Physiological Ecology Of Soil Organisms

Given the physiological ecology of the microbes and fauna involved, are long food chains energetically possible? There are several theoretical reasons why long food chains could be expected. Let us take, as an example, the energetically most dominant interactions between microbes and fauna, which occur in many terrestrial ecosystems, summarized by Hunt et al. (1987) (Fig. 6.2). The flow of organic carbon or nitrogen moves from initial organic substrates (labile or resistant) to the primary decomposer, either bacteria or fungi, and then on into micro-

FIGURE 6.2. Representation of detrital food web in shortgrass prairie. Fungal-feeding mites are separated into two groups (I and II) to distinguish the slow-growing Oribatids from faster-growing taxa. Flows omitted from the figure for the sake of clarity include transfers from every organism to the substrate pools (death) and transfers from every animal to the substrate pools (defecation) and to inorganic nitrogen (N) (ammonification) (from Hunt et al., 1987, reprinted with permission).

FIGURE 6.2. Representation of detrital food web in shortgrass prairie. Fungal-feeding mites are separated into two groups (I and II) to distinguish the slow-growing Oribatids from faster-growing taxa. Flows omitted from the figure for the sake of clarity include transfers from every organism to the substrate pools (death) and transfers from every animal to the substrate pools (defecation) and to inorganic nitrogen (N) (ammonification) (from Hunt et al., 1987, reprinted with permission).

bivorous microfauna (flagellates and amoebae) or microbivorous meso-fauna (feeding on fungi) and, in turn, to omnivorous or predaceous nematodes, and on to nematode feeding mites and predaceous mites. Further predation upon the mites by ants (E. O. Wilson, personal communication) or lithobiomorph Chilopods (centipedes) is possible, although not explicitly represented by Hunt et al. (1987). There are at least eight links in the bacterial-based detrital food chain, with considerable evidence of omnivory. For example, many fungivorous mites require a nematode "supplement" to complete their life cycles (Walter et al., 1991). Note that Figure 6.2 is a rather ecosystem-specific diagram. One could draw another for decomposition in a coniferous or oak/beech forest, with a significant proportion of the total decomposition being mediated by ectotrophic mycorrhizae, operating perhaps in competition with the saprophytic fungi (Gadgil and Gadgil, 1975).

For desert and estuarine food webs, reviewed by Hall and Raffaelli (1993), the detrital food chain length noted previously is comparable to the average length of five to seven links (Polis, 1991), with maximal recorded of eight. In contrast, Hairston and Hairston (1993) assert that the usual food chain length in detrital systems seldom exceeds three. As noted later in this chapter, these long chain-lengths of five to seven links are not only feasible, but also thermodynamically possible at several times and in several locations in the soil matrix, particularly the rhizo-sphere and other "hot spots" of activity.

What levels of taxonomic resolution are both most useful and appropriate for detrital food web studies? Our inability to sort out the details of microbial taxonomy in situ (see Furlong et al., 2002, and other references in Chapter 3 for insight into molecular probing techniques in agroecosystems) and limited knowledge of many of the soil invertebrates, particularly the immature stages (Behan-Pelletier and Bissett, 1993), requires use of rather coarse functional groups for taxonomy of the soil biota. Interestingly, this sort of separation enabled Wardle and Yeates (1993) to identify competition and predation forces operative in an assemblage of detritus-microbial-nematode trophic groups in an agricultural field. Using a correlation analysis, they noted that predatory nematodes reflected most closely the changes in primary production, and the microbivorous nematodes seemed to be more dependent on substrate quality in the microbial (bacterial and fungal) community.

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