生物多样性与生态系统功能:进展与争论

生物多样性与生态系统功能的关系已成为当前人类社会面临的一个重大科学问题,生物多样性的空前丧失,促使人们开展了大量研究工作来描述物种多样性-生态系统功能关系,并试图揭示多样性与系统功能关系的内在机制,本文将多样性对生态系统功能作用机制的有关假说分为统计学与生物学两大类：前者是从统计学角度来解释观察到的多样性-系统功能模式,包括抽样效应,统计均衡效应等;而后者是基于多样性的生物学效应给出的,包括生态位互补,种间正相互作用,保险效应等,本文较为详细地介绍了该领域内有代表性的实验工作,包括“生态箱”实验,Cedar Creek草地多样性实验,微宇宙实验,欧洲草地实验,以及在这些实验结果解释上的激烈争论。     

植物功能性状、功能多样性与生态系统功能: 进展与展望

植物功能性状与生态系统功能是生态学研究的一个重要领域和热点问题。开展植物功能性状与生态系统功能的研究不仅有助于人类更好地应对全球变化情景下生物多样性丧失的生态学后果,而且能为生态恢复实践提供理论基础。近二十年来,该领域的研究迅速发展,并取得了一系列的重要研究成果,增强了人们对植物功能性状-生态系统功能关系的认识和理解。本文首先明确了植物功能性状的概念, 评述了近年来植物功能性状-生态系统功能关系领域的重要研究结果, 尤其是植物功能性状多样性-生态系统功能关系研究现状; 提出了未来植物功能性状与生态系统功能关系研究中应加强植物地上和地下性状之间关系及其与生态系统功能、植物功能性状与生态系统多功能性、不同时空尺度上植物功能性状与生态系统功能, 以及全球变化和消费者的影响等方面。     

种、种的多样性及退化生态系统功能的恢复和维持研究

物种多样性是生态系统的重要特征并维持系统的功能支行,生物种和不同种类构成的群落为人类提供诸如营养物质循环、生物生产力、营养功能等形式的重要生态服务,特种多样性与生态系统抵御逆境和干扰的能力紧密相关,多样性的提高会增加系统的稳定性,与单个种和种类的数量相比,功能群和功能多样性对生态系统功能的影响效应要大得多,且易于被用来测度稳定性和预测群落变化,本文提出并探讨了种对生态系统功能作用的几种形式,理解物种多样性与生态系统的功能关系能指导退化生态系统恢复和维持其功能的实践活动,尤其为恢复的初始阶段进行群落的“种类组装”提供生态理论基础。     

基于植物功能群的生态系统服务形成与维持机制研究

生态系统可以从“结构-功能-服务”3个层次来理解,其中服务是人类的主观感受或效用。维持高质量的生态系统服务还需从生态系统的自然属性入手。结构和功能是生态系统服务形成和维持的内在机制,而植物是生态系统结构和功能的核心驱动力。植物功能群具有特定功能的植物组合,其中优势植物功能群控制着生态系统的结构和功能。生态系统服务的概念、分类与植物功能群密切相关,植物功能群是生态系统与生态系统服务间更直接的桥梁。建立生态系统服务与植物功能群间的联系,使相关研究有更明确的指向,在深化理论研究的同时使研究成果更容易落地。生态系统服务的形成、维持机制与植物功能群的内在联系主要体现在4个主要方面：(1)植物功能群的内在适应性特征和外在效应是生态系统服务形成的基础;(2)植物功能群的多功能特性为生态系统服务协同提供了可能;(3)植物功能群间替代和互补效应可以提升生态系统服务质量和稳定性;(4)植物功能群能够提供基于自然的生态系统服务修复问题解决方案。     

植物遗传多样性与生态系统功能关系的研究进展

全球变化和人类活动导致物种生境的萎缩, 造成很多植物种群数量缩减, 遗传多样性快速丧失。对于物种多样性低的生态系统, 优势种的遗传多样性可能比物种多样性对生态系统功能产生更大的影响。因此, 了解遗传多样性和生态系统功能的关系(GD-EF)及其机制对生物多样性保护、应对环境变化和生态修复具有指导意义。该文综述了植物遗传多样性对生态系统结构(高营养级生物群落结构)和生态系统功能(初级生产力、养分循环和稳定性)的影响及机制、功能多样性对GD-EF的影响、遗传多样性效应和物种多样性效应的比较, 以及GD-EF在生态修复等实际应用的研究进展。最后指出当前研究的不足之处, 以期为后续研究提供参考: 1)还需深入研究GD-EF机制; 2)未评估遗传多样性对生态系统多功能性的影响; 3)不同遗传多样性测度对生态系统功能的影响不明确; 4)缺少长期的和多空间尺度结合的GD-EF实验; 5)遗传多样性效应相对于其他因子的作用不清楚。     

