Developing X-ray Computed Tomography to non-invasively image 3-D root systems architecture in soil

Background

The positive relationship between biodiversity and ecosystem functioning (BEF) is due mainly to complementarity between species. Most BEF studies primarily focused on plant interactions; however, plants are embedded in a dense network of multitrophic interactions above and below the ground, which are likely to play a crucial role in BEF relationships.

Scope

In the present review I point out the relevance of aboveground–belowground interactions as a source of complementarity effects in grassland biodiversity experiments. A review of the current knowledge on the role of decomposers, arbuscular mycorrhizal fungi, rhizobia, plant growth promoting rhizobacteria, invertebrate ecosystem engineers, herbivores, pathogens and predators in biodiversity experiments, indicates that soil biota can drive both positive and negative complementarity between plant species via a multitude of mechanisms.

Conclusions

I pose four main processes by which aboveground–belowground interactions determine positive complementarity effects: enlarging biotope space, mediating legume effects, increasing plant community resistance, and maintaining plant diversity. By contrast, soil biota may also reinforce negative complementarity effects by competing with plants for nutrients or by exerting herbivore or pathogen pressure, thereby reducing community productivity. Thus, considering aboveground–belowground interactions as well as interactions between antagonistic and mutualistic consumers may improve the mechanistic understanding of complementarity effects in plant diversity–ecosystem functioning experiments and should inspire future research.     

Functional trait diversity across trophic levels determines herbivore impact on plant community biomass

Understanding the consequences of trophic interactions for ecosystem functioning is challenging, as contrasting effects of species and functional diversity can be expected across trophic levels. We experimentally manipulated functional identity and diversity of grassland insect herbivores and tested their impact on plant community biomass. Herbivore resource acquisition traits, i.e. mandible strength and the diversity of mandibular traits, had more important effects on plant biomass than body size. Higher herbivore functional diversity increased overall impact on plant biomass due to feeding niche complementarity. Higher plant functional diversity limited biomass pre‐emption by herbivores. The functional diversity within and across trophic levels therefore regulates the impact of functionally contrasting consumers on primary producers. By experimentally manipulating the functional diversity across trophic levels, our study illustrates how trait‐based approaches constitute a promising way to tackle existing links between trophic interactions and ecosystem functioning.     

Placing biodiversity and ecosystem functioning in context: environmental perturbations and the effects of species richness in a stream field experiment

Greater biodiversity is often associated with increased ecosystem process rates, and is expected to enhance the stability of ecosystem functioning under abiotic stress. However, these relationships might themselves be altered by environmental factors, complicating prediction of the effects of species loss in ecosystems subjected to abiotic stress. In boreal streams, we investigated effects of biodiversity and two abiotic perturbations on three related indices of ecosystem functioning: leaf decomposition, detritivore leaf processing efficiency (LPE) and detritivore growth. Replicate field enclosures containing leaves and detritivore assemblages were exposed to liming and nutrient enrichment, raising pH and nutrient levels. Both treatments constitute perturbations for our naturally acidic and nutrient-poor streams. We also varied detritivore species richness and density. The effects of the abiotic and diversity manipulations were similar in magnitude, but whereas leaf decomposition increased by 18% and 8% following liming and nutrient enrichment, respectively, increased detritivore richness reduced leaf decomposition (6%), detritivore LPE (19%) and detritivore growth (12%). The detritivore richness effect on growth was associated with negative trait-independent complementarity, indicating interspecific interference competition. These interactions were apparently alleviated in both enriched and limed enclosures, as trait-independent complementarity became less negative. LPE increased with detritivore density in the monocultures, indicating benefits of intra-specific aggregation that outweighed the costs of intra-specific competition, and dilution of these benefits probably contributed to lowered leaf decomposition in the species mixtures. Finally, the effects of liming were reduced in most species mixtures relative to the monocultures. These results demonstrate how environmental changes might regulate the consequences of species loss for functioning in anthropogenically perturbed ecosystems, and highlight potential influences of biodiversity on functional stability. Additionally, the negative effects of richness and positive effects of density in our field study were opposite to previous laboratory observations, further illustrating the importance of environmental context for biodiversity–ecosystem functioning relationships.     

