Partitioning the effects of algal species identity and richness on benthic marine primary production

Influential research in terrestrial habitats indicates that several ecosystem processes are related to plant biodiversity, yet these links remain poorly studied in marine ecosystems. We conducted one field and one mesocosm experiment to quantify the relative effects of macroalgal species identity and richness on primary production in coral reef macroalgal communities off the north coast of Jamaica. We measured production as the net accumulation of algal biomass in the absence of consumers and as photosynthetic rate using oxygen probes in sealed aquaria. We used two recently developed techniques to attribute deviations in expected relative yield to components associated with species identity or diversity and then to further partition diversity effects into mechanistic components based on dominance, trait-dependent complementarity, and trait-independent complementarity. Our results indicate that algal identity had far greater effects on absolute net growth and photosynthesis than richness. The most diverse mixture of macroalgae did not outperform the most productive monoculture or the average monoculture in either measure of primary production (i.e. we did not find evidence of either transgressive or non-transgressive overyielding). Trait-independent complementarity effects were positive but dominance and trait-dependent complementarity were both negative and became stronger when richness was increased. Thus the potentially positive influence of species interactions and niche partitioning on production were negated by dominance and other negative selection effects. These results demonstrate that the counteracting influence of component effects can diminish the net richness effects on production. This could explain frequently observed weak net richness effects in other aquatic and terrestrial systems and suggests that life history tradeoffs greatly reduce the potential for ecologically relevant plant biodiversity effects on ecosystem properties.     

Effects of macroalgal species identity and richness on primary production in benthic marine communities

Plant biodiversity can enhance primary production in terrestrial ecosystems, but biodiversity effects are largely unstudied in the ocean. We conducted a series of field and mesocosm experiments to measure the relative effects of macroalgal identity and richness on primary productivity (net photosynthetic rate) and biomass accumulation in hard substratum subtidal communities in North Carolina, USA. Algal identity consistently and strongly affected production; species richness effects, although often significent, were subtle. Partitioning of the net biodiversity effect indicated that complementarity effects were always positive and species were usually more productive in mixtures than in monoculture. Surprisingly, slow growing species performed relatively better in the most diverse treatments than the most productive species, thus selection effects were consistently negative. Our results suggest that several basic mechanisms underlying terrestrial plant biodiversity effects also operate in algal-based marine ecosystems, and thus may be general.     

Reciprocal causality between marine macroalgal diversity and productivity in an upwelling area

The relationship between biodiversity and ecosystem functioning has become a prominent topic in the ecological literature. However, the contemporary approach that species diversity controls primary productivity contrasts with the historical perspective that species diversity responds to productivity. Moreover, previous experimental results have not been consistent with the patterns observed in nature. To resolve these questions, the multivariate productivity–diversity (MPD) hypothesis proposes a bidirectional relationship between diversity and productivity. It predicts that the resource supply, expressed in terms of resource availability and imbalance, establishes the number of species that can locally coexist. Simultaneously, the resource supply also indirectly affects biomass production, determining the form and cause of the effects of species richness on resource use and biomass. To test the MPD hypothesis, we conducted three field experiments with a subtidal marine macroalgal community using a seasonal upwelling process as a driver of distinct levels of nutrient supply. Seasonally, macroalgal species richness and biomass were assessed and experimental manipulations conducted to investigate the relative importance of species richness and identity effects on biomass production and the mechanisms underlying these. Changes in macroalgal biomass and species richness were observed in response to the nutrient supply. Stronger effects of species identity were detected for all periods investigated, although species richness effects also occurred to some extent. The magnitudes of the net biodiversity and of the complementarity effects were a unimodal function of nutrient supply, whereas a concave‐up curve was observed for selection effects. The nutrient supply directly affected the number of species that dominated the local community and, consequently, determined the efficiency with which resources were exploited and converted to biomass. Our results provide evidence consistent with the MPD hypothesis and aids in explaining the discrepancies between experimental results and natural patterns through the merging of two contrasting perspectives in ecology.     