植物功能性状与森林生态系统服务的关系研究综述

植物功能性状途径是揭示生物多样性与生态系统服务关系的重要视角，尽管植物功能性状与生态系统服务的关系在单一地点的研究取得突出进展，但对于植物功能性状与生态系统服务的关系仍缺乏整体认识。以森林生态系统为对象，通过系统文献检索及筛选，收集了216篇文献，应用整合分析和二分网络分析等方法，探讨了植物功能性状对森林生态系统服务及其权衡与协同关系的影响。结果表明：植物叶片功能性状关注最多，占研究性状数量的48%，生态系统服务中关注最多是生物量、土壤肥力、病虫害控制和固碳服务；81.1%的植物功能性状与生态系统服务关系组表现为稳定的正向或负向关系，而关联植物功能性状多的生态系统服务（生物量、固碳服务、土壤水分、土壤肥力和病虫害控制）往往与植物功能性状表现为不稳定关系；森林生态系统中存在6组"植物功能性状-生态系统服务簇（简称"性状-服务簇"）：水循环相关的性状-服务簇、土壤保持相关的性状-服务簇、物质生产相关的性状-服务簇、灾害控制相关的性状-服务簇、养分循环相关的性状-服务簇和授粉相关的性状-服务簇，揭示了各性状-服务簇内生态系统服务的权衡或协同关系以及与各性状-服务簇关系密切的植物功能性状。该研究从总体上阐明了植物功能性状与森林生态系统服务关系的研究重点和进展、揭示了植物功能性状对森林生态系统服务影响效应的方向和强度，可为深化森林生态系统服务形成机制认识以及协调生态系统服务权衡关系提供科学依据。     

陆地生态系统植物功能群研究进展

从植物功能群角度探讨陆地生态系统功能与稳定性维持机理、植物对环境变化的适应与响应以及水分、养分利用效率等成为当前生态系统生态学研究的主要技术路线.植物功能群的提出和研究,为研究复杂的生态系统提供了一个良好的方法和途径.综述了陆地生态系统功能群方面的最新研究进展,介绍了植物功能群定义的发展历程,详细比较了植物功能群划分的依据及方法,对于植物功能群与群落稳定性之间的关系、植物功能群对群落生产力的影响以及植物功能群与环境因子的动态关系等进行了深入讨论.这些研究资料表明,植物功能群整合了功能及对环境响应相似的一类植物,但植物功能特征不是绝对的、单一的,所以对植物功能群就会有不同的理解,会有不同的定义及划分方法.许多研究者从不同的角度、尺度来对植物功能群进行研究,这些研究结果有不同的针对方向和目的,使人们可以从不同的角度更全面的理解复杂的陆地生态系统.学者们在研究生态系统时,或多或少地总要与植物功能群相联系,这大大拓宽了植物功能群的应用范围.所有前人的研究使植物功能群的概念、划分、方向、应用等诸方面越来越清晰.这要求应有一个规范、统一、明确的植物功能群研究方案,这样能使对植物功能群的研究更加深入,能整合全球所有植物功能群的相关研究.     

功能多样性及其研究方法

功能多样性是联系生物多样性和生态系统功能的关键性因素。植物群落内功能多样性的量化描述对于研究生物多样性响应环境变化及其对生物多样性-生态系统功能关系的影响至关重要。由于功能多样性的概念含混不清,使得对其范围的确定、定量描述及其对生态系统功能的作用的评价存在较大的难度。在查阅大量文献的基础上,以植物群落为例,介绍了功能多样性的定义、计算方法及研究中应注意的问题。在植物群落功能多样性指数的选择和测定过程中,对获取相关物种必要的功能特征信息、确定其权重、确定不同物种在群落中的相对丰富度、利用所得的功能多样性指数解释和预测生态系统过程的变化等问题,进行了研究。     

