A.M. Neutel

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.

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.

Biodiversity in soil ecosystems: the role of energy flow and community stability

In a series of community food webs from native and agricultural soils, we modeled energetics and stability, and evaluated the role of the various groups of organisms and their interactions in energy flow and community stability. Species were aggregated into functional groups based on their trophic position in the food webs. Energy flow rates among the groups were calculated by a model using observations on population sizes, death rates, specific feeding preferences and energy conversion efficiencies. From the energetic organization of the communities we derived the strengths of the mutual effects among the populations. These interaction strengths were found to be patterned in a way that is important to community stability. The patterning consisted of the simultaneous occurrence of strong top down effects at lower trophic levels and strong bottom up effects at higher trophic levels. These patterns resulted directly from the empirical data we used to parameterize the model, as we found no stabilizing patterns with random but plausible parameter values. Also, the impact of each individual interaction on community stability was established. This analysis showed that some interactions had a relatively strong impact on stability, whereas other interactions had only a small impact. These impacts on stability were neither correlated with energy flow nor with interaction strength. Comparison of the seven food webs showed that these impacts were sometimes connected to particular groups of organisms involved in the interaction, but sometimes they were not, which might be due to different trophic positions in the food webs. We argue that future research should be directed to answer the question which energetic properties of the organisms form the basis of the patterning of the interaction strengths, as this would improve our understanding of the interrelationships between energetics, community stability, and hence the maintenance of biological diversity.

C and N mineralization in sandy and loamy grassland soils: The role of microbes and microfauna

Abstract A food web model was used to evaluate the role of soil microorganisms, protozoa and nematodes and their interactions on carbon and nitrogen mineralization in sandy and loamy grassland soils. The differences in C mineralization between the sites did not correspond with the differences in N mineralization. To analyse this discrepancy between the mineralization patterns, C and N mineralization rates were calculated using the observed densities of soil organisms. The C mineralization pattern could be explained from differences in the bacterial biomass estimated on each site. Food web calculations carried out to explain the discrepancy between C and N mineralization patterns indicated that faunal activity played a minor role but that the observed C and N mineralization rates could satisfactorily be calculated using different bacterial C:N ratios as determined for the soil types (8 for the sandy soils; 4.5 and 6 for the loams).

Energetics and Stability in Belowground Food Webs

The analysis of energy and material flow is considered to be fundamental to understanding the patterns and dynamics in ecosystems and the way ecosystems are organized (e.g., DeAngelis (1992) and Ulanowicz (1986)). The availability of energy to food webs has long been recognized to be an important factor influencing food web structure (e.g., Elton (1927), Lindeman (1942), McNaughton et al. (1989), and Wright (1990)), and has also been found to be related to food web stability (e.g., DeAngelis et al. (1989), DeAngelis (1992), and Moore et al. (1993)). Within food webs, energetics and population dynamics are deeply interrelated in that they both reflect the interactions among the populations (e.g., Hairston and Hairston (1993) and Moore et al. (1993)). These interactions influence population dynamics and represent transfer rates of energy and matter. It is by means of the population-dynamic descriptions of the interactions that we can analyze the stability of food web models (May, 1973). In real food webs, interactions are not random, but patterned, and these patterns influence the stability of the webs (e.g., May (1972), Pimm (1980), and Pimm et al. (1991)). In the same way, the nature and strength of the interactions are not random but patterned (e.g., Yodzis (1980, 1981)). An example of such a pattern is given by Moore and Hunt (1988) showing compartmentation along dominant flows of energy in the belowground food web from a short-grass prairie system. Compartmentation implies that the interactions among trophic groups within a compartment are frequent or strong relative to the interactions among compartments. This compartmentation along energy channels in real food webs corresponds with the Pidea of May (1972) that compartmented ecosystems are more likely to be stable than systems without compartmentation. How precisely the organization and transfer of energy within food webs are related to food web stability is yet unclear. Results of experiments establishing population-dynamic effects of a disturbance in interactions among trophic groups have not revealed a straightforward relationship with the energetics. Perturbations of interactions representing relatively small rates of material transfer sometimes lead to a large response, and perturbations of interactions representing a major pathway of material flow sometimes lead to a relatively small response (e.g., Paine (1980, 1992)). These experimental findings question whether a necessary link exists between the energetics in a food web and its stability.

Biological Diversity and Function in Soils: The balance between productivity and food web structure in soil ecosystems

PART III Patterns and drivers of soil biodiversity 5 The use of model Pseudomonas fluorescens populations to study the causes and consequences of microbial diversity 83 Paul B. Rainey, Michael Brockhurst, Angus Buckling, David J. Hodgson and Rees Kassen 6 Patterns and determinants of soil biological diversity 100 Richard D. Bardgett, Gregor W. Yeates and Jonathan M. Anderson 7 How plant communities influence decomposer communities 119 David A. Wardle 8 The balance between productivity and food web structure in soil ecosystems 139 Peter C. de Ruiter, Anje-Margriet Neutel and John Moore

ADDENDUM Reconciling complexity with stability in naturally assembling food webs

This corrects the article DOI: 10.1038/nature06154