Trophic complementarity drives the biodiversity–ecosystem functioning relationship in food webs

The biodiversity–ecosystem functioning (BEF) relationship is central in community ecology. Its drivers in competitive systems (sampling effect and functional complementarity) are intuitive and elegant, but we lack an integrative understanding of these drivers in complex ecosystems. Because networks encompass two key components of the BEF relationship (species richness and biomass flow), they provide a key to identify these drivers, assuming that we have a meaningful measure of functional complementarity. In a network, diversity can be defined by species richness, the number of trophic levels, but perhaps more importantly, the diversity of interactions. In this paper, we define the concept of trophic complementarity (TC), which emerges through exploitative and apparent competition processes, and study its contribution to ecosystem functioning. Using a model of trophic community dynamics, we show that TC predicts various measures of ecosystem functioning, and generate a range of testable predictions. We find that, in addition to the number of species, the structure of their interactions needs to be accounted for to predict ecosystem productivity.     

Plant Diversity Impacts Decomposition and Herbivory via Changes in Aboveground Arthropods

Loss of plant diversity influences essential ecosystem processes as aboveground productivity, and can have cascading effects on the arthropod communities in adjacent trophic levels. However, few studies have examined how those changes in arthropod communities can have additional impacts on ecosystem processes caused by them (e.g. pollination, bioturbation, predation, decomposition, herbivory). Therefore, including arthropod effects in predictions of the impact of plant diversity loss on such ecosystem processes is an important but little studied piece of information. In a grassland biodiversity experiment, we addressed this gap by assessing aboveground decomposer and herbivore communities and linking their abundance and diversity to rates of decomposition and herbivory. Path analyses showed that increasing plant diversity led to higher abundance and diversity of decomposing arthropods through higher plant biomass. Higher species richness of decomposers, in turn, enhanced decomposition. Similarly, species-rich plant communities hosted a higher abundance and diversity of herbivores through elevated plant biomass and C:N ratio, leading to higher herbivory rates. Integrating trophic interactions into the study of biodiversity effects is required to understand the multiple pathways by which biodiversity affects ecosystem functioning.     

Territorial ants depress plant growth through cascading non‐trophic effects in an alpine meadow

Understanding non‐trophic interactions is critical to mechanistically linking community structure and ecosystem functioning. Despite the widespread occurrence of territoriality across animal taxa and ecosystems, the cascading ecological consequences of non‐trophic interactions between territorial animals and intruders have been poorly studied. We experimentally investigated the non‐trophic interaction between territorial ants and members of a dung decomposer community (i.e. predatory arthropods, maggots and coprophagous beetles) in an alpine meadow. We further examined how this non‐trophic interaction cascaded to influence ecosystem properties including dung removal rate, soil nutrient status and aboveground plant biomass surrounding dung pats. Results indicated that territorial interference of ants on key decomposers cascaded to affect plant growth. Specifically, ants significantly decreased the abundance of coprophagous beetles at the time of their peak‐abundance and hence decreased dung removal rates and soil nitrogen concentrations, ultimately decreasing aboveground plant biomass. The strength of this non‐trophic cascading effect was comparable to those reported in studies addressing trophic cascades triggered by predator–prey interactions. Our findings suggest that the non‐trophic interactions and associated cascading effects stemming from territorial behavior should be incorporated into ecological network modeling and research addressing biodiversity–ecosystem functioning relationships.     

生物多样性与生态系统功能:最新的进展与动向

生物多样性与生态系统功能的关系及其内在机制是当前生态学领域的重大科学问题。 2 0 0 2年以来人们不再过多地纠缠于“抽样 -互补之争” ,对这一世纪课题的认识又有了新的进展。 (1)人们开始运用已有的知识揭示更大时间和空间尺度上的物种多样性 -生态系统功能关系。多样性作用机制可能存在着动态变化———“抽样向互补转型” :群落建立初期 ,抽样效应是主要的多样性作用机制 ;随时间推移 ,生态位互补成为主要机制。理论研究则预测 :局域尺度上生态系统功能与物种多样性呈现单峰曲线关系 ,在区域尺度上为单调上升关系 ;(2 )非生物因素与多样性 -生产力的交互关系吸引了许多实验研究。人们发现 :物种多样性 -生产力关系可能会受到资源供给率和环境扰动的修正 ,环境因素可能是多样性 -生产力关系的幕后操纵者 ;(3)人们开始重视营养级相互作用对于多样性 -生态系统功能关系的影响 ,生态位互补和抽样假说开始被扩展运用到消费者营养级上 ;(4 )人们开始认真思考物种共存机制在多样性 -生态系统功能关系的形成中所扮演的角色。理论模型研究表明 ,不同的物种共存机制会导致不同的多样性 -生产力关系     