消费者多样性对食物网结构和生态系统功能的影响

前所未有的生物多样性丧失使人们越来越关注生物多样性的生态系统功能.现有的绝大多数研究都是局限在单一营养级别上,主要是植物上,但是今天越来越多的证明表明消费者的多样性对生态系统结构和功能具有深刻影响.综述了消费者多样性对相邻或非相邻营养级的种群密度、物种多样性和生产力等方面影响的最新进展,同时也提出了若干研究展望.总体上.消费者多样性,无论是草食动物还是肉食动物,都倾向于增加该消费者所在营养级的养分和能量利用效率,以及生产力.这可能源于取样效应,或者物种之间的互补作用,类似于植物物种多样性影响初级生产力的机制.草食动物可能降低或者提高植物物种多样性,或者没有显著影响,其具体效应取决于生态系统生产力水平和草食动物的大小.捕食者哌能通过直接抑制草食动物而间接提高植物的多样性和生产力,但这种效应的大小差异很大,甚至效应的方向,都可能随团体内捕食者所占的比例而改变.未来的研究,应该考虑应用较大尺度的实验来检测食物网复杂营养关系对生态系统特性的影响,继续探讨消费者对生态系统功能的影响机制.认为异速生长法则和生态化学计量学在食物网组分关系研究中的应用将有利于增强人们对消费者.生态系统功能关系的理解.另外,全球变暖和转基因植物对食物网中消费者结构和生态系统的功能的影响也将是未来的一个重要研究方向.     

物种多样性和功能群多样性与生态系统生产力的关系——以内蒙古短花针茅草原为例

生物多样性与生态系统生产力之间的关系是当前生态学领域的热点问题。短花针茅(Stipa breviflora)草原是内蒙古荒漠草原的主要类型, 生态系统脆弱, 气候波动剧烈, 研究内蒙古短花针茅草原生物多样性与生产力的关系具有十分重要的意义。该研究在内蒙古短花针茅草原区设置了202个样地进行群落调查, 在干旱区及半干旱区两种资源供给下, 分析了物种丰富度、功能群丰富度与生产力的关系, 旨在解决两个科学问题: 1)物种多样性和功能群多样性中, 哪一种与生产力关系更为密切?2)资源供给对多样性和生产力关系的影响。结果表明: 1)物种丰富度、群落生产力与年降水量呈正相关关系, 而功能群丰富度与年降水量之间不存在显著相关性; 2)群落生产力随物种丰富度的增加而增加, 且两者间呈正线性关系, 功能群丰富度与生产力之间不存在显著相关关系; 3)资源供给会影响多样性与生产力之间的关系, 资源供给低时, 多样性对生产力贡献较低, 资源供给高时, 多样性对生产力的贡献较高。该研究丰富了多样性与生产力关系的研究, 同时, 考虑到植物功能性状的研究在近几年受到生态学家的重视, 且多数研究集中于小尺度的人工控制实验, 因此, 在大尺度自然生态系统中开展功能性状多样性与生态系统功能关系的研究将十分必要。     

植物群落功能多样性计算方法

以性状为基础的功能多样性指数在预测生态系统功能中起到越来越重要的作用.本文对近年来陆续出现的植物群落功能多样性指数进行综述.依据物种多样性指数的组成,功能多样性指数分为功能丰富度、功能均匀度和功能离散度指数.介绍了这3类指数的计算方法,有助于更好、更准确地理解“生物多样性-环境-生态系统功能”的关系.     

Conventional functional classification schemes underestimate the relationship with ecosystem functioning

Studies linking the functional diversity of a biota to ecosystem functioning typically employ a priori classifications of species into hypothetically complementary groups. However, multiple alternate classifications exist in which the number of functional groups, the number of species per functional group, and the grouping of species differ from the a priori scheme. Without assessing the relative precision, or ability of an a priori scheme to accurately predict ecosystem functioning relative to its many alternatives, the validity and utility of analyses based on a single a priori classification scheme remains unclear. We examine the precision of a priori classifications used in 10 experimental grassland systems in Europe and the United States that have found evidence for a significant role of functional plant diversity in governing ecosystem function. The predictive precision of the a priori classifications employed in these studies was seldom significantly higher than the precision of random classifications. Post-hoc classification schemes that performed well in predicting ecosystem function resembled each other more with regard to species composition than average classifications, but there was still considerable variability in the manner in which these classification schemes grouped species. These results suggest that we need a more nuanced understanding of how the diversity of functional traits of species in an assemblage affects ecosystem functioning.     

