counterbalanced competitive exclusion by superior competitors in the grass family. We conclude that patterns of species relatedness created by competitive exclusion in Iberian Atlantic grasslands depend on species niche differences. Competitive exclusion may cause differential patterns of phylogenetic assembly.学界普遍认为，亲缘关系较近的物种比远缘物种拥有更为相似的生态位，这会限制它们的共存能力，并最终形成系统发育过度分散（phylogenetic over-dispersion）的格局。与之相反，近年的理论研究指出，竞争排除（competitive exclusion）可能导致系统发育过度分散、聚集或随机三种格局，具体结果取决于生态位差异与种间竞争能力差异的相对重要性。本研究依托半天然草地群落，检验了"竞争排除所产生的物种亲缘关系格局取决于物种生态位差异"这一假说。本研究并未通过观测格局反推过程，而是通过实验操控草地样地来检验竞争排除的效应：将放牧样地（放牧作为均等化机制，抑制了地上竞争）与经过9年实验禁牧的邻近样地（未阻止地上竞争）进行对比。当物种种间生态位差异较弱时，竞争排除会导致系统发育聚集（phylogenetic clustering）格局：具有系统发育保守性的植物冠层高度与侧向扩张能力，使得禾本科物种拥有更强的竞争能力，从而排挤所有双子叶分支类群。与之相对，当物种种间生态位差异适中时，竞争排除会形成系统发育物种组装（phylogenetic species assembly）的随机格局：此时物种的生态位差异能够部分抵消禾本科优势竞争者带来的竞争排除效应。本研究得出结论：伊比利亚大西洋沿岸草地中，竞争排除所塑造的物种亲缘关系格局取决于物种生态位差异。竞争排除可引发不同类型的系统发育组装格局。