Heikki Setälä

EFFECTS OF BIODIVERSITY ON ECOSYSTEM FUNCTIONING: A CONSENSUS OF CURRENT KNOWLEDGE

Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earths biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the re- lationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are struc- tured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earths ecosystems and the diverse biota they contain.

Sensitivity of primary production to changes in the architecture of belowground food webs

The proposed mechanisms for the species diversity-function relationship in plant communities stress the recognition of functional properties of species, and interactions between plants and soil processes. As resource availability to plants is influenced by the architecture of decomposer food webs, it has been hypothesised that the diversity of decomposers can also control ecosystem processes, including primary production. We manipulated the complexity of soil animal communities in a miniecosysten experiment in which a boreal forest floor with birch seedlings infected with mycorrhizal fungi was created. The soil animal diversity ranged from zero to typical species richness of soil fauna (approximately 50 taxa) in coniferous forests. Between these extremes was a nested factorial design with manually assembled communities consisting of two sets of one-species, and two sets of five-species animal communities within fungivorous and microbi-detritivorous trophic groups. To investigate the role of predators in system functioning, the miniecosystems with fungivorous and microbidetritivorous fauna were established either with or without mesostigmatid mites as top predators. The miniecosystems were incubated in a climate chamber with varying illumination and temperature regimes for 40 weeks. Our experiment provides evidence that primary productivity is generally insensitive to variation at the species level or even at the level of trophic groups. Although top predators generally reduced prey population size, no effect was found on systen functioning. However, the removal of microbe- or detritus-feeding fauna, especially the microbi-detritivore Cognettia sphagnetorum reduced plant N uptake and accumulation of plant biomass. The functional importance of soil fauna was inversely related to the trophic position of the group. Our results suggest that ecosystem functioning is robust against species extinctions in belowground food webs, and that primary production is predominantly controlled by organisms at low trophic positions in the decomposer food web.

Soil food web properties explain ecosystem services across European land use systems

Intensive land use reduces the diversity and abundance of many soil biota, with consequences for the processes that they govern and the ecosystem services that these processes underpin. Relationships between soil biota and ecosystem processes have mostly been found in laboratory experiments and rarely are found in the field. Here, we quantified, across four countries of contrasting climatic and soil conditions in Europe, how differences in soil food web composition resulting from land use systems (intensive wheat rotation, extensive rotation, and permanent grassland) influence the functioning of soils and the ecosystem services that they deliver. Intensive wheat rotation consistently reduced the biomass of all components of the soil food web across all countries. Soil food web properties strongly and consistently predicted processes of C and N cycling across land use systems and geographic locations, and they were a better predictor of these processes than land use. Processes of carbon loss increased with soil food web properties that correlated with soil C content, such as earthworm biomass and fungal/bacterial energy channel ratio, and were greatest in permanent grassland. In contrast, processes of N cycling were explained by soil food web properties independent of land use, such as arbuscular mycorrhizal fungi and bacterial channel biomass. Our quantification of the contribution of soil organisms to processes of C and N cycling across land use systems and geographic locations shows that soil biota need to be included in C and N cycling models and highlights the need to map and conserve soil biodiversity across the world.

Decomposition rate of organic substrates in relation to the species diversity of soil saprophytic fungi

Despite the great interest concerning the relationship between species diversity and ecosystem functioning, there is virtually no knowledge as to how the diversity of decomposer microbes influences the decomposition rate of soil organic matter. We established a microcosm study in which the number of soil fungi was investigated in relation to the system’s ability to (i) degrade raw coniferous forest humus, and (ii) use resources that were either added to the systems or released into the soils after a disturbance (drought). With the exception of the most diverse treatment, in each of the six replicates of each of the six diversity treatments (1, 3, 6, 12, 24 or 43 taxa), fungal taxa were randomly chosen from a pool of 43 commonly isolated fungal species of raw humus. Two months after initiation of the study CO2 production increased as fungal diversity increased, but in the species-poor end of the diversity gradient only. Addition of various energy resources to the microcosms generally increased the level of soil respiration but did not affect the shape of the diversity-CO2-production curve. Rewetting the soil after severe drought resulted in a rapid flush of CO2, particularly in the most diverse communities. The biomass of the fungi in the non-disturbed soils, and soil NH4-N concentration and soil pH in both disturbed and non-disturbed systems were slightly but significantly higher in the diverse than in the simple systems. Fungal species richness had no influence on the organic matter content of the humus at the end of the experiment. The results suggest that the functional efficiency of fungal communities can increase with the number of fungal taxa. This diversity effect was, however, significant at the species-poor end of the diversity gradient only, which implies considerable functional equivalency (redundancy) among the decomposer fungi.