基于功能性状的生态系统服务研究框架

功能性状通过影响生态系统的属性和过程及其维持来影响生态系统服务。功能多样性-生态系统功能关系的研究有助于深入探讨生态系统服务形成机制, 也为生态系统服务研究提供了一个切入点。该文对目前的功能性状和生态系统服务研究框架进行了介绍, 回顾了功能多样性-生态系统功能关系的研究现状, 总结了目前功能性状在生态系统服务研究中的应用, 提出了基于功能性状的生态系统服务研究框架。在这个研究框架中, 首先选取对生态系统功能影响显著的非生物因子和功能多样性指数, 然后量化非生物因子和功能多样性与生态系统功能, 以及生态系统功能-生态系统服务之间的关系, 进而构建功能多样性与生态系统服务的数量关系。与此同时, 利用群落构建理论和物种共存机制分析功能多样性-生态系统功能变化的机制联系, 以研究生态系统服务形成和变化机制, 为生态系统服务管理决策提供科学依据。     

Interactive effects of species richness and species traits on functional diversity and redundancy

The importance of species diversity for ecosystem function has emerged as a key question for conservation biology. Recently, there has been a shift from examining the role of species richness in isolation towards understanding how species interact to effect ecosystem function. Here, we briefly review theoretical predictions regarding species contributions to functional diversity and redundancy and further use simulated data to test combined effects of species richness, number of functional traits, and species differences within these traits on unique species contributions to functional diversity and redundancy, as well as on the overall functional diversity and redundancy within species assemblages. Our results highlighted that species richness and species functional attributes interact in their effects on functional diversity. Moreover, our simulations suggested that functional differences among species have limited effects on the proportion of redundancy of species contributions as well as on the overall redundancy within species assemblages, but that redundancy rather was determined by number of traits and species richness. Our simulations finally indicated scale dependence in the relative effects of species richness and functional attributes, which suggest that the relative influence of these factors may affect individual contributions differently compared to the overall ecosystem function of species assemblages. We suggest that studies on the relationship between biological diversity and ecosystem function will benefit from focusing on multiple processes and ecological interactions, and that the relative functional attributes of species will have pivotal roles for the ecosystem function of a given species assembly.     

Ecosystem services provided by semi-natural and intensified grasslands: Synergies,trade-offs and linkages to plant traits and functional richness

Question

Semi-natural grasslands (SNG) are important for maintaining biodiversity and ecological processes in farmland. Current pasture-based livestock production mainly occurs on intensified grasslands (IG) that have been agronomically improved. Although it is documented that SNG and IG differ in terms of plant diversity, their ability to provide ecosystem services (ES) in farmland is less explored. Here, we review the role of SNG and IG in delivering ES, illustrate their trade-offs and synergies, and examine how ES can be assessed by using plant traits and functional richness.

Results

We found that SNG generate a wider range of ES than IG. Trade-offs exist between ES that appear more pronounced in IG between high biomass production and other ES. SNG are good in providing habitat for biodiversity, supporting pollination and cultural services. SNG also have a significantly wider range of plant functional traits and a higher functional richness, suggesting that the potential to supply ES in SNG is partly driven by higher number of species and their functional diversity.

Conclusion

Clearer trade-offs were found in IG compared with SNG, supported both by the literature and the functional richness. This suggests that functional knowledge could be a good complement to understand the mechanisms behind ES generation and could help with tailoring grassland management to sustain biodiversity, ecological functions and ES. Although both IG and SNG are likely needed for the long-term sustainability of food production, both could aim for a more balanced generation of ES, increasing biodiversity and functional redundancy at the landscape scale.     

西藏草地植物功能性状与多项生态系统服务关系

针对植被功能性状与生态系统服务功能之间的相互关系,构建了西藏草地株高和可食性两种功能性状的9项指标,并基于土壤和植物采样,分析了9项植物功能性状指标和5项生态系统服务指标间的相关性,探讨了4种机制(Mass ratio,Selection,Niche complementarity及Insurance)在西藏草地的适用性。结果表明,9项功能性状指标中,株高Rao和可食种与所有种株高CWM比分别与土壤有机碳、土壤全氮和土壤含水率3项生态系统服务指标呈显著负相关及显著正相关。说明群落植被对光能竞争的互补性及可食性状植株在群落中的光能资源相对竞争力,与土壤固碳、肥力供给及水源涵养有显著相关关系。而群落可食种、优势种、优势种与次优势种对光能资源竞争力水平,可食植株多样性、可食植株在群落中的优势度及其光能资源竞争力均值,对草地生态系统服务无显著影响。西藏草地植物功能性状对多项生态系统服务的影响机制从光能资源竞争角度更符合Niche complementarity和Insurance理论,而从可食功能性状角度更符合Mass ratio和Selection理论。     

