The relationship between tree biodiversity and biomass dynamics changes with tropical forest succession

Theory predicts shifts in the magnitude and direction of biodiversity effects on ecosystem function (BEF) over succession, but this theory remains largely untested. We studied the relationship between aboveground tree biomass dynamics (Δbiomass) and multiple dimensions of biodiversity over 8–16 years in eight successional rainforests. We tested whether successional changes in diversity–Δbiomass correlations reflect predictions of niche theories. Diversity–Δbiomass correlations were positive early but weak later in succession, suggesting saturation of niche space with increasing diversity. Early in succession, phylogenetic diversity and functional diversity in two leaf traits exhibited the strongest positive correlations with Δbiomass, indicating complementarity or positive selection effects. In mid‐successional stands, high biodiversity was associated with greater mortality‐driven biomass loss, i.e. negative selection effects, suggesting successional niche trade‐offs and loss of fast‐growing pioneer species. Our results demonstrate that BEF relationships are dynamic across succession, thus successional context is essential to understanding BEF in a given system.     

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.     

Biodiversity and ecosystem functioning at local and regional spatial scales

Local niche complementarity among species (the partitioning of species based upon niche differentiation) is predicted to affect local ecosystem functioning positively. However, recent theory predicts that greater local diversity may hinder local ecosystem functioning when diversity is enhanced through source–sink dynamics. We suggest community assembly as a way to incorporate both the local and regional processes that determine biodiversity and its consequent effects on ecosystem functioning. From this, we propose a hump-shaped relationship between diversity and ecosystem functioning at local scales, but a linear increase of functioning with diversity at regional scales due to regional complementarity.     

Scale‐dependent effects of neighborhood biodiversity on individual tree productivity in a coniferous and broad‐leaved mixed forest in China

The relationship between biodiversity and productivity has stimulated an increasing body of research over the past decades, and this topic still occupies a central place in ecology. While most studies have focused on biomass production in quadrats or plots, few have investigated the scale‐dependent relationship from an individual plant perspective. We present an analysis of the effects of biodiversity (species diversity and functional diversity) on individual tree growth with a data set of 16,060 growth records from a 30‐ha temperate forest plot using spatially explicit individual tree‐based methods. A significant relationship between species diversity and tree growth was found at the individual tree level in our study. The magnitude and direction of biodiversity effects varies with the spatial scale. We found positive effects of species diversity on tree growth at scales exceeding 9 m. Individual tree growth rates increased when there was a greater diversity of species in the neighborhood of the focal tree, which provides evidence of a niche complementarity effect. At small scales (3–5 m), species diversity had negative effects on tree growth, suggesting that competition is more prevalent than complementarity or facilitation in these close neighborhoods. The results also revealed many confounding factors which influence tree growth, such as elevation and available sun light. We conclude that the use of individual tree‐based methods may lead to a better understanding of the biodiversity‐productivity relationship in forest communities.     

Bioturbating space enhances the effects of non‐additive interactions among benthic ecosystem engineers on cross‐habitat nutrient regeneration

Understanding the mechanisms that drive complementary interactions among species is key to the progress of the debate over the role of biodiversity in ecosystem functioning. In addition, interspecific interactions among physical ecosystem engineers have rarely been framed in the context of biodiversity experiments. Here, we provide an empirical test of how the physical niche space of species influences the effects of the biodiversity of bioturbators on cross‐habitat nutrient fluxes in benthic sediments. In the laboratory, we orthogonally manipulated the number and composition of three benthic invertebrate bioturbator species that differ in the dimensions of their bioturbating space niche; i.e. their vertical distribution in the sediment over a gradient of sediment depth and volume. The ammonium (NH₄‐N) flux from the sediment to the water was positively related to bioturbator species richness only in the sediments with the deepest depth and greatest volume. The non‐additive effects of bioturbator species richness on the benthic–pelagic NH₄‐N flux increased linearly with sediment depth and volume, but only in the three‐species mixtures. Furthermore, no individual species dominated the rates of H₄‐N fluxes, indicating that biodiversity effects were mainly driven by complementarity. These results suggest that sediment bioturbating space mediates the magnitude of non‐additive effects among the three invertebrate species and sheds light on the importance of physical niche space in modulating the positive effects of biodiversity on ecosystem functioning.     