Biodiversity influences ecosystem functioning in aquatic angiosperm communities

Knowledge of the connection between aquatic plant diversity and ecosystem processes is still limited. To examine how plant species diversity affects primary productivity, plant nutrient use, functional diversity of secondary producers and population/community stability, we manipulated submerged angiosperm species diversity in a field experiment lasting 15 weeks. Plant richness increased the shoot density for three of four species. Polyculture biomass production was enhanced by increasing richness, with positive complementarity and selection effects causing positive biodiversity effects. Species richness enhanced the community stability for biomass production and shoot density. Sediment ammonium availability decreased with plant diversity, suggesting improved nutrient usage with increasing plant richness. Interestingly, positive multitrophic effects of plant species richness on structural and functional diversity of macrobenthic secondary producers were recorded. The results suggest that mixed seagrass meadows play an important role for ecosystem functioning and thus contribute to the provision of goods and services in coastal areas.     

The Effect of Genetic Diversity on Ecosystem Functioning in Vegetated Coastal Ecosystems

In species-poor communities, genetic diversity potentially plays an important role for ecosystem functioning, though this is still largely unexplored in marine and estuarine ecosystems. We studied how genetic diversity (sensu genotypic diversity and/or allelic richness) affects ecosystem functioning in marine habitat-forming plant communities. First, we conducted a 15-month field experiment in the highly seasonal Baltic Sea and established mono- and polycultures of different genotypes and genotype combinations of Zostera marina. Second, we reviewed existing literature and performed a meta-analysis of 12 studies including this study. We found no evidence of positive genetic diversity effects on shoot production in the field experiment, but diversity enhanced community stability over time. The literature review revealed that a majority of the included studies observed positive effects of genetic diversity on ecosystem functions such as primary production and nutrient uptake. The results from the meta-analysis support the hypothesis that genetic diversity effects on productivity are stronger during or after periods of stress. These diversity effects were also more positive in the field compared to mesocosm studies. Our results indicate that genetic diversity has positive effects on ecosystem functioning, particularly during increased environmental stress. Thus, local genetic diversity should be preserved especially in species-poor ecosystems, where it potentially provides insurance against environmental change.     

Species traits and environmental conditions govern the relationship between biodiversity effects across trophic levels

Changing environments can have divergent effects on biodiversity–ecosystem function relationships at alternating trophic levels. Freshwater mussels fertilize stream foodwebs through nutrient excretion, and mussel species-specific excretion rates depend on environmental conditions. We asked how differences in mussel diversity in varying environments influence the dynamics between primary producers and consumers. We conducted field experiments manipulating mussel richness under summer (low flow, high temperature) and fall (moderate flow and temperature) conditions, measured nutrient limitation, algal biomass and grazing chironomid abundance, and analyzed the data with non-transgressive overyielding and tripartite biodiversity partitioning analyses. Algal biomass and chironomid abundance were best explained by trait-independent complementarity among mussel species, but the relationship between biodiversity effects across trophic levels (algae and grazers) depended on seasonal differences in mussel species’ trait expression (nutrient excretion and activity level). Both species identity and overall diversity effects were related to the magnitude of nutrient limitation. Our results demonstrate that biodiversity of a resource-provisioning (nutrients and habitat) group of species influences foodweb dynamics and that understanding species traits and environmental context are important for interpreting biodiversity experiments.     

Determining the mechanism by which fish diversity influences production

Understanding the ability of biodiversity to govern ecosystem function is essential with current pressures on natural communities from species invasions and extirpations. Changes in fish communities can be a major determinant of food web dynamics, and even small shifts in species composition or richness can translate into large effects on ecosystems. In addition, there is a large information gap in extrapolating results of small-scale biodiversity–ecosystem function experiments to natural systems with realistic environmental complexity. Thus, we tested the key mechanisms (resource complementarity and selection effect) for biodiversity to influence fish production in mesocosms and ponds. Fish diversity treatments were created by replicating species richness and species composition within each richness level. In mesocosms, increasing richness had a positive effect on fish biomass with an overyielding pattern indicating species mixtures were more productive than any individual species. Additive partitioning confirmed a positive net effect of biodiversity driven by a complementarity effect. Productivity was less affected by species diversity when species were more similar. Thus, the primary mechanism driving fish production in the mesocosms was resource complementarity. In the ponds, the mechanism driving fish production changed through time. The key mechanism was initially resource complementarity until production was influenced by the selection effect. Varying strength of intraspecific interactions resulting from differences in resource levels and heterogeneity likely caused differences in mechanisms between the mesocosm and pond experiments, as well as changes through time in the ponds. Understanding the mechanisms by which fish diversity governs ecosystem function and how environmental complexity and resource levels alter these relationships can be used to improve predictions for natural systems.     