Assessing the functional importance of large‐bodied invertebrates in experimental headwater streams

Recent theoretical advances in food web ecology emphasize the importance of body size disparities among species for the structure, stability and functions of ecosystems. Experimental confirmations of the functional importance of large species, independent of their trophic position, are scarce. We specifically examine the multiple ecological roles of large invertebrates from two distinct trophic levels in headwater streams. We experimentally manipulated the presence of large predatory invertebrates (two Perlid stoneflies) or detritivores (a limnephilid caddisfly and a Pteronarcys stonefly) in a two‐by‐two design in stream channels open to immigration/emigration of smaller biota. We assessed treatment effects on the trophic structure of the benthic invertebrate community, dynamics of basal resources (benthic algae and leaf litter of cedar and alder), and stability of litter decomposition rates against an experimental pulse perturbation (fine sediment input). The presence of the large invertebrates was associated with a ten‐fold decrease in the biomass of invertebrate filterers whereas other trophic groups were unaffected by the large species. The biomass of benthic algae was lower and the rate of mass loss of alder litter was higher in channels lacking the large predators, thus revealing trophic cascades operating along both algal‐based and detritus‐based food chains. The large predators had no detectable effect on the decomposition of cedar whereas both cedar and alder disappeared faster in the presence of the large detritivores. Furthermore, the large predators and large detritivores interactively influenced the decomposition of the cedar–alder mixture through a litter diversity effect and the variability of the rate of alder decomposition after a pulse of fine sediment. Because the large invertebrates affected multiple ecosystem properties, and as their absence was not rapidly compensated for by small immigrant species, our findings support the notion that large species could be critically important in controlling ecosystem structure and functioning.     

Woody plant phylogenetic diversity mediates bottom–up control of arthropod biomass in species-rich forests

Global change is predicted to cause non-random species loss in plant communities, with consequences for ecosystem functioning. However, beyond the simple effects of plant species richness, little is known about how plant diversity and its loss influence higher trophic levels, which are crucial to the functioning of many species-rich ecosystems. We analyzed to what extent woody plant phylogenetic diversity and species richness contribute to explaining the biomass and abundance of herbivorous and predatory arthropods in a species-rich forest in subtropical China. The biomass and abundance of leaf-chewing herbivores, and the biomass dispersion of herbivores within plots, increased with woody plant phylogenetic diversity. Woody plant species richness had much weaker effects on arthropods, but interacted with plant phylogenetic diversity to negatively affect the ratio of predator to herbivore biomass. Overall, our results point to a strong bottom–up control of functionally important herbivores mediated particularly by plant phylogenetic diversity, but do not support the general expectation that top–down predator effects increase with plant diversity. The observed effects appear to be driven primarily by increasing resource diversity rather than diversity-dependent primary productivity, as the latter did not affect arthropods. The strong effects of plant phylogenetic diversity and the overall weaker effects of plant species richness show that the diversity-dependence of ecosystem processes and interactions across trophic levels can depend fundamentally on non-random species associations. This has important implications for the regulation of ecosystem functions via trophic interaction pathways and for the way species loss may impact these pathways in species-rich forests.     

