Species pool distributions along functional trade-offs shape plant productivity-diversity relationships

Livestock grazing and biodiversity: Effects on CO2 exchange in semi-arid Karoo ecosystems, South Africa

Livestock use in semi-arid South African ecosystems has not been extensively studied in relation to the Net Ecosystem Exchange (NEE) of carbon dioxide (CO2). We present four years of measurements from twinned eddy-covariance towers in Nama-Karoo, South Africa, to investigate the carbon fluxes and the impact of grazing intensity on NEE. The design contrasted NEE at a long-term site grazed at recommended levels (LG) with a long-term heavily grazed (EG) site that had been rested for 10 years, and was monitored for two years after which intensive grazing was reintroduced for this experiment. This allowed for the quantification of long-term NEE trends on "recovering" vegetations (years I, II) and short-term responses to an intensified land use (years III, IV). The results showed that the net release of CO2 was slightly higher at LG than on "recovering" vegetation at the EG site, where near-neutral exchange was observed during years I and II. However, after grazing was reintroduced to the EG site, differences between sites was reduced but not eliminated. These findings suggest that there is a somewhat higher carbon sequestration potential at the resting EG site than at the LG site, apparently associated with the dominance of unpalatable drought-tolerant grass species and local elimination of many palatable shrubs. Reduction of this sink potential by reintroduction of high-intensity grazing indicates the sensitivity of C-sequestration in this "recovering" system to heavy grazing, but underlines continued resilience of NEE under far heavier grazing than in the LG system. These data suggest notable trade-offs in these ecosystems between carbon storage, biodiversity, and livestock production with rainfall variability being a critical inter-annual driver. PLAIN LANGUAGE SUMMARY: This study suggests that long-term resting of previously over-utilized southern African semi-arid vegetation supports enhanced carbon sequestration potential, even if over-utilization has transformed vegetation composition (i.e. has caused degradation through reduced plant species richness). However, this enhanced carbon sequestration potential can be quickly negated by the reintroduction of grazing, even after 10 years of resting. Achievement of carbon sequestration is dependent on average to above-average precipitation and its distribution throughout the year, with sink activity evident mainly after seasonal rains during the warm season.

Interspecific trait variability and local soil conditions modulate grassland model community responses to climate

Medium-to-high elevation grasslands provide critical services in agriculture and ecosystem stabilization, through high biodiversity and providing food for wildlife. However, these ecosystems face elevated risks of disruption due to predicted soil and climate changes. Separating the effects of soil and climate, however, is difficult in situ, with previous experiments focusing largely on monocultures instead of natural grassland communities. We experimentally exposed model grassland communities, comprised of three species grown on either local or reference soil, to varied climatic environments along an elevational gradient in the European Alps, measuring the effects on species and community traits. Although species-specific biomass varied across soil and climate, species' proportional contributions to community-level biomass production remained consistent. Where species experienced low survivorship, species-level biomass production was maintained through increased productivity of surviving individuals; however, maximum species-level biomass was obtained under high survivorship. Species responded directionally to climatic variation, spatially separating differentially by plant traits (including height, reproduction, biomass, survival, leaf dry weight, and leaf area) consistently across all climates. Local soil variation drove stochastic trait responses across all species, with high levels of interactions occurring between site and species. This soil variability obscured climate-driven responses: we recorded no directional trait responses for soil-corrected traits like observed for climate-corrected traits. Our species-based approach contributes to our understanding of grassland community stabilization and suggests that these communities show some stability under climatic variation.

Tumors (re)shape biotic interactions within ecosystems: Experimental evidence from the freshwater cnidarian Hydra

While it is often assumed that oncogenic processes in metazoans can influence species interactions, empirical evidence is lacking. Here, we use the cnidarian Hydra oligactis to experimentally explore the consequences of tumor associated phenotypic alterations for its predation ability, relationship with commensal ciliates and vulnerability to predators. Unexpectedly, hydra's predation ability was higher in tumorous polyps compared to non-tumorous ones. Commensal ciliates colonized preferentially tumorous hydras than non-tumorous ones, and had a higher replication rate on the former. Finally, in a choice experiment, tumorous hydras were preferentially eaten by a fish predator. This study, for the first time, provides evidence that neoplastic growth has the potential, through effect(s) on host phenotype, to alter biotic interactions within ecosystems and should thus be taken into account by ecologists.

Linking functional traits and demography to model species-rich communities

It has long been anticipated that relating functional traits to species demography would be a cornerstone for achieving large-scale predictability of ecological systems. If such a relationship existed, species demography could be modeled only by measuring functional traits, transforming our ability to predict states and dynamics of species-rich communities with process-based community models. Here, we introduce a new method that links empirical functional traits with the demographic parameters of a process-based model by calibrating a transfer function through inverse modeling. As a case study, we parameterize a modified Lotka–Volterra model of a high-diversity mountain grassland with static plant community and functional trait data only. The calibrated trait–demography relationships are amenable to ecological interpretation, and lead to species abundances that fit well to the observed community structure. We conclude that our new method offers a general solution to bridge the divide between trait data and process-based models in species-rich ecosystems.