Species richness destabilizes ecosystem functioning in experimental aquatic microcosms

The relationship between species diversity and ecosystem stability has long interested ecologists, yet no consensus has been reached and the underlying mechanisms remain unclear. We used five unicellular algal species, cultured in all possible combinations, to assemble microcosms containing 1 to 5 algal species, on which a cold perturbation was imposed. Our aim was to find whether and how species richness begets ecosystem resistance and resilience. In the experiment, the species-rich communities produced more biomass than the species-poor ones, either in pre-, under- or post-perturbation conditions. The positive diversity–biomass relationship was weakened by the perturbation, and fully restored one week after the perturbation. The diverse communities showed greater absolute biomass reduction during the perturbation than did species-poor systems. Resistance of community, measured by the relative change in biomass from pre- to under-perturbation, decreased with species richness. All the species showed significant reduction in biomass when stressed, without any density compensation among species in diverse communities; and the ratio of biomass change in each species was independent of diversity. The overyielding effect, measured as relative yield total, remained constant from pre- to under-perturbation; and the selection and complementarity effects played equal roles for the biodiversity effect on biomass production, and their relative importance was not altered by the perturbation. These results suggest that similar responses of different species to environmental perturbations may limit the insurance effect of biodiversity, and lead to an inverse diversity–resistance relationship.     

Environmental context determines multi‐trophic effects of consumer species loss

Loss of biodiversity and nutrient enrichment are two of the main human impacts on ecosystems globally, yet we understand very little about the interactive effects of multiple stressors on natural communities and how this relates to biodiversity and ecosystem functioning. Advancing our understanding requires the following: (1) incorporation of processes occurring within and among trophic levels in natural ecosystems and (2) tests of context‐dependency of species loss effects. We examined the effects of loss of a key predator and two groups of its prey on algal assemblages at both ambient and enriched nutrient conditions in a marine benthic system and tested for interactions between the loss of functional diversity and nutrient enrichment on ecosystem functioning. We found that enrichment interacted with food web structure to alter the effects of species loss in natural communities. At ambient conditions, the loss of primary consumers led to an increase in biomass of algae, whereas predator loss caused a reduction in algal biomass (i.e. a trophic cascade). However, contrary to expectations, we found that nutrient enrichment negated the cascading effect of predators on algae. Moreover, algal assemblage structure varied in distinct ways in response to mussel loss, grazer loss, predator loss and with nutrient enrichment, with compensatory shifts in algal abundance driven by variation in responses of different algal species to different environmental conditions and the presence of different consumers. We identified and characterized several context‐dependent mechanisms driving direct and indirect effects of consumers. Our findings highlight the need to consider environmental context when examining potential species redundancies in particular with regard to changing environmental conditions. Furthermore, non‐trophic interactions based on empirical evidence must be incorporated into food web‐based ecological models to improve understanding of community responses to global change.     