A general biodiversity–function relationship is mediated by trophic level

Species diversity affects the functioning of ecosystems, including the efficiency by which communities capture limited resources, produce biomass, recycle and retain biologically essential nutrients. These ecological functions ultimately support the ecosystem services upon which humanity depends. Despite hundreds of experimental tests of the effect of biodiversity on ecosystem function (BEF), it remains unclear whether diversity effects are sufficiently general that we can use a single relationship to quantitatively predict how changes in species richness alter an ecosystem function across trophic levels, ecosystems and ecological conditions. Our objective here is to determine whether a general relationship exists between biodiversity and standing biomass. We used hierarchical mixed effects models, based on a power function between species richness and biomass production (Y = a × S^b), and a database of 374 published experiments to estimate the BEF relationship (the change in biomass with the addition of species), and its associated uncertainty, in the context of environmental factors. We found that the mean relationship (b = 0.26, 95% CI: 0.16, 0.37) characterized the vast majority of observations, was robust to differences in experimental design, and was independent of the range of species richness levels considered. However, the richness–biomass relationship varied by trophic level and among ecosystems; in aquatic systems b was nearly twice as large for consumers (herbivores and detritivores) compared to primary producers; in terrestrial ecosystems, b for detritivores was negative but depended on few studies. We estimated changes in biomass expected for a range of changes in species richness, highlighting that species loss has greater implications than species gains, skewing a distribution of biomass change relative to observed species richness change. When biomass provides a good proxy for processes that underpin ecosystem services, this relationship could be used as a step in modeling the production of ecosystem services and their dependence on biodiversity.     

Seed limitation interacts with biotic and abiotic factors to constrain novel species' impact on community biomass and richness

Seed limitation can narrow down the number of coexisting plant species, limit plant community productivity, and also constrain community responses to changing environmental and biotic conditions. In a 10-year full-factorial experiment of seed addition, fertilisation, warming and herbivore exclusion, we tested how seed addition alters community richness and biomass, and how its effects depend on seed origin and biotic and abiotic context. We found that seed addition increased species richness in all treatments, and increased plant community biomass depending on nutrient addition and warming. Novel species, originally absent from the communities, increased biomass the most, especially in fertilised plots and in the absence of herbivores, while adding seeds of local species did not affect biomass. Our results show that seed limitation constrains both community richness and biomass, and highlight the importance of considering trophic interactions and soil nutrients when assessing novel species immigrations and their effects on community biomass.     

Trophic interactions and abiotic factors drive functional and phylogenetic structure of vertebrate herbivore communities across the Arctic tundra biome

Communities are assembled from species that evolve or colonise a given geographic region, and persist in the face of abiotic conditions and interactions with other species. The evolutionary and colonisation histories of communities are characterised by phylogenetic diversity, while functional diversity is indicative of abiotic and biotic conditions. The relationship between functional and phylogenetic diversity infers whether species functional traits are divergent (differing between related species) or convergent (similar among distantly related species). Biotic interactions and abiotic conditions are known to influence macroecological patterns in species richness, but how functional and phylogenetic diversity of guilds vary with biotic factors, and the relative importance of biotic drivers in relation to geographic and abiotic drivers is unknown. In this study, we test whether geographic, abiotic or biotic factors drive biome‐scale spatial patterns of functional and phylogenetic diversity and functional convergence in vertebrate herbivores across the Arctic tundra biome. We found that functional and phylogenetic diversity both peaked in the western North American Arctic, and that spatial patterns in both were best predicted by trophic interactions, namely vegetation productivity and predator diversity, as well as climatic severity. Our results show that both bottom–up and top–down trophic interactions, as well as winter temperatures, drive the functional and phylogenetic structure of Arctic vertebrate herbivore assemblages. This has implications for changing Arctic ecosystems; under future warming and northward movement of predators potential increases in phylogenetic and functional diversity in vertebrate herbivores may occur. Our study thus demonstrates that trophic interactions can determine large‐scale functional and phylogenetic diversity just as strongly as abiotic conditions.     

Herbivore and detritivore effects on rainforest plant production are altered by disturbance

Consumer effects on rainforest primary production are often considered negligible because herbivores and macrodetritivores usually consume a small fraction of annual plant and litter production, even though consumers are known to have effects on plant production and composition in nontropical systems. Disturbances, such as treefall gaps, however, often increase resources to understory food webs, thereby increasing herbivory and feeding rates of detritivores. This increase in consumption could lead to more prominent ecosystem‐level effects of consumers after disturbances, such as storms that cause light gaps. We determined how the effects of invertebrate herbivores (walking sticks) and detritivores (litter snails) on understory plant growth may be altered by disturbances in a Puerto Rican rainforest using an enclosure experiment. Consumers had significant effects on plant growth, but only in light gaps. Specifically, herbivores increased plant growth by 60%, and there was a trend for detritivores to reduce plant growth. Additionally, plant biomass tended to be 50% higher with both consumers in combination, suggesting that herbivores may mediate the effects of detritivores by altering the resources available to detritivore food webs. This study demonstrates that disturbance alters the effects of rainforest consumers, and, furthermore, that consumer activity has the potential to change rainforest successional processes.     