Soil Fauna Increase Betula Pendula Growth: Laboratory Experiments With Coniferous Forest Floor

The effects of soil fauna on the growth and nutrient contents of birch seedlings were studied in laboratory microcosms simulating the complexity of a coniferous forest floor. In one set of microcosms partially sterilized soil was reinoculated with soil microorganisms only; another set was reinoculated with microorganisms and a diverse soil fauna. A birch seedling (Betula pendula) was planted in each microcosm, all of which were incubated in a climate chamber with various temperature and illumination regimes. During two growing periods birch—leaf, stem, and root biomasses were 70%, 53%, and 38% greater, respectively, in the presence of fauna. The N and P contents of leaves were °3— and 1.5—times higher, respectively, in the refaunted microcosmms than in the controls with microorganisms only. The amount of K, Ca. and Mg in leaf tissue were almost the same in both treatments. Despite the greater leaf biomass, and therefore more effective transpiration, the water content of the soil remained significantly higher in the refaunted microcosms. The results clearly indicate that soil fauna, via enhanced nutrient mobilization and favorable changes in the structural soil properties, expert a positive influence on plant growth.

Relating species diversity to ecosystem functioning : mechanistic backgrounds and experimental approach with a decomposer food web

Hypotheses have recently been formulated to elucidate the relationship between species diversity and ecosystem functioning. Using previously published mechanisms as a starting point we suggest that common mechanisms can be provided for this relationship by using the concepts of niche and trophic-level dynamics. The reasoning is the following: if remaining species within a trophic level can modify their niches as other species disappear, production within the level remains unchanged, whereas in the absence of niche modification production decreases. Decreased production within a trophic level affects biomass and production at other levels as predicted by trophic-dynamic models. Changes in biomass and production finally bring about changes in ecosystem functioning. In the redundant species hypothesis remaining species can modify their niches, and so functioning remains unchanged. In the predictable change hypothesis (our counterpart for the rivet hypothesis), and in the idiosyncratic response hypothesis, remaining species cannot modify their niches, leading to predictable and unpredictable changes in functioning, respectively. Unpredictable changes are due to differences in the characteristics of species and indirect interactions between populations. We tested the hypotheses and the suggested mechanisms using a soil food web with three trophic levels: microbes, microbivorous nematodes and a predatory nematode. We established one diverse (3 bacterivores and 3 fungivores) and three simple (1 bacterivore and 1 fungivore) food webs and found that differences in trophic-level biomasses between the diverse and simple food webs were idiosyncratic. Unpredictability resulted from differences in microbivore characteristics - their efficiency in resource utilisation and vulnerability to predation and competition. Changes in microbial respiration and total mineralisation of C and N, i.e., system functioning, were also idiosyncratic rather than redundant or predictable when diversity was reduced, although idiosyncracy was not as clear as in the case of trophic-level biomasses. We conclude that predicting the influence of declining species diversity on trophic-level dynamics and ecosystem processes is difficult, at least in food webs with a small initial number of species, unless the characteristics of species and the nature of their interactions are known.

Productivity and trophic-level biomasses in a microbial-based soil food web

Basic trophic-dynamic models using prey-dependent prey-predator interactions typically predict (1) that the limiting factors, resources and predation, should alternate at adjacent trophic levels, and (2) that only biomasses at resource limited levels should increase when the productivity of a system is increased. However, experimental studies on aquatic systems have shown that biomasses tend to respond to increased productivity at all trophic levels. To test the predictions in a terrestrial environment. we performed an experiment with a soil food web. We established three food webs with one, two, or three trophic levels in microcosms containing an initially sterilized mixture of leaf litter and raw humus, and increased the productivity with additional glucose in half of the replicates of each food web. The first trophic level contained 22 species of bacteria and fungi, the second level a bacterivorous nematode (Caenorhabditis elegans) and a fungivorous nematode (Aphelenchoides sp.), and the third level a predatory nematode (Prionchulus punctatus). We sampled the microcosms destructively four times during the 22-week experiment to estimate the trophic-level biomasses and soil NH 4 + -N concentration. Evolution of CO 2 was measured to estimate microbial productivity. Microbial productivity was greater and the amount of NH 4 + -N lower in the communities provided with additional energy. The presence of microbivores also resulted in greater microbial productivity than in the pure microbial community. The biomass of microbes and microbivores increased when provided with supplementary energy independently of the number of trophic levels in the food web, while the biomass of the predatory nematode did not significantly respond to additional energy. The predatory and the bacterial feeding nematodes limited the biomass of their resources, whereas the fungal biomass was unaffected by the fungivore. The results infer that the biomasses at the first and second trophic level were simultaneously limited by resources and predation. which contradicts basic prey-dependent models. Prey refuges provided by soil structure may explain the inability of predators to control their preys as effectively as predicted by these models. Moreover, the results suggest that the nature of trophic interactions may differ at the bottom and top of soil food webs, and between the fungal and bacterial channels.