Consumer and resource diversity effects on marine macroalgal decomposition

Few manipulative experiments that explicitly test the relationship between biodiversity and ecosystem function have focussed on regenerative processes such as decomposition and nutrient cycling. Of the studies that have taken place, most have concentrated on the effects of leaf litter diversity rather than the effects of consumer diversity on decomposition. In the present study, we established an in‐situ mesocosm experiment on an intertidal mudflat in the Ythan Estuary, Scotland, to investigate the interactive effects of consumer diversity, resource diversity and microbial activity on algal consumption and decomposition. We assembled communities of three commonly occurring macrofaunal species (Hediste diversicolor, Hydrobia ulvae and Littorina littorea) in monoculture and all possible combinations of two and three species mixtures and supplied them with single vs two‐species mixtures of the algae Fucus spiralis and Ulva intestinalis. Further, we also investigated whether algal decomposition changes nutrient remineralisation within the sediment by determining the C:N ratio of the surficial sediment. Data were analysed using extended linear regression with generalized least squares estimation to characterise the variance structure. We found that consumer species diversity effects are best explained by compositional effects and that species richness per se may not be the single most important determinant of resource use and decomposition in this community. Algal identity and invertebrate identity effects underpin the observed response and reflect species‐specific traits associated with algal consumption and processing. The role of the microbial community is comparatively weak, but strongly linked to faunal activities and behaviour. The C:N ratio of the sediment increased with consumer species richness, indicating increased mineralisation in more diverse communities. Overall, our results suggest that although consumer species richness effects per se are weak, decomposition and subsequent incorporation of resources is nevertheless dependent on the composition of the decomposer community, which, in turn, has important implications for biogeochemical nutrient cycling in marine coastal habitats.     

Effects of land-use change on productivity depend on small-scale plant species diversity

Understanding the interplay between land-use change, species diversity and ecosystem function is critical for the prediction of global change impacts on ecosystem services. Biodiversity experiments with artificial species assemblages have shown that community-scale species richness may affect ecosystem productivity and spatial stability. However, the use of synthetic communities with controlled levels of species density for biodiversity experiments has been criticised and their relevance for natural communities has been questioned. Here, we use a land-use change experiment to investigate the biodiversity effects on production within managed, upland grasslands. We examine species diversity and productivity at both the small plant-neighbourhood scale (14×14 cm) and the field scale (15 m×25 m) for two land-use trajectories under field conditions: intensification through fertilisation, and extensification through the cessation of mowing. Both intensification and extensification were associated with a decrease in species number, but the magnitude of this decrease was greater at the small scale. Extensification was associated with a decrease in small-scale productivity whereas intensification had no significant effect on small-scale productivity. Effects of land-use treatments on biomass production were mediated by variation in small-scale species number; species number showed a significant positive relationship with small-scale productivity within each land-use treatment. Furthermore, species number was associated with a decrease in the variance of small-scale green biomass. In contrast, no species diversity effects were found on productivity at the field scale. Instead, field-scale species diversity decreased with increase in the total above-ground biomass (green biomass+litter). This study demonstrates that biodiversity effects can be observed under field conditions at the small scale and may play an important role for ecosystem functioning and stability even in low-diversity plant communities.     

Effects of algal diversity on the production of biomass in homogeneous and heterogeneous nutrient environments: a microcosm experiment

Background

One of the most common questions addressed by ecologists over the past decade has been-how does species richness impact the production of community biomass? Recent summaries of experiments have shown that species richness tends to enhance the production of biomass across a wide range of trophic groups and ecosystems; however, the biomass of diverse polycultures only rarely exceeds that of the single most productive species in a community (a phenomenon called ‘transgressive overyielding’). Some have hypothesized that the lack of transgressive overyielding is because experiments have generally been performed in overly-simplified, homogeneous environments where species have little opportunity to express the niche differences that lead to ‘complementary’ use of resources that can enhance biomass production. We tested this hypothesis in a laboratory experiment where we manipulated the richness of freshwater algae in homogeneous and heterogeneous nutrient environments.

Methodology/Principal Findings

Experimental units were comprised of patches containing either homogeneous nutrient ratios (16∶1 nitrogen to phosphorus (N∶P) in all patches) or heterogeneous nutrient ratios (ranging from 4∶1 to 64∶1 N∶P across patches). After allowing 6–10 generations of algal growth, we found that algal species richness had similar impacts on biomass production in both homo- and heterogeneous environments. Although four of the five algal species showed a strong response to nutrient heterogeneity, a single species dominated algal communities in both types of environments. As a result, a ‘selection effect’–where diversity maximizes the chance that a competitively superior species will be included in, and dominate the biomass of a community–was the primary mechanism by which richness influenced biomass in both homo- and heterogeneous environments.