Plant diversity induces shifts in the functional structure and diversity across trophic levels

Changes to primary producer diversity can cascade up to consumers and affect ecosystem processes. Although the effect of producer diversity on higher trophic groups have been studied, these studies often quantify taxonomy‐based measures of biodiversity, like species richness, which do not necessarily reflect the functioning of these communities. In this study, we assess how plant species richness affects the functional composition and diversity of higher trophic levels and discuss how this might affect ecosystem processes, such as herbivory, predation and decomposition. Based on six different consumer traits, we examined the functional composition of arthropod communities sampled in experimental plots that differed in plant species richness. The two components we focused on were functional variation in the consumer community structure (functional structure) and functional diversity, expressed as functional richness, evenness and divergence. We found a consistent positive effect of plant species richness on the functional richness of herbivores, carnivores, and omnivores, but not decomposers, and contrasting patterns for functional evenness and divergence. Increasing plant species richness shifted the omnivore community to more predatory and less mobile species, and the herbivore community to more specialized and smaller species. This was accompanied by a shift towards more species occurring in the vegetation than in the ground layer. Our study shows that plant species richness strongly affects the functional structure and diversity of aboveground arthropod communities. The observed shifts in body size (herbivores), specialization (herbivores), and feeding mode (omnivores) together with changes in the functional diversity may underlie previously observed increases in herbivory and predation in plant communities of higher diversity.     

Structural and taxonomic diversity predict above-ground biomass better than functional measures of maximum height in mixed-species forests

Aims: Mixed-species forests are known to be highly productive systems because of their high species diversity, including taxonomic diversity (species richness) and structural diversity. Recent empirical evidence also points to plant maximum height, as a functional trait that potentially drives forest above-ground biomass (AGB). However, the interrelations between these biotic variables are complex, and it is not always predictable if structural diversity attributes or functional metrics of plant maximum height would act as the most important determinant of stand biomass. Here we evaluated the relative importance of structural diversity attributes and functional metrics of plant maximum height (Hmax) in predicting and mediating AGB response to variation in species richness in mixed-species forests, while also accounting for fine-scale environmental variation. Location: Northern Benin. Methods: We used forest inventory data from mixed-species stands of native and exotic species. We quantified structural diversity as coefficient of variation of tree diameter at breast height (CVdbh) and of height (CVHt). For plant Hmax, we computed three metrics: functional range (FRHmax), functional divergence (FDHmax) and community-weighted mean (CWMHmax). We used topographical variables such as elevation and slope to account for possible environmental effects. Simple and multiple mixed-effects models, and structural equation models were performed to assess the direct and indirect links of AGB with species richness through structural diversity attributes and functional metrics of plant Hmax. Results: Species richness and CVdbh were positively related to AGB, while functional metrics of plant Hmax were not. Structural equation models revealed that species richness influenced AGB indirectly via CVdbh, which alone strongly promoted AGB. Elevation only had a positive direct effect on AGB. While increasing species richness enhanced CVdbh and functional measures of plant Hmax, there was no support for the latter mediating the effects of species richness on AGB. Conclusion: Structural diversity has a significant advantage in predicting and mediating the positive effect of species richness on AGB more so than functional measures of plant Hmax. We argue that structural diversity acts as a mechanism for the species richness–AGB relationship, and that maintaining high structural diversity would enhance biomass in mixed-species forests.     

Biodiversity effects on ecosystem functioning in a 15-year grassland experiment: Patterns,mechanisms, and open questions

