A framework for soil food web diagnostics: extension of the nematode faunal analysis concept

Abstract

Nematodes, the earth’s most abundant metazoa, are ubiquitous in the soil environment. They are sufficiently large to be identifiable by light microscopy and sufficiently small to inhabit water films surrounding soil particles. They aggregate around and in food sources. They include component taxa of the soil food web at several trophic levels. They can be categorized into functional guilds whose members respond similarly to food web enrichment and to environmental perturbation and recovery. Indices derived through nematode faunal analysis provide bioindicators for disturbance of the soil environment and condition of the soil food web. We enhance the resolution of faunal analyses by providing a weighting system for the indicator importance of the presence and abundance of each functional guild in relation to enrichment and structure of the food web. Graphical representations of food web structure, based on nematode faunal analyses, allow diagnostic interpretation of its condition. Simple ratios of the weighted abundance of representatives of specific functional guilds provide useful indicators of food web structure, enrichment, and decomposition channels.

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.