Conclusions/Significance

Our study suggests that spatial heterogeneity, by itself, is not sufficient to generate strong effects of biodiversity on productivity. Rather, heterogeneity must be coupled with variation in the relative fitness of species across patches in order for spatial niche differentiation to generate complementary resource use.     

Functional group richness: implications of biodiversity for light use and lipid yield in microalgae

Currently, very few studies address the relationship between diversity and biomass/lipid production in primary producer communities for biofuel production. Basic studies on the growth of microalgal communities, however, provide evidence of a positive relationship between diversity and biomass production. Recent studies have also shown that positive diversity–productivity relationships are related to an increase in the efficiency of light use by diverse microalgal communities. Here, we hypothesize that there is a relationship between diversity, light use, and microalgal lipid production in phytoplankton communities. Microalgae from all major freshwater algal groups were cultivated in treatments that differed in species richness and functional group richness. Polycultures with high functional group richness showed more efficient light use and higher algal lipid content with increasing species richness. There was a clear correlation between light use and lipid production in functionally diverse communities. Hence, a powerful and cost‐effective way to improve biofuel production might be accomplished by incorporating diversity related, resource‐use‐dynamics into algal biomass production.     

Seaweed chemical diversity: an additional and efficient tool for coastal evaluation

The assessment of human impacts on marine ecosystems is usually done by assessing changes in species diversity and abundance. Here, we add to this approach the assessment of primary and secondary metabolites from macroalgal communities in urban and protected areas in south Brazil and investigate whether the chemical diversity of marine macroalgae is affected by environmental changes, such as those caused by coastal urbanization, through the use of thin-layer chromatography. Additionally, we compare the chemical and biological diversity of macroalgal communities within urban and undeveloped sites along the southern Brazilian coast. Coastlines within protected areas had greater species richness and higher amounts of substances such as chlorophylls, carotenoids and lipids as well as a greater chemical diversity than coasts subjected to multiple stressors from urbanization. We conclude that the composition and abundance of primary and secondary metabolites provide useful additional information about the ecological status of coastal environments and improve our understanding of the effects of coastal biodiversity loss due to coastal urbanization.     

Is the relationship between algal diversity and biomass in North American lakes consistent with biodiversity experiments?

Over the past few decades, a large body of research has examined how biodiversity loss influences the functioning of ecosystems, as well as the cascading impacts on the goods and services ecosystems provide to humanity. The relationship between biodiversity and ecosystem functions quantified in prior experiments suggests that initial losses of biodiversity have relatively small impacts on properties like community biomass production; however, beyond some threshold, increasing losses lead to accelerating declines in function. Some have questioned whether a saturating relationship between diversity and community biomass production is an artifact of overly simplified experiments that manipulate diversity in homogeneous conditions over short time‐scales in which niche differences may not be realized. Others have questioned whether even the modest effects of biodiversity observed in experiments would be discernible in natural systems where they could be over‐ridden by the stronger influence of abiotic factors. Here, we used a biogeographic dataset to assess how the taxonomic richness of aquatic primary producers relates to community biomass in unmanipulated lake ecosystems in the US, and then compared these findings to prior experiments. We used structural equation modeling to evaluate hypotheses about the effects of algal richness on community biomass while accounting for covariance with environmental parameters measured in the USEPA's National Lakes Assessment (NLA), which sampled 1157 freshwater lakes. These analyses converged on a single best‐fit model (χ²= 0.31, p = 0.58) wherein community algal biomass was a function of three explanatory variables – nitrogen, phosphorus, and algal richness. The quantitative magnitude of the algal diversity (x) – biomass (y) relationship in the NLA dataset is statistically greater than that documented in the average biodiversity experiment. It did, however, lie at approximately the 75th percentile of experimental relationships, indicating the diversity–biomass relationship in unmanipulated lakes is within the range that has been characterized experimentally.     

