A framework for soil food web diagnostics: extension of the nematode faunal analysis concept
Introduction
The many functions of soil food webs are defined in terms of key ecosystem processes or characterized relative to subjective perspectives (Table 1). At global, societal and ecosystem levels, organic matter decomposition, mineral and nutrient cycling, and carbon sequestration are major components of resource conservation, environmental maintenance and even mediation of global climate change. The soil food web provides reservoirs of minerals and nutrients, detoxifies pollutants, modifies soil structure, and regulates abundance of pest and other opportunistic species (Doran and Parkin, 1994, Kennedy and Smith, 1995, van Straalen and van Gestel, 1998). Knowledge of the function of the soil food web in relation to the presence and abundance of its component organisms is a basic requirement for soil stewardship.
Since large amounts of the carbon and energy assimilated by each trophic guild are dissipated through metabolic activity (De Ruiter et al., 1998, Moore, 1994), the abundance and, perhaps, diversity of organisms in food webs may be regulated by the resource supply rate. The supply rate represents a “bottom-up” constraint on the size and activity of the web. Predation and competition among trophic levels provide “top-down” regulation of food web structure and function. Both regulators may control all, or different parts of, a food web (De Ruiter et al., 1995). Trophic cascade effects result from top-down regulation in a linear chain of trophic exchanges. In soil food webs, except at a local patch level or during successional recovery from extreme disturbance, there is probably sufficient connectance among guilds that trophic cascades are unlikely. More likely is that each guild has more than one food source and that several guilds may share a common predator. The effects of change in abundance of a guild in such systems are much less predictable. The high degree of connectance provides functional redundancy and, consequently, functional resilience to perturbation, through many direct and indirect interactions (Menge, 1995, Strong, 1992; Wardle et al., 1995a, Wardle et al., 1995b; Yeates and Wardle, 1996).
To avoid contributing to the inconsistent usage of terminology in description of food webs (Wilson, 1999), we provide our working definitions (Table 2).
Alteration of the structure or function of the soil food web may be a consequence of environmental perturbation. It may also be an explicit management objective of environmental conservation and restoration programs, or of agricultural production practices. Evaluation of the state of the web may be accomplished by structural or functional analysis. In both cases, interpretation of the analysis involves knowledge of the relationship between the indicator and a characteristic of interest. Interpretation usually requires consideration of reliability, temporal and spatial variability, sensitivity and resolution of the measure, and technical sophistication (National Soil Survey Center, 1996).
Structural analysis of the soil food web through determination of the presence and abundance of individual taxa presents challenges. Sampling, capture, identification and assessment may be difficult for some taxa and technologically daunting for a whole fauna. Alternative measures are emerging; for example, biochemical analysis of microbial communities provides opportunities for indicating characteristic fatty acid or DNA fingerprints (Bossio and Scow, 1995, Pankhurst, 1997). Functional analysis of soil food web condition may include rates of soil respiration, organic matter decomposition, biologically-mediated mineralization, and other processes (e.g. Griffiths et al., 2001, Gunapala et al., 1998). Functional analysis may not indicate how those functions are being accomplished, or their sustainability.
An alternative to complete structural analysis is provided by assessment of the presence and abundance of indicator guilds. There is considerable evidence that nematode faunal analysis provides a useful tool in assessing the structure, function, and probably the resilience of the soil food web (Ferris et al., 1999, Ritz and Trudgill, 1999, Wardle et al., 1995b). The characteristics of the nematode fauna that make it a good bioindicator have been well documented. In summary, nematodes are the most abundant of the Metazoa, occupy key positions at most trophic levels in soil food webs, can be captured and enumerated by standardized extraction procedures and are readily identified from morphological and anatomical characters. Further, since their feeding habits are clearly related to oral structure, their trophic roles are readily inferred. Each soil sample contains an abundance and diversity of nematodes and, consequently, has high intrinsic information value (Bongers, 1999, Bongers and Bongers, 1998, Bongers and Ferris, 1999, Yeates et al., 1993). In this paper, we develop a framework for weighted nematode faunal analysis as an indicator of the condition of the soil food web.
Section snippets
Background and rationale
Conceptual developments in the ecology of soil nematodes have progressed through several interesting and definable stages during the last few decades. Faced with the diversity and abundance of soil organisms, ecologists often resort to trophic or functional group categories based on feeding habit. The abundance and dynamics of nematode trophic groups have been described for various habitats and levels of disturbance. Results are often ambiguous, and conclusions few, since the trophic groups
The faunal profile
A graphic representation of the “basal”, “structure” and “enrichment” condition of the soil food web, the faunal profile (Fig. 2A and B), is based on the relative weighted abundance of nematode guilds (Fig. 1). An earlier attempt at diagramming the nematode fauna was the cp triangle (De Goede et al., 1993) which depicts the proportional representation of unweighted cp-1, cp-2 and cp-3–5 groups (Fig. 2C). Disadvantages of that system were that the unweighted data did not provide satisfactory
Results and discussion
Ecological weighting of indicator species is commonly used in freshwater biology but has seldom been applied in terrestrial ecology. Exceptions are the MI for nematodes (Bongers, 1990) and the weighted coenotic index (WCI) developed with testate and ciliate protozoa but applicable to other potential indicator groups (Wodarz et al., 1992). The WCI collapses an enormous amount of both community and species level information, including species richness, dominance, abundance and ecological
Summary and conclusion
Nematode faunal analysis provides a powerful tool for diagnosis of the complexity and status of soil food webs (Ritz and Trudgill, 1999, Wardle et al., 1995b). The functional diversity of soil nematodes includes activities at many nodes and at many trophic levels in the web. Consequently, the presence and abundance of specific taxa is an indicator of the complexity of the web at the trophic levels indicated by those taxa. Since related taxa, with similar morphological, anatomical and
Acknowledgements
We acknowledge and appreciate the vision of researchers who recognizing that their data sets may be invaluable resources for addressing a range of ecological questions, have published nematode faunal analyses in toto. We thank Dr. Gerard Korthals for supplying the complete data set from the pH/Cu study and Bart Verschoor for his insights on the relationship of nematode biomass to the weighting system.
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