Species diversity and decomposition in laboratory aquatic systems: the role of species interactions

1. Interest in the effects of biodiversity on ecosystem processes is increasing, stimulated by the global species decline. Different hypotheses about the biodiversity‐ecosystem functioning (BEF) relationship have been put forward and various underlying mechanisms proposed for different ecosystems. 2. We investigated BEF relationships and the role of species interactions in laboratory experiments focussing on aquatic decomposition. Species richness at three different trophic levels (leaf detritus, detritus‐colonising fungi and invertebrate detritivores) was manipulated, and its effects on leaf mass loss and fungal growth were assessed in two experiments. In the first, monocultures and mixtures of reed (Phragmites australis), alder (Alnus glutinosa) and oak (Quercus cerris) leaf disks were incubated with zero, one or eight fungal species. Leaf mixtures were also incubated with combinations of three and five fungal species. In the second experiment, reed leaf disks were incubated with all eight fungal species and offered to combinations of one, two, three, four or five macroinvertebrate detritivores with different feeding modes. 3. Results from the first experiment showed that leaf mass loss was directly related to fungal mass and varied unimodally with the number of fungi, with a maximum rate attained at intermediate diversity in oak and reed and at maximum diversity in alder (the fastest decomposing leaf). 4. Mixing litter species stimulated fungal growth but interactions between species of fungi slowed down decomposition. In contrast, mixtures of macroinvertebrate detritivores reduced fungal mass and accelerated leaf decomposition. Possible explanations of the positive relationship between detritivore diversity and decomposition are a reduction in fungal dominance and a differentiation in the use of different resource patches promoted by higher fungal diversity. 5. In conclusion, the results show a general increase in decomposition rate with increasing biodiversity that is controlled by within‐ and between‐trophic level interactions, and support the hypothesis of both bottom‐up and top‐down effects of diversity on this process.     

Ecosystem consequences of diversity depend on food chain length in estuarine vegetation

Biodiversity and food chain length each can strongly influence ecosystem functioning, yet their interactions rarely have been tested. We manipulated grazer diversity in seagrass mesocosms with and without a generalist predator and monitored community development. Changing food chain length altered biodiversity effects: higher grazer diversity enhanced secondary production, epiphyte grazing, and seagrass biomass only with predators present. Conversely, changing diversity altered top‐down control: predator impacts on grazer and seagrass biomass were weaker in mixed‐grazer assemblages. These interactions resulted in part from among‐species trade‐offs between predation resistance and competitive ability. Despite weak impact on grazer abundance at high diversity, predators nevertheless enhanced algal biomass through a behaviourally mediated trophic cascade. Moreover, predators influenced every measured variable except total plant biomass, suggesting that the latter is an insensitive metric of ecosystem functioning. Thus, biodiversity and trophic structure interactively influence ecosystem functioning, and neither factor's impact is predictable in isolation.     

Detritivory: stoichiometry of a neglected trophic level

Previous syntheses have identified the key roles that phylogeny, body size, and trophic level play in determining arthropod stoichiometry. To date, however, detritivores have been largely omitted from such syntheses, despite their importance in nutrient cycling, biodiversity, and food web interactions. Here, we report on a compiled database of the allometry and nutritional stoichiometry (N and P) of detritivorous arthropods. Overall, both N and P content for detritivores varied among major phylogenetic lineages. Detritivore N content was similar to the N content of herbivores, but below that of predators. By contrast, detritivore P content was independent of trophic level. Contrary to previous reports, neither nutrient varied with body size. This analysis places detritivores in the context of related herbivores and predators, and as such, sets the stage for future investigations into the causes and consequences of elemental (mis)matches between detritivores and their detrital resources. Holly M. Martinson and Katie Schneider are co-first author.     

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.     