Ecospace: A unified framework for understanding variation in terrestrial biodiversity

Understanding patterns in biodiversity is a core ambition in ecological research. Existing ecological theories focusing on individual species, populations, communities, or niches aid in understanding the determinants of biodiversity patterns, yet very few general models for biodiversity have emerged from simplistic approaches. We propose that a systematic, low-dimensional representation of environmental space with building blocks adopted from gradient, niche, metapopulation and assembly theory may unite old and new aspects of biodiversity theory and improve our understanding of variation in terrestrial biodiversity.We propose the term ecospace to cover the local conditions and resources underlying diversity. Our definition of ecospace encompasses abiotic position, biotic expansion and spatiotemporal continuity, which all affect the biodiversity of a biotope (α-diversity). Position refers to placement along abiotic gradients such as temperature, soil pH and fertility, leading to environmental filtering known from classical community theory. Expansion represents the build-up and diversification of organic matter that are not strictly given by position. Continuity refers to the spatiotemporal extension of position and expansion.Biodiversity is scale dependent. The contribution of one biotope to large scale diversity must be estimated by considering its unique contribution to the species richness of the surrounding landscape or region or to the biodiversity of the entire planet. In addition to the relationship between ecospace and biotope richness (α-diversity), we also propose a relation between the uniqueness of the biotope ecospace and the unique contribution of species to the surrounding larger-scale richness.Whereas the impacts of ecospace position and continuity on biodiversity have been studied in isolation, studies comparing or combining them are rare. Furthermore, biotic expansion has never been fully developed as a determinant of biodiversity, ignoring the megadiverse carbon-depending groups of insects and fungi. Precursors of the ecospace concept have been presented over the last 70 years, but they were never fully developed conceptually for terrestrial biodiversity or applied to prediction of biodiversity.Ecospace unites classical and – at times – contradicting theories such as niche theory, island biogeography theory and a suite of community assembly theories into one framework for further development of a general theory of terrestrial biodiversity.     

Functional traits reveal the expansion and packing of ecological niche space underlying an elevational diversity gradient in passerine birds

Variation in species richness across environmental gradients may be associated with an expanded volume or increased packing of ecological niche space. However, the relative importance of these alternative scenarios remains unknown, largely because standardized information on functional traits and their ecological relevance is lacking for major diversity gradients. Here, we combine data on morphological and ecological traits for 523 species of passerine birds distributed across an Andes-to-Amazon elevation gradient. We show that morphological traits capture substantial variation in species dietary (75%) and foraging niches (60%) when multiple independent trait dimensions are considered. Having established these relationships, we show that the 14-fold increase in species richness towards the lowlands is associated with both an increased volume and density of functional trait space. However, we find that increases in volume contribute little to changes in richness, with most (78%) lowland species occurring within the range of trait space occupied at high elevations. Taken together, our results suggest that high species richness is mainly associated with a denser occupation of functional trait space, implying an increased specialization or overlap of ecological niches, and supporting the view that niche packing is the dominant trend underlying gradients of increasing biodiversity towards the lowland tropics.     

Can root trait diversity explain complementarity effects in a grassland biodiversity experiment?

Aims The positive relationship between plant biodiversity and community productivity is well established. However, our knowledge about the mechanisms underlying these positive biodiversity effects is still limited. One of the main hypotheses is that complementarity in resource uptake is responsible for the positive biodiversity effects: plant species differ in resource uptake strategy, which results in a more complete exploitation of the available resources in space and time when plant species are growing together. Recent studies suggest that functional diversity of the community, i.e. the diversity in functional characteristics ('traits') among species, rather than species richness per se, is important for positive biodiversity effects. However, experimental evidence for specific trait combinations underlying resource complementarity is scarce. As the root system is responsible for the uptake of nutrients and water, we hypothesize that diversity in root traits may underlie complementary resource use and contribute to the biodiversity effects.Methods In a common garden experiment, 16 grassland species were grown in monoculture, 4-species mixtures differing in root trait diversity and 16-species mixtures. The 4-species mixtures were designed to cover a gradient in average rooting depth. Above-ground biomass was cut after one growing season and used as a proxy for plant productivity to calculate biodiversity effects.Important findings Overall, plant mixtures showed a significant increase in biomass and complementarity effects, but this varied greatly between communities. However, diversity in root traits (measured in a separate greenhouse experiment and based on literature) could not explain this variation in complementarity effects. Instead, complementarity effects were strongly affected by the presence and competitive interactions of two particular species. The large variation in complementarity effects and significant effect of two species emphasizes the importance of community composition for positive biodiversity effects. Future research should focus on identifying the traits associated with the key role of particular species for complementarity effects. This may increase our understanding of the links between functional trait composition and biodiversity effects as well as the relative importance of resource complementarity and other underlying mechanisms for the positive biodiversity effects.     