Macroalgal blooms alter community structure and primary productivity in marine ecosystems

Eutrophication, coupled with loss of herbivory due to habitat degradation and overharvesting, has increased the frequency and severity of macroalgal blooms worldwide. Macroalgal blooms interfere with human activities in coastal areas, and sometimes necessitate costly algal removal programmes. They also have many detrimental effects on marine and estuarine ecosystems, including induction of hypoxia, release of toxic hydrogen sulphide into the sediments and atmosphere, and the loss of ecologically and economically important species. However, macroalgal blooms can also increase habitat complexity, provide organisms with food and shelter, and reduce other problems associated with eutrophication. These contrasting effects make their overall ecological impacts unclear. We conducted a systematic review and meta‐analysis to estimate the overall effects of macroalgal blooms on several key measures of ecosystem structure and functioning in marine ecosystems. We also evaluated some of the ecological and methodological factors that might explain the highly variable effects observed in different studies. Averaged across all studies, macroalgal blooms had negative effects on the abundance and species richness of marine organisms, but blooms by different algal taxa had different consequences, ranging from strong negative to strong positive effects. Blooms' effects on species richness also depended on the habitat where they occurred, with the strongest negative effects seen in sandy or muddy subtidal habitats and in the rocky intertidal. Invertebrate communities also appeared to be particularly sensitive to blooms, suffering reductions in their abundance, species richness, and diversity. The total net primary productivity, gross primary productivity, and respiration of benthic ecosystems were higher during macroalgal blooms, but blooms had negative effects on the productivity and respiration of other organisms. These results suggest that, in addition to their direct social and economic costs, macroalgal blooms have ecological effects that may alter their capacity to deliver important ecosystem services.     

Ultraviolet radiation and consumer effects on a field-grown intertidal macroalgal assemblage in Antarctica

Ultraviolet radiation (UVR) research on marine macroalgae has hithero focussed on physiological effects at the organism level, while little is known on the impact of UV radiation on macroalgal assemblages and even less on interactive effects with other community drivers, e.g. consumers. Field experiments on macrobenthos are scarce, particularly in the Antarctic region. Therefore, the effects of UVR and consumers (mainly limpets were excluded) on early successional stages of a hard bottom macroalgal community on King George Island, Antarctica, were studied. In a two‐factorial design experimental units [(1) ambient radiation, 280–700 nm; (2) ambient minus UVB, 320–700 nm and (3) ambient minus UVR, 400–700 nm vs. consumer–no consumer] were installed between November 2004 and March 2005 (n= 4 plus controls). Dry mass, species richness, diversity and composition of macroalgal assemblages developing on ceramic tiles were followed. Consumers significantly suppressed green algal recruits and total algal biomass but increased macroalgal richness and diversity. Both UVA and UVB radiation negatively affected macroalgal succession. UVR decreased the density of Monostroma hariotii germlings in the first 10 weeks of the experiment, whereas the density of red algal recruits was significantly depressed by UVR at the end of the study. After 106 days macroalgal diversity was significantly higher in UV depleted than in UV‐exposed assemblages. Furthermore, species richness was significantly lower in the UV treatments and species composition differed significantly between the UV‐depleted and the UV‐exposed treatment. Marine macroalgae are very important primary producers in coastal ecosystems, serving as food for herbivores and as habitat for many organisms. Both, UVR and consumers significantly shape macroalgal succession in the Antarctic intertidal. Consumers, particularly limpets can mediate negative effects of ambient UVR on richness and diversity till a certain level. UVB radiation in general and an increase of this short wavelength due to stratospheric ozone depletion in particular may have the potential to affect the zonation, composition and diversity of Antarctic intertidal seaweeds altering trophic interactions in this system.     