In the past two decades, a large number of studies have investigated the relationship between biodiversity and ecosystem functioning, most of which focussed on a limited set of ecosystem variables. The Jena Experiment was set up in 2002 to investigate the effects of plant diversity on element cycling and trophic interactions, using a multi-disciplinary approach. Here, we review the results of 15 years of research in the Jena Experiment, focussing on the effects of manipulating plant species richness and plant functional richness. With more than 85,000 measures taken from the plant diversity plots, the Jena Experiment has allowed answering fundamental questions important for functional biodiversity research.First, the question was how general the effect of plant species richness is, regarding the many different processes that take place in an ecosystem. About 45% of different types of ecosystem processes measured in the ‘main experiment’, where plant species richness ranged from 1 to 60 species, were significantly affected by plant species richness, providing strong support for the view that biodiversity is a significant driver of ecosystem functioning. Many measures were not saturating at the 60-species level, but increased linearly with the logarithm of species richness. There was, however, great variability in the strength of response among different processes. One striking pattern was that many processes, in particular belowground processes, took several years to respond to the manipulation of plant species richness, showing that biodiversity experiments have to be long-term, to distinguish trends from transitory patterns. In addition, the results from the Jena Experiment provide further evidence that diversity begets stability, for example stability against invasion of plant species, but unexpectedly some results also suggested the opposite, e.g. when plant communities experience severe perturbations or elevated resource availability. This highlights the need to revisit diversity–stability theory.Second, we explored whether individual plant species or individual plant functional groups, or biodiversity itself is more important for ecosystem functioning, in particular biomass production. We found strong effects of individual species and plant functional groups on biomass production, yet these effects mostly occurred in addition to, but not instead of, effects of plant species richness.Third, the Jena Experiment assessed the effect of diversity on multitrophic interactions. The diversity of most organisms responded positively to increases in plant species richness, and the effect was stronger for above- than for belowground organisms, and stronger for herbivores than for carnivores or detritivores. Thus, diversity begets diversity. In addition, the effect on organismic diversity was stronger than the effect on species abundances.Fourth, the Jena Experiment aimed to assess the effect of diversity on N, P and C cycling and the water balance of the plots, separating between element input into the ecosystem, element turnover, element stocks, and output from the ecosystem. While inputs were generally less affected by plant species richness, measures of element stocks, turnover and output were often positively affected by plant diversity, e.g. carbon storage strongly increased with increasing plant species richness. Variables of the N cycle responded less strongly to plant species richness than variables of the C cycle.Fifth, plant traits are often used to unravel mechanisms underlying the biodiversity–ecosystem functioning relationship. In the Jena Experiment, most investigated plant traits, both above- and belowground, were plastic and trait expression depended on plant diversity in a complex way, suggesting limitation to using database traits for linking plant traits to particular functions.Sixth, plant diversity effects on ecosystem processes are often caused by plant diversity effects on species interactions. Analyses in the Jena Experiment including structural equation modelling suggest complex interactions that changed with diversity, e.g. soil carbon storage and greenhouse gas emission were affected by changes in the composition and activity of the belowground microbial community. Manipulation experiments, in which particular organisms, e.g. belowground invertebrates, were excluded from plots in split-plot experiments, supported the important role of the biotic component for element and water fluxes.Seventh, the Jena Experiment aimed to put the results into the context of agricultural practices in managed grasslands. The effect of increasing plant species richness from 1 to 16 species on plant biomass was, in absolute terms, as strong as the effect of a more intensive grassland management, using fertiliser and increasing mowing frequency. Potential bioenergy production from high-diversity plots was similar to that of conventionally used energy crops. These results suggest that diverse ‘High Nature Value Grasslands’ are multifunctional and can deliver a range of ecosystem services including production-related services.A final task was to assess the importance of potential artefacts in biodiversity–ecosystem functioning relationships, caused by the weeding of the plant community to maintain plant species composition. While the effort (in hours) needed to weed a plot was often negatively related to plant species richness, species richness still affected the majority of ecosystem variables. Weeding also did not negatively affect monoculture performance; rather, monocultures deteriorated over time for a number of biological reasons, as shown in plant-soil feedback experiments.To summarize, the Jena Experiment has allowed for a comprehensive analysis of the functional role of biodiversity in an ecosystem. A main challenge for future biodiversity research is to increase our mechanistic understanding of why the magnitude of biodiversity effects differs among processes and contexts. It is likely that there will be no simple answer. For example, among the multitude of mechanisms suggested to underlie the positive plant species richness effect on biomass, some have received limited support in the Jena Experiment, such as vertical root niche partitioning. However, others could not be rejected in targeted analyses. Thus, from the current results in the Jena Experiment, it seems likely that the positive biodiversity effect results from several mechanisms acting simultaneously in more diverse communities, such as reduced pathogen attack, the presence of more plant growth promoting organisms, less seed limitation, and increased trait differences leading to complementarity in resource uptake. Distinguishing between different mechanisms requires careful testing of competing hypotheses. Biodiversity research has matured such that predictive approaches testing particular mechanisms are now possible.