Complementarity among species in horizontal versus vertical rooting space

Aims: Many experiments have shown a positive effect of species richnesson productivity in grassland plant communities. However, itis poorly understood how environmental conditions affect thisrelationship. We aimed to test whether deep soil and limitingnutrient conditions increase the complementarity effect (CE)of species richness due to enhanced potential for resource partitioning. Methods: We grew monocultures and mixtures of four common grassland speciesin pots on shallow and deep soil, factorially combined withtwo nutrient levels. Soil volume was kept constant to avoidconfounding soil depth and volume. Using an additive partitioningmethod, we separated biodiversity effects on plant productivityinto components due to species complementarity and dominance. Important findings: Net biodiversity and complementarity effects were consistentlyhigher in shallow pots, which was unexpected, and at the lownutrient level. These two results suggest that although belowgroundpartitioning of resources was important, especially under lownutrient conditions, it was not due to differences in rootingdepths. We conclude that in our experiment (i) horizontal rootsegregation might have been more important than the partitioningof rooting depths and (ii) that the positive effects of deepsoil found in other studies were due to the combination of deepersoil with larger soil volume.     

Experimental evaluation of diversity–productivity relationships in a coral reef fish assemblage

The global decline in biodiversity is causing increasing concern about the effects of biodiversity loss on ecosystem services such as productivity. Biodiversity has been hypothesised to be important in maintaining productivity of biological assemblages because niche complementarity and facilitation among the constituent species can result in more efficient use of resources. However, these conclusions are primarily based on studies with plant communities, and the relationship between diversity and productivity at higher trophic levels is largely unknown, especially in the marine environment. Here, we used a manipulative field experiment to test the effects of species richness and species identity on biomass accumulation in coral reef fish assemblages at Lizard Island. Small patch reefs were stocked with a total of 30 juveniles belonging to three planktivorous damselfish (genus Pomacentrus) according to three different levels of fish species richness (one, two and three species) and seven different combinations of fish species. Species richness had no effect on the relative growth in this assemblage after 18 days, but relative growth differed among individual fish species and the different combinations of species. Patterns of increase in biomass were best explained by species-specific differences and variable effects of intra- and interspecific competition on growth. These results suggest that niche complementarity and facilitation are not the most influential drivers of total productivity within this guild of planktivorous fishes. Total productivity may be resilient to declining reef fish biodiversity, but this will depend on which species are lost and on the life-history traits of remaining species.     

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。     

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     

Environmental heterogeneity increases complementarity in experimental grassland communities

Previous grassland biodiversity experiments were carried out in uniform environments. It is conceivable, however, that biodiversity effects on community characteristics such as primary productivity might be enhanced under more realistic levels of environmental heterogeneity, if this allows complementary resource use by different species in mixture. Therefore, we would expect larger complementarity effects between species in a heterogeneous environment than in a uniform environment. We tested these hypotheses with experiments in four non-overlapping species pools containing the three functional groups grasses, herbs and legumes. We established all species in monoculture, 3- and 6-species mixture on plots with horizontally heterogeneous or uniform distribution of the same total amount of soil nutrients. The positive net biodiversity effects on aboveground biomass production were similar in both heterogeneous and uniform environment. When the net biodiversity effects were partitioned into components, however, it became clear that in the heterogeneous environment they were due to increased complementarity among species whereas in the uniform environment dominance of species with high monoculture yield played also an important role.     

Positive effects of plant species diversity on productivity in the absence of legumes

We investigated the effect of species richness on productivity in randomly assembled grassland communities without legumes. Aboveground biomass increased with increasing species richness and different measures of complementarity showed strong increases with plant species richness. Increasing productivity could not be attributed to a relative increase of highly productive species. Instead, the increase appeared to be caused by the increased performance of several low‐productive species. Our results provide evidence that niche complementarity can strongly increase productivity in grasslands, although the communities contained only grasses and forbs.     

