Peter C. de Ruiter

Spatial vegetation patterns and imminent desertification in Mediterranean arid ecosystems

Humans and climate affect ecosystems and their services, which may involve continuous and discontinuous transitions from one stable state to another. Discontinuous transitions are abrupt, irreversible and among the most catastrophic changes of ecosystems identified. For terrestrial ecosystems, it has been hypothesized that vegetation patchiness could be used as a signature of imminent transitions. Here, we analyse how vegetation patchiness changes in arid ecosystems with different grazing pressures, using both field data and a modelling approach. In the modelling approach, we extrapolated our analysis to even higher grazing pressures to investigate the vegetation patchiness when desertification is imminent. In three arid Mediterranean ecosystems in Spain, Greece and Morocco, we found that the patch-size distribution of the vegetation follows a power law. Using a stochastic cellular automaton model, we show that local positive interactions among plants can explain such power-law distributions. Furthermore, with increasing grazing pressure, the field data revealed consistent deviations from power laws. Increased grazing pressure leads to similar deviations in the model. When grazing was further increased in the model, we found that these deviations always and only occurred close to transition to desert, independent of the type of transition, and regardless of the vegetation cover. Therefore, we propose that patch-size distributions may be a warning signal for the onset of desertification.

Soil invertebrate fauna enhances grassland succession and diversity

One of the most important areas in ecology is to elucidate the factors that drive succession in ecosystems and thus influence the diversity of species in natural vegetation. Significant mechanisms in this process are known to be resource limitation and the effects of aboveground vertebrate herbivores. More recently, symbiotic and pathogenic soil microbes have been shown to exert a profound effect on the composition of vegetation and changes therein. However, the influence of invertebrate soil fauna on succession has so far received little attention. Here we report that invertebrate soil fauna might enhance both secondary succession and local plant species diversity. Soil fauna from a series of secondary grassland succession stages selectively suppress early successional dominant plant species, thereby enhancing the relative abundance of subordinate species and also that of species from later succession stages. Soil fauna from the mid-succession stage had the strongest effect. Our results clearly show that soil fauna strongly affects the composition of natural vegetation and we suggest that this knowledge might improve the restoration and conservation of plant species diversity.

CARBON SEQUESTRATION IN ECOSYSTEMS: THE ROLE OF STOICHIOMETRY

The fate of carbon (C) in organisms, food webs, and ecosystems is to a major extent regulated by mass-balance principles and the availability of other key nutrient elements. In relative terms, nutrient limitation implies excess C, yet the fate of this C may be quite different in autotrophs and heterotrophs. For autotrophs nutrient limitation means less fixation of inorganic C or excretion of organic C, while for heterotrophs nutrient limitation means that more of ingested C will “go to waste” in the form of egestion or respiration. There is in general a mismatch between autotrophs and decomposers that have flexible but generally high C:element ratios, and consumers that have lower C:element ratios and tighter stoichiometric regulation. Thus, C-use efficiency in food webs may be governed by the element ratios in autotroph biomass and tend to increase when C:element ratios in food approach those of consumers. This tendency has a strong bearing on the sequestration of C in ecosystems, since more C will be di...

Reconciling complexity with stability in naturally assembling food webs

Understanding how complex food webs assemble through time is fundamental both for ecological theory and for the development of sustainable strategies of ecosystem conservation and restoration. The build-up of complexity in communities is theoretically difficult, because in random-pattern models complexity leads to instability. There is growing evidence, however, that nonrandom patterns in the strengths of the interactions between predators and prey strongly enhance system stability. Here we show how such patterns explain stability in naturally assembling communities. We present two series of below-ground food webs along natural productivity gradients in vegetation successions. The complexity of the food webs increased along the gradients. The stability of the food webs was captured by measuring the weight of feedback loops of three interacting ‘species’ locked in omnivory. Low predator–prey biomass ratios in these omnivorous loops were shown to have a crucial role in preserving stability as productivity and complexity increased during succession. Our results show the build-up of food-web complexity in natural productivity gradients and pin down the feedback loops that govern the stability of whole webs. They show that it is the heaviest three-link feedback loop in a network of predator–prey effects that limits its stability. Because the weight of these feedback loops is kept relatively low by the biomass build-up in the successional process, complexity does not lead to instability.

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.

