Hierarchical Approach To Organisms In Soils

Because of the need to deal with soil heterogeneity in space and time, arenas of interest, noted in the previous section, are represented in

Figure 6.4 (Beare et al., 1995) showing the volumes and biotic groups of concern. The aggregatusphere shows bacteria, amoebae, and some nematodes, having varying degrees of success in gaining access to the prey biota of interest (Vargas and Hattori, 1986). Moving up to a coarser level of resolution, to the rhizosphere, a few millimeters or less in scale one sees the microbes and fauna associated with them, and the considerable feeding and activity which has been documented numerous times. The activities are strongly influenced by abiotic, i.e., wetting and drying events, and the intrusion of new organic substances from growing root tips (Cheng et al., 1993; Kuzyakov, 2002), or deposited feces from microarthropods, enchytraeids, or other mesofauna. The next level of resolution expands from many centimeters to several meters across the landscape, when any of the macrofauna such as earthworms or burrowing beetles come into play. There is then a qualitative shift, brought about by the ingestion of soil, which includes considerable amounts of micro- and mesobiota, that is, protozoa and nematodes (Yeates, 1981; Piearce and Phillips, 1980) as food. Interestingly, even with earthworms, the drilosphere sensu stricto is only 2-3 millimeters in thickness (Bouché, 1975) but the burrow extends laterally for many centimeters or meters through the soil. As a consequence of this activity, there can be major short-term decreases in viability of the existing biota, but possibly longer-term stimulation by enhanced microbial activity, as noted previously, and also from the considerable input of mucopolysaccharide-containing mucus (Marinissen and Dexter, 1990).

An additional aspect of altered species makeup of bacteria in earthworm-influenced soils has been explored using molecular probing techniques. Using 16S rRNA probes and "libraries" of soil bacteria at the Horseshoe Bend site in Athens, GA, Furlong et al. (2002) and Singleton et al. (2002) found enhanced percentage occurrences of Actinobacteria, Firmicutes, and gamma-Proteobacteria in castings of Lumbricus rubel-lus, an epigeic earthworm.

In addition, considerable amounts of ammonia and urea, as nitrogenous end-products of metabolism, may be voided either externally through nephridiopores, or internally into the gut cavities of earthworm genera that have that mode of nitrogen excretion (Lavelle et al., 1992). In tropical regions, certain endogeic earthworms will process and assimilate end-products of the breakdown (from 2 to 9%) of soil organic matter in a wide range of ecosystems (Lavelle and Martin, 1992).

Similar sorts of activities may be catalyzed by certain termites, particularly those in the advanced family Termitidae, which are truly geophagous. These geophages utilize soil organic matter, deriving significant amounts of nutrition from this low-quality substrate by processing the organic matter in a high-pH chemical milieu in the region between the midgut and the first proctodaeal segment (see the Isoptera section in Chapter 4) (Bignell, 1984; Bignell et al., 2000). The additional influence of microbial enzymes on insect digestive processes and, indeed, enhancement of nitrogen fixation in downed branches and logs (Martin, 1984) are well known. Finally, the impacts of ant and termite nests are significant, and certainly have an influence at the landscape scale. The impacts of the macrofauna, sometimes termed "ecosystem engineers" (Jones et al., 1994), can extend for many meters beyond the immediate zones that they occupy. It has been contrasted with the impacts of smaller fauna, with smaller fauna more influential in energy flow and immediate nutrient recycling, noted previously, versus the longer-term effect of the "engineering" by the macrofauna (Scheu and Setala, 2002) (Fig. 6.5). Scheu and Setala (2002) and Wardle (2002) note that "trophic cascades," the term denoting the effects of predation on the biomass of organisms at least two trophic levels removed, occur in soil systems. Although developed principally for systems with living net primary production as the energy base, there are numerous examples in soil systems, particularly ones dominated by fungi. Scheu and Setala (2002) comment on the limited number of studies of trophic cascades in soil systems to date, and that the fungal-based energy channel may be

MICROFLORA MICROFAUNA MESOFAUNA MACROFAUNA Body diameter < 2 |im 2-100 |im 0.1-2 mm > 2 mm

Trophic interaction actual

Predation

Predation

Grazing

Predation

Substrate processing

Energy flow

"Engineering" long-term

Habitat formation

Grazing

Substrate processing

Pore formation Litter fragmentation, Pore formation

Litter fragmentation, bioturbation

Pore formation Litter fragmentation, Pore formation

Litter fragmentation, bioturbation

FIGURE 6.5. Size dependent interactions among soil organisms. Trophic interactions and interactions caused by "engineering" are separated; both are indicated by arrows. Note that trophic interactions and interactions caused by engineering are strongly size dependent but complement each other (tapering and widening triangles). Both function at different scales: trophic interactions drive the current energy flow, engineering sets the conditions for the existence of the soil biota community in the long term (from Scheu and Setala, 2002).

much more prone to trophic cascades than the bacterial-based channel. This assertion is certainly a candidate for further experiments in the future.

Conceptualizations of detrital food webs are undergoing a considerable shift early in the third millennium. Following up on earlier ideas of Wardle (1995) and Lavelle et al. (1999), Pokarzhevskii et al. (2003) note that a definite nested element exists such that different compartments feed into others. For example, the bacteria-algae-protozoa compartment is nested inside a fungi-microarthropod compartment, and this in turn is contained within an earthworm-rhizosphere compartment. Animals at higher levels consume communities of the lower levels as a whole (Pokarzhevskii et al., 2003) (Fig. 6.6). This arises from the dependence of all animals on microorganisms for their supply of proteins and

FIGURE 6.6. A conceptual scheme illustrating the nested structure of detrital food webs. A distinction is made between bacteria-algae-protozoa communities (left), fungi-microarthropod communities (right), and earthworm-plant communities (top). Communities of the higher levels consume communities of the lower levels as a whole (indicated by arrows) (from Pokarzhevskii et al., 2003).

FIGURE 6.6. A conceptual scheme illustrating the nested structure of detrital food webs. A distinction is made between bacteria-algae-protozoa communities (left), fungi-microarthropod communities (right), and earthworm-plant communities (top). Communities of the higher levels consume communities of the lower levels as a whole (indicated by arrows) (from Pokarzhevskii et al., 2003).

scarce minerals. The concept of "ecological stoichiometry," which concerns the roles of interactions between several major nutrients such as nitrogen, phosphorus, and/or sulfur, has been discussed at length by Sterner and Elser (2002). Much of Pokarzhevskii et al.'s (2003) paper discusses the need to consider the effects of limiting nutrients, which may be in shorter supply than the carbon or energy that characterize the outlook of many of the previously developed detrital food webs.

Was this article helpful?

0 0
Oplan Termites

Oplan Termites

You Might Start Missing Your Termites After Kickin'em Out. After All, They Have Been Your Roommates For Quite A While. Enraged With How The Termites Have Eaten Up Your Antique Furniture? Can't Wait To Have Them Exterminated Completely From The Face Of The Earth? Fret Not. We Will Tell You How To Get Rid Of Them From Your House At Least. If Not From The Face The Earth.

Get My Free Ebook


Post a comment