Biodiversity and ecosystem functioning in naturally assembled communities

Approximately 25 years ago, ecologists became increasingly interested in the question of whether ongoing biodiversity loss matters for the functioning of ecosystems. As such, a new ecological subfield on Biodiversity and Ecosystem Functioning (BEF) was born. This subfield was initially dominated by theoretical studies and by experiments in which biodiversity was manipulated, and responses of ecosystem functions such as biomass production, decomposition rates, carbon sequestration, trophic interactions and pollination were assessed. More recently, an increasing number of studies have investigated BEF relationships in non‐manipulated ecosystems, but reviews synthesizing our knowledge on the importance of real‐world biodiversity are still largely missing. I performed a systematic review in order to assess how biodiversity drives ecosystem functioning in both terrestrial and aquatic, naturally assembled communities, and on how important biodiversity is compared to other factors, including other aspects of community composition and abiotic conditions. The outcomes of 258 published studies, which reported 726 BEF relationships, revealed that in many cases, biodiversity promotes average biomass production and its temporal stability, and pollination success. For decomposition rates and ecosystem multifunctionality, positive effects of biodiversity outnumbered negative effects, but neutral relationships were even more common. Similarly, negative effects of prey biodiversity on pathogen and herbivore damage outnumbered positive effects, but were less common than neutral relationships. Finally, there was no evidence that biodiversity is related to soil carbon storage. Most BEF studies focused on the effects of taxonomic diversity, however, metrics of functional diversity were generally stronger predictors of ecosystem functioning. Furthermore, in most studies, abiotic factors and functional composition (e.g. the presence of a certain functional group) were stronger drivers of ecosystem functioning than biodiversity per se. While experiments suggest that positive biodiversity effects become stronger at larger spatial scales, in naturally assembled communities this idea is too poorly studied to draw general conclusions. In summary, a high biodiversity in naturally assembled communities positively drives various ecosystem functions. At the same time, the strength and direction of these effects vary highly among studies, and factors other than biodiversity can be even more important in driving ecosystem functioning. Thus, to promote those ecosystem functions that underpin human well‐being, conservation should not only promote biodiversity per se, but also the abiotic conditions favouring species with suitable trait combinations.     

Tree litter functional diversity and nitrogen concentration enhance litter decomposition via changes in earthworm communities

Biodiversity is a major driver of numerous ecosystem functions. However, consequences of changes in forest biodiversity remain difficult to predict because of limited knowledge about how tree diversity influences ecosystem functions. Litter decomposition is a key process affecting nutrient cycling, productivity, and carbon storage and can be influenced by plant biodiversity. Leaf litter species composition, environmental conditions, and the detritivore community are main components of the decomposition process, but their complex interactions are poorly understood. In this study, we tested the effect of tree functional diversity (FD) on litter decomposition in a field experiment manipulating tree diversity and partitioned the effects of litter physiochemical diversity and the detritivore community. We used litterbags with different mesh sizes to separate the effects of microorganisms and microfauna, mesofauna, and macrofauna and monitored soil fauna using pitfall traps and earthworm extractions. We hypothesized that higher tree litter FD accelerates litter decomposition due to the availability of complementary food components and higher activity of detritivores. Although we did not find direct effects of tree FD on litter decomposition, we identified key litter traits and macrodetritivores that explained part of the process. Litter mass loss was found to decrease with an increase in leaf litter carbon:nitrogen ratio. Moreover, litter mass loss increased with an increasing density of epigeic earthworms, with most pronounced effects in litterbags with a smaller mesh size, indicating indirect effects. Higher litter FD and litter nutrient content were found to increase the density of surface‐dwelling macrofauna and epigeic earthworm biomass. Based on structural equation modeling, we conclude that tree FD has a weak positive effect on soil surface litter decomposition by increasing the density of epigeic earthworms and that litter nitrogen‐related traits play a central role in tree composition effects on soil fauna and decomposition.     

Biodiversity effects on ecosystem functioning change along environmental stress gradients

Positive relationship between biodiversity and ecosystem functioning has been observed in many studies, but how this relationship is affected by environmental stress is largely unknown. To explore this influence, we measured the biomass of microalgae grown in microcosms along two stress gradients, heat and salinity, and compared our results with 13 published case studies that measured biodiversity–ecosystem functioning relationships under varying environmental conditions. We found that positive effects of biodiversity on ecosystem functioning decreased with increasing stress intensity in absolute terms. However, in relative terms, increasing stress had a stronger negative effect on low‐diversity communities. This shows that more diverse biotic communities are functionally less susceptible to environmental stress, emphasises the need to maintain high levels of biodiversity as an insurance against impacts of changing environmental conditions and sets the stage for exploring the mechanisms underlying biodiversity effects in stressed ecosystems.