The relationship between species richness and biomass changes from boreal to subtropical forests in China

Diversity‐manipulation experiments suggest a positive effect of biodiversity on ecosystem properties (EPs), but variable relationships between species richness and EPs have been reported in natural communities. An explanation for this discrepancy is that observed richness–EPs relationships in natural communities change with environment and species functional identities. But how the relationships change along broad‐scale climatic gradients has rarely been examined. In this paper, we sampled 848 plots of 20 × 30 m² from boreal to tropical forests across China. We examined plot biomass with respect to environmental factors, tree species richness and functional group identity (FGI, i.e. evergreen vs deciduous, and coniferous vs broadleaf). Variation partitioning was used to evaluate the relative effects of the three classes of factors. We found that, most of the ‘effects’ (percentage of variation explained) of richness and FGI on forest biomass were shared with environmental factors, but species richness and FGI still revealed significant effects in addition to environment for plots across China. Richness and FGI explained biomass mainly through their shared effects instead of independent effects, suggesting that the positive biodiversity effect is closely associated with a sampling effect. The relative effects of richness, FGI and environment varied latitudinally: the independent effects of environment and richness decreased from boreal to subtropical forests, whereas the total effect of FGI increased. We also found that the slope of richness–biomass relationship decreased monotonically from boreal to subtropical forests, possibly because of decreasing complementarity and increasing competition with increasing productivity. Our results suggest that while species richness does have significant effects on forest biomass it is less important than environmental gradients and other biotic factors in shaping large‐scale biomass patterns. We suggest that understanding how and why the diversity–EPs relationships change along climatic gradient would be helpful for a better understanding of real biodiversity effects in natural communities.     

Diversity-dependent stability under mowing and nutrient addition: evidence from a 7-year grassland experiment

Anthropogenic perturbations may affect biodiversity and ecological stability as well as their relationships. However, diversity-stability patterns and associated mechanisms under human disturbances have rarely been explored. We conducted a 7-year field experiment examining the effects of mowing and nutrient addition on the diversity and temporal stability of herbaceous plant communities in a temperate steppe in northern China. Mowing increased population and community stability, whereas nutrient addition had the opposite effects. Stability exhibited positive relationships with species richness at population, functional group and community levels. Treatments did not alter these positive diversity-stability relationships, which were associated with the stabilising effect of species richness on component populations, species asynchrony and portfolio effects. Despite the difficulty of pinpointing causal mechanisms of diversity-stability patterns observed in nature, our results suggest that diversity may still be a useful predictor of the stability of ecosystems confronted with anthropogenic disturbances.     

Community Biomass and Bottom up Multivariate Nutrient Complementarity Mediate the Effects of Bioturbator Diversity on Pelagic Production

Tests of the biodiversity and ecosystem functioning (BEF) relationship have focused little attention on the importance of interactions between species diversity and other attributes of ecological communities such as community biomass. Moreover, BEF research has been mainly derived from studies measuring a single ecosystem process that often represents resource consumption within a given habitat. Focus on single processes has prevented us from exploring the characteristics of ecosystem processes that can be critical in helping us to identify how novel pathways throughout BEF mechanisms may operate. Here, we investigated whether and how the effects of biodiversity mediated by non-trophic interactions among benthic bioturbator species vary according to community biomass and ecosystem processes. We hypothesized that (1) bioturbator biomass and species richness interact to affect the rates of benthic nutrient regeneration [dissolved inorganic nitrogen (DIN) and total dissolved phosphorus (TDP)] and consequently bacterioplankton production (BP) and that (2) the complementarity effects of diversity will be stronger on BP than on nutrient regeneration because the former represents a more integrative process that can be mediated by multivariate nutrient complementarity. We show that the effects of bioturbator diversity on nutrient regeneration increased BP via multivariate nutrient complementarity. Consistent with our prediction, the complementarity effects were significantly stronger on BP than on DIN and TDP. The effects of the biomass-species richness interaction on complementarity varied among the individual processes, but the aggregated measures of complementarity over all ecosystem processes were significantly higher at the highest community biomass level. Our results suggest that the complementarity effects of biodiversity can be stronger on more integrative ecosystem processes, which integrate subsidiary “simpler” processes, via multivariate complementarity. In addition, reductions in community biomass may decrease the strength of interspecific interactions so that the enhanced effects of biodiversity on ecosystem processes can disappear well before species become extinct.