Contrasting Effects of Intraspecific Trait Variation on Trait-Based Niches and Performance of Legumes in Plant Mixtures

Niche differentiation, assumed to be a key mechanism of species coexistence, requires that species differ in their functional traits. So far it remains unclear to which extent trait plasticity leads to niche shifts of species at higher plant diversity, thereby increasing or decreasing niche overlap between species. To analyse this question it is convenient to measure niches indirectly via the variation in resource-uptake traits rather than directly via the resources used. We provisionally call these indirectly measured niches trait-based niches. We studied shoot- and leaf-morphological characteristics in seven legume species in monoculture and multi-species mixture in experimental grassland. Legume species varied in the extent of trait variation in response to plant diversity. Trait plasticity led to significant shifts in species niches in multiple dimensions. Single-species niches in several traits associated with height growth and filling of canopy space were expanded, while other niche dimensions were compressed or did not change with plant diversity. Niche separation among legumes decreased in dimensions related to height growth and space filling, but increased in dimensions related to leaf size and morphology. The total extent of occupied niche space was larger in mixture than in the combined monocultures for dimensions related to leaf morphology and smaller for dimensions related to whole-plant architecture. Taller growth, greater space filling and greater plasticity in shoot height were positively, while larger values and greater plasticity in specific leaf area were negatively related with increased performance of species in mixture. Our study shows that trait variation in response to plant diversity shifts species niches along trait axes. Plastically increased niche differentiation is restricted to niche dimensions that are apparently not related to size-dependent differences between species, but functional equivalence (convergence in height growth) rather than complementarity (divergence in traits associated with light acquisition) explains increased performance of legumes in mixture.     

C,N and P fertilization in an Amazonian rainforest supports stoichiometric dissimilarity as a driver of litter diversity effects on decomposition

Plant leaf litter generally decomposes faster as a group of different species than when individual species decompose alone, but underlying mechanisms of these diversity effects remain poorly understood. Because resource C : N : P stoichiometry (i.e. the ratios of these key elements) exhibits strong control on consumers, we supposed that stoichiometric dissimilarity of litter mixtures (i.e. the divergence in C : N : P ratios among species) improves resource complementarity to decomposers leading to faster mixture decomposition. We tested this hypothesis with: (i) a wide range of leaf litter mixtures of neotropical tree species varying in C : N : P dissimilarity, and (ii) a nutrient addition experiment (C, N and P) to create stoichiometric similarity. Litter mixtures decomposed in the field using two different types of litterbags allowing or preventing access to soil fauna. Litter mixture mass loss was higher than expected from species decomposing singly, especially in presence of soil fauna. With fauna, synergistic litter mixture effects increased with increasing stoichiometric dissimilarity of litter mixtures and this positive relationship disappeared with fertilizer addition. Our results indicate that litter stoichiometric dissimilarity drives mixture effects via the nutritional requirements of soil fauna. Incorporating ecological stoichiometry in biodiversity research allows refinement of the underlying mechanisms of how changing biodiversity affects ecosystem functioning.     

Biodiversity and ecosystem functioning: recent theoretical advances

The relationship between biodiversity and ecosystem functioning has emerged as a major scientific issue today. As experiments progress, there is a growing need for adequate theories and models to provide robust interpretations and generalisations of experimental results, and to formulate new hypotheses. This paper provides an overview of recent theoretical advances that have been made on the two major questions in this area: (1) How does biodiversity affect the magnitude of ecosystem processes (short‐term effects of biodiversity)? (2) How does biodiversity contribute to the stability and maintenance of ecosystem processes in the face of perturbations (long‐term effects of biodiversity)?
Positive short‐term effects of species diversity on ecosystem processes, such as primary productivity and nutrient retention, have been explained by two major types of mechanisms: (1) functional niche complementarity (the complementarity effect), and (2) selection of extreme trait values (the selection effect). In both cases, biodiversity provides a range of phenotypic trait variation. In the complementarity effect, trait variation then forms the basis for a permanent association of species that enhances collective performance. In the selection effect, trait variation comes into play only as an initial condition, and a selective process then promotes dominance by species with extreme trait values. Major differences between within‐site effects of biodiversity and across‐site productivity–diversity patterns have also been clarified. The local effects of diversity on ecosystem processes are expected to be masked by the effects of varying environmental parameters in across‐site comparisons.
A major reappraisal of the paradigm that has dominated during the last decades seems necessary if we are to account for long‐term effects of biodiversity on ecosystem functioning. The classical deterministic, equilibrium approaches to stability do not explain the reduced temporal variability of aggregate ecosystem properties that has been observed in more diverse systems. On the other hand, stochastic, nonequilibrium approaches do show two types of biodiversity effects on ecosystem productivity in a fluctuating environment: (1) a buffering effect, i.e., a reduction in the temporal variance; and (2) a performance‐enhancing effect, i.e., an increase in the temporal mean. The basic mechanisms involved in these long‐term insurance effects are very similar to those that operate in short‐term biodiversity effects: temporal niche complementarity, and selection of extreme trait values. The ability of species diversity to provide an insurance against environmental fluctuations and a reservoir of variation allowing adaptation to changing conditions may be critical in a long‐term perspective.
These recent theoretical developments in the area of biodiversity and ecosystem functioning suggest that linking community and ecosystem ecology is a fruitful avenue, which paves the way for a new ecological synthesis.     