Temporal and spatial heterogeneity of trophic interactions within below-ground food webs☆

Abstract Real food webs are dynamic multi-dimensional systems, whereas the descriptions of real food webs often do not capture this complexity in that they have been confined to a single habitat (the community web sensu Cohen, 1978) and do not represent changes in time (Paine, 1988). We present an analytical approach that uses univariate and multivariate statistics and simulation modeling to study pattern within food webs. As an illustration of the approach, we compared below-ground food webs from natural and agricultural ecosystems in terms of their architecture, temporal dynamics of the biomass of functional groups, and the temporal and spatial dynamics of energy channels. The complexity and diversity of below-ground food webs are similar to the detritus-based food webs of other terrestrial and aquatic habitats. The pattern of the flow of nitrogen through the below-ground food webs of the Shortgrass Steppe of North America is similar to that of the food web of agricultural soils of reclaimed marine sediments in The Netherlands. The webs are compartmented along dominant flows of energy (energy channels) originating from primary production and detritus. Comparisons of the connectedness descriptions, and implementations of cluster analysis, canonical discriminant analysis and analysis of variance of temporal biomass of functional groups within food webs of soils from North America and The Netherlands, indicate that the detritus energy channel can be further compartmented into a fungal and bacterial channel. For winter wheat soils in The Netherlands, the degree of compartmentalization appears to depend on management practice. Consumers of fungi were separated in time from consumers of bacteria in the integrated management practice, while little separation was observed in conventional practice. Our study indicates (1) that analyses of food webs should aim to project the web onto the principal niche dimensions food, habitat and time, and (2) that quantitative measures of community structure — identification of functional groups, the biomass and productivity of functional groups, and the flow of nutrients within energy channels — are useful measures of food web structure.

Microbial numbers and activity in dried and rewetted arable soil under integrated and conventional management

Abstract During an 8-week microplot experiment, effects of moisture regime and farm management on microbial numbers and activity were studied. Under integrated management (reduced input farming), bacterial numbers, O 2 consumption and N mineralization, respectively, were 1.6, 2.1 and 1.8 times higher than under conventional management (high input farming). These differences may be attributed to 1.3 and 1.4 times higher contents of organic matter and total N in the integrated microplots. One month of drying from a water potential of −0.03 to −0.12 MPa, and subsequent rewetting to −0.01 MPa, did not affect bacterial numbers significantly. However, the relatively small decrease in water potential caused a significant decrease in O 2 consumption and N mineralization. After rewetting, respiration increased from 1.3 to 1.5 fold, and N mineralization from 3 to 5 fold. Concurrently, the frequency of dividing-divided cells (FDDC) increased from 10 to 16% in the conventional and to 23% in the integrated microplots. This suggests that the FDDC, which is determined by direct microscopy and requires no incubation, can be used as an index of in situ bacterial growth rate in soil. For marine bacteria, mathematical relationships have been established between specific growth rate (μ) and FDDC. If it is assumed that these relationships are also valid for soil bacteria, FDDCs of 16 and 23%, respectively, may indicate specific growth rates of about 1 and 2 day −1 . Bacterial production rates based on FDDC (8.5–45 μg C g −1 day −1 ) were 3–8 times higher than those based on O 2 consumption rates determined by 2-week incubations. Uncertainties of the methods are discussed.

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.

Modelling food webs and nutrient cycling in agro-ecosystems

Agricultural practices affect the spatial patterns and dynamics of the decomposition of soil organic matter and the availability of plant-limiting nutrients. The biological processes underlying these patterns and dynamics are the trophic interactions among the organisms in the soil community food web. Food web models simulate nutrient flow rates close to observed rates and clarify the role of the various groups of organisms in the cycling of nutrients. Several large interdisciplinary programs are currently focusing on these interactions, with a view to developing and managing sustainable forms of agriculture.

Nutrients and hydrology indicate the driving mechanisms of peatland surface patterning.

Peatland surface patterning motivates studies that identify underlying structuring mechanisms. Theoretical studies so far suggest that different mechanisms may drive similar types of patterning. The long time span associated with peatland surface pattern formation, however, limits possibilities for empirically testing model predictions by field manipulations. Here, we present a model that describes spatial interactions between vegetation, nutrients, hydrology, and peat. We used this model to study pattern formation as driven by three different mechanisms: peat accumulation, water ponding, and nutrient accumulation. By on‐and‐off switching of each mechanism, we created a full‐factorial design to see how these mechanisms affected surface patterning (pattern of vegetation and peat height) and underlying patterns in nutrients and hydrology. Results revealed that different combinations of structuring mechanisms lead to similar types of peatland surface patterning but contrasting underlying patterns in nutrients and hydrology. These contrasting underlying patterns suggest that the presence or absence of the structuring mechanisms can be identified by relatively simple short‐term field measurements of nutrients and hydrology, meaning that longer‐term field manipulations can be circumvented. Therefore, this study provides promising avenues for future empirical studies on peatland patterning.