Data from: Species richness, but not phylogenetic diversity, influences community biomass production and temporal stability in a re-examination of 16 grassland biodiversity studies

1.Hundreds of experiments have now manipulated species richness of various groups of organisms and examined how this aspect of biological diversity influences ecosystem functioning. Ecologists have recently expanded this field to look at whether phylogenetic diversity among species, often quantified as the sum of branch lengths on a molecular phylogeny leading to all species in a community, also predicts ecological function. Some have hypothesized that phylogenetic divergence should be a superior predictor of ecological function than species richness because evolutionary relatedness represents the degree of ecological and functional differentiation among species. But studies to date have provided mixed support for this hypothesis. 2.Here, we re-analyze data from 16 experiments that have manipulated plant species richness in grassland ecosystems and examined the impact on aboveground biomass production over multiple time points. Using a new molecular phylogeny of the plant species used in these experiments, we quantified how the phylogenetic diversity of plants impacts average community biomass production as well as the stability of community biomass production through time. 3.Using four complementary analyses we show that, after statistically controlling for variation in species richness, phylogenetic diversity (the sum of branches in a molecular phylogenetic tree connecting all species in a community) is neither related to mean community biomass nor to the temporal stability of biomass. These results run counter to past claims. However, after controlling for species richness, phylogenetic diversity was positively related to variation in community biomass over time due to an increase in the variances of individual species, but this relationship was not strong enough to influence community stability. 4.In contrast to the non-significant relationships between phylogenetic diversity, biomass, and stability, our analyses show that species richness per se tends to increase the mean biomass production of plant communities, after controlling for phylogenetic diversity. The relationship between species richness and temporal variation in community biomass was either positive, non-significant or negative depending on which analysis was used. However, the increases in community biomass with species richness, independently of phylogenetic diversity, always led to increased stability. These results suggest that phylogenetic diversity is no better as a predictor of ecosystem functioning than species richness. 5.Synthesis. Our study on grasslands offers a cautionary tale when trying to relate phylogenetic diversity to ecosystem functioning suggesting that there may be ecologically important trait and functional variation among species that is not explained by phylogenetic relatedness. Our results fail to support the hypothesis that the conservation of evolutionarily distinct species would be more effective than the conservation of species richness as a way to maintain productive and stable communities under changing environmental conditions.1. 目前已有数百项实验对不同类群生物的物种丰富度（species richness）进行操控，并探究这一生物多样性维度如何影响生态系统功能。近期，生态学家将该研究领域拓展至物种间的系统发育多样性（phylogenetic diversity）——其通常被量化为群落内所有物种所在的分子系统发育树（molecular phylogeny）上的分支长度之和——是否同样能预测生态功能。部分学者提出假说：相较于物种丰富度，系统发育分化程度应是更优的生态功能预测因子，因为进化亲缘关系能够反映物种间生态与功能分化的程度。但截至目前的相关研究对该假说的支持结果并不一致。 2. 本研究重新分析了16项草地生态系统中植物物种丰富度操控实验的数据，这些实验探究了多个时间节点下地上生物量生产的变化情况。本研究利用实验中所用植物物种的全新分子系统发育数据，量化了植物系统发育多样性对群落平均生物量生产以及群落生物量生产随时间变化的稳定性的影响。 3. 本研究通过四项互补性分析表明：在统计控制物种丰富度变异的前提下，系统发育多样性（即连接群落内所有物种的分子系统发育树的分支长度之和）与群落平均生物量、生物量的时间稳定性均无关联。这一结果与此前的相关研究结论相悖。不过，在控制物种丰富度后，由于单个物种的方差增大，系统发育多样性与群落生物量随时间的变异呈正相关，但该关联强度不足以影响群落稳定性。 4. 与系统发育多样性、生物量及稳定性之间无显著关联的结果不同，本研究分析显示：在控制系统发育多样性的前提下，物种丰富度本身往往会提升植物群落的平均生物量生产。物种丰富度与群落生物量的时间变异之间的关联因分析方法不同而有所差异，可呈正相关、无显著相关或负相关。但无论采用何种分析方法，不依赖于系统发育多样性、随物种丰富度提升而增加的群落生物量，始终会带来稳定性的提升。本研究结果表明，系统发育多样性在预测生态系统功能方面并不优于物种丰富度。 5. 综合讨论。本项草地生态研究为探究系统发育多样性与生态系统功能之间的关联提供了警示案例，表明物种间可能存在无法通过进化亲缘关系解释的、具有生态重要性的性状与功能变异。此前有假说提出：在环境变化的背景下，保护进化独特物种比保护物种丰富度更能维持群落的高生产力与稳定性，而本研究结果并不支持这一假说。