Leaving uncut refuges during meadow harvesting increases the functional diversity of Orthoptera

In the past, insect species richness was high in Central European seminatural grasslands, which were characterized by low‐intensity land use. Currently, however, the hay in most of these grasslands is mechanically harvested, which negatively impacts insect biodiversity. One way to reduce this negative effect is to leave unmown patches as refuges. In the current research we evaluated the short‐term effects of leaving an unmown patch on the taxonomic and functional diversity of the Orthoptera assemblage in a meadow. We found that orthopteran species richness and abundance were significantly reduced by mowing, whether or not a patch was left uncut. In contrast, functional evenness, indicating distribution of species abundances in a niche space, was reduced by mowing only if the plot lacked an uncut refuge. Functional richness, indicating the amount of niche space occupied by species, was elevated if the plot had an uncut refuge. Larger species were negatively affected by mowing, while habitat specialists, mobile species and soil‐ovipositing species benefitted from it. We infer that the presence of an uncut patch increased the diversity of habitats available to orthopterans and maintained even distribution of species among niche space. In summary, leaving an unmown refuge in grasslands could increase the functional diversity of orthopterans, even if it does not preserve taxonomic diversity.     

Does niche overlap control relative abundance in French lacustrine fish communities? A new method incorporating functional traits

1. The mechanisms that structure biological communities hold the key to understanding ecosystem functioning and the maintenance of biodiversity. Patterns of species abundances have been proposed as a means of differentiation between niche-based and neutral processes, but abundance information alone cannot provide unequivocal discrimination. 2. We combined species niche information and species' relative abundances to test the effects of two opposing structuring mechanisms (environmental filtering and niche complementarity) on species' relative abundances in French lacustrine fish communities. The test involved a novel method comparing the abundance-weighted niche overlap within communities against that expected when relative abundances were randomized among species within the community. 3. Observed overlap was consistently significantly lower than expected at random for two (swimming ability and trophic status) of four primary niche axes across lakes of differing physical environments. Thus, for these niche axes, pairs of abundant species tended to have relatively low niche overlap, while rare species tended to have relatively high niche overlap with abundant species. 4. This suggests that niche complementarity may have acted to enhance ecosystem function and that it is important for species coexistence in these fish communities. The method used may be easily applied to any sort of biological community and thus may have considerable potential for determining the generality of niche complementarity effects on community structure.     

Fertilization and plant diversity accelerate primary succession and restoration of dune communities

Plant species richness can increase primary production because plants occupy different niches or facilitate each other (“complementarity effects”) or because diverse mixtures have a greater chance of having more productive species (“selection effects”). To determine how complementarity and selection influence dune restoration, we established four types of plant communities [monocultures of sea oats (Uniola paniculata), bitter panicgrass (Panicum amarum) and saltmeadow cordgrass (Spartina patens) and the three-species mixture] under different soil treatments typical of dune restorations (addition of soil organic material, nutrients, both, or neither). This fully factorial design allowed us to determine if plant identity, diversity and soil treatments influenced the yield of both the planted species and species that recruited naturally (volunteers). Planted species responses in monocultures and mixtures varied among soil treatments. The composition of the plantings and soils also influenced the abundance of volunteers. The mixture of the three species had the lowest cover of volunteers. We also found that the effect of diversity on production increased with fertilizer. We partitioned the biodiversity effect into complementarity and selection effects and found that the increase in the diversity effect occurred because increased nutrients decreased dominance by the largest species and increased complementarity among species. Our findings suggest that different planting schemes can be used to meet specific goals of restoration (e.g., accelerate plant recovery while suppressing colonization of non-planted species).