Advances in process-based community ecology models are hindered by the challenge of linking functional traits to demography in species-rich systems, where a high number of parameters need to be estimated from limited data. Here the authors propose a new Bayesian framework to calibrate community models via functional traits, and validate it in a species-rich plant community.

A landscape-scale assessment of the relationship between grassland functioning, community diversity, and functional traits

Livestock farmers rely on a high and stable grassland productivity for fodder production to sustain their livelihoods. Future drought events related to climate change, however, threaten grassland functionality in many regions across the globe. The introduction of sustainable grassland management could buffer these negative effects. According to the biodiversity-productivity hypothesis, productivity positively associates with local biodiversity. The biodiversity-insurance hypothesis states that higher biodiversity enhances the temporal stability of productivity. To date, these hypotheses have mostly been tested through experimental studies under restricted environmental conditions, hereby neglecting climatic variations at a landscape-scale. Here, we provide a landscape-scale assessment of the contribution of species richness, functional composition, temperature, and precipitation on grassland productivity. We found that the variation in grassland productivity during the growing season was best explained by functional trait composition. The community mean of plant preference for nutrients explained 24.8% of the variation in productivity and the community mean of specific leaf area explained 18.6%, while species richness explained only 2.4%. Temperature and precipitation explained an additional 22.1% of the variation in productivity. Our results indicate that functional trait composition is an important predictor of landscape-scale grassland productivity.

Resource-use efficiency drives overyielding via enhanced complementarity

Overyielding, the primary metric for assessing biodiversity effects on ecosystem functions, is often partitioned into "complementarity" and "selection" components, but this reveals nothing about the role of increased resource use, resource-use efficiency, or trait plasticity. We obtained multiple overyielding values by comparing productivity in a five-species mixture to expected values from its component monocultures at a) six levels of nitrogen addition (spanning 0-500 kg N ha^-1 year^-1) and b) across four seasons. We also measured light, water, and nitrogen use, resource-use efficiency, and three functional traits-leaf nitrogen content, specific leaf area, and leaf area ratio-n mixtures and monocultures. We found strong evidence for non-transgressive overyielding. This was strongest in spring, with mixture productivity exceeding expected values by 20 kg dry matter ha^-1 day^-1. Peak overyielding was driven by enhanced complementarity, with the two non-N₂-fixing forb species far exceeding expected productivity in mixtures. Peak overyielding also coincided with higher water use in the mixture than for any monoculture, and enhanced mixture-resource-use efficiency. There was only weak evidence that trait plasticity influenced overyielding or resource use. Our findings suggest that when complementarity drives overyielding in grassland mixtures, and this is made possible both by increased water use and enhanced efficiency in water, nitrogen, and light use. Our results also suggest that mixtures offer a viable compromise between productivity, resource-use efficiency, and reduced environmental impacts (i.e., nitrate leaching) from intensive agriculture.

The productivity-biodiversity relationship varies across diversity dimensions

Understanding the processes that drive the dramatic changes in biodiversity along the productivity gradient remains a major challenge. Insight from simple, bivariate relationships so far has been limited. We combined >11,000 community plots in the French Alps with a molecular phylogeny and trait information for >1200 plant species to simultaneously investigate the relationships between all major biodiversity dimensions and satellite-sensed productivity. Using an approach that tests for differential effects of species dominance, species similarity and the interplay between phylogeny and traits, we demonstrate that unimodal productivity-biodiversity relationships only dominate for taxonomic diversity. In forests, trait and phylogenetic diversity typically increase with productivity, while in grasslands, relationships shift from unimodal to declining with greater land-use intensity. High productivity may increase trait/phylogenetic diversity in ecosystems with few external constraints (forests) by promoting complementary strategies, but under external constraints (managed grasslands) successful strategies are similar and thus the best competitors may be selected.

Both mass ratio effects and community diversity drive biomass production in a grassland experiment

The maintenance of biodiversity is crucial for ecosystem processes such as plant biomass production, as higher species richness is associated with increased biomass production in plant communities. However, the effects of evenness and functional diversity on biomass production are understudied. We manipulated the composition of an experimental grassland by sowing various seed mixtures and examined the effects of diversity and evenness on biomass production after three years. We found that biomass production increased with greater species and functional richness but decreased with greater species and functional evenness. Standing biomass increased but species number and functional richness decreased with increasing proportion of perennial grasses. Our findings emphasise the importance of productive dominant species, as the proportion of perennial grasses had a positive effect on standing biomass, while species and functional evenness had a negative effect on it. Thus, our findings support the theory that, besides diversity, dominance effects and the so-called mass ratio hypothesis may also play a key role in explaining primary biomass production.