Divergent composition but similar function of soil food webs of individual plants: plant species and community effects

Soils are extremely rich in biodiversity, and soil organisms play pivotal roles in supporting terrestrial life, but the role that individual plants and plant communities play in influencing the diversity and functioning of soil food webs remains highly debated. Plants, as primary producers and providers of resources to the soil food web, are of vital importance for the composition, structure, and functioning of soil communities. However, whether natural soil food webs that are completely open to immigration and emigration differ underneath individual plants remains unknown. In a biodiversity restoration experiment we first compared the soil nematode communities of 228 individual plants belonging to eight herbaceous species. We included grass, leguminous, and non-leguminous species. Each individual plant grew intermingled with other species, but all plant species had a different nematode community. Moreover, nematode communities were more similar when plant individuals were growing in the same as compared to different plant communities, and these effects were most apparent for the groups of bacterivorous, carnivorous, and omnivorous nematodes. Subsequently, we analyzed the composition, structure, and functioning of the complete soil food webs of 58 individual plants, belonging to two of the plant species, Lotus corniculatus (Fabaceae) and Plantago lanceolata (Plantaginaceae). We isolated and identified more than 150 taxa/groups of soil organisms. The soil community composition and structure of the entire food webs were influenced both by the species identity of the plant individual and the surrounding plant community. Unexpectedly, plant identity had the strongest effects on decomposing soil organisms, widely believed to be generalist feeders. In contrast, quantitative food web modeling showed that the composition of the plant community influenced nitrogen mineralization under individual plants, but that plant species identity did not affect nitrogen or carbon mineralization or food web stability. Hence, the composition and structure of entire soil food webs vary at the scale of individual plants and are strongly influenced by the species identity of the plant. However, the ecosystem functions these food webs provide are determined by the identity of the entire plant community.

Intensive agriculture reduces soil biodiversity across Europe

Soil biodiversity plays a key role in regulating the processes that underpin the delivery of ecosystem goods and services in terrestrial ecosystems. Agricultural intensification is known to change the diversity of individual groups of soil biota, but less is known about how intensification affects biodiversity of the soil food web as a whole, and whether or not these effects may be generalized across regions. We examined biodiversity in soil food webs from grasslands, extensive, and intensive rotations in four agricultural regions across Europe: in Sweden, the UK, the Czech Republic and Greece. Effects of land-use intensity were quantified based on structure and diversity among functional groups in the soil food web, as well as on community-weighted mean body mass of soil fauna. We also elucidate land-use intensity effects on diversity of taxonomic units within taxonomic groups of soil fauna. We found that between regions soil food web diversity measures were variable, but that increasing land-use intensity caused highly consistent responses. In particular, land-use intensification reduced the complexity in the soil food webs, as well as the community-weighted mean body mass of soil fauna. In all regions across Europe, species richness of earthworms, Collembolans, and oribatid mites was negatively affected by increased land-use intensity. The taxonomic distinctness, which is a measure of taxonomic relatedness of species in a community that is independent of species richness, was also reduced by land-use intensification. We conclude that intensive agriculture reduces soil biodiversity, making soil food webs less diverse and composed of smaller bodied organisms. Land-use intensification results in fewer functional groups of soil biota with fewer and taxonomically more closely related species. We discuss how these changes in soil biodiversity due to land-use intensification may threaten the functioning of soil in agricultural production systems.

Does urban vegetation mitigate air pollution in northern conditions

It is generally accepted that urban vegetation improves air quality and thereby enhances the well-being of citizens. However, empirical evidence on the potential of urban trees to mitigate air pollution is meager, particularly in northern climates with a short growing season. We studied the ability of urban park/forest vegetation to remove air pollutants (NO2, anthropogenic VOCs and particle deposition) using passive samplers in two Finnish cities. Concentrations of each pollutant in August (summer; leaf-period) and March (winter, leaf-free period) were slightly but often insignificantly lower under tree canopies than in adjacent open areas, suggesting that the role of foliage in removing air pollutants is insignificant. Furthermore, vegetation-related environmental variables (canopy closure, number and size of trees, density of understorey vegetation) did not explain the variation in pollution concentrations. Our results suggest that the ability of urban vegetation to remove air pollutants is minor in northern climates.