It is becoming more and more imperative to bring small working groups, or teams of investigators, together to make further progress in food web studies. The real breakthroughs are certain to come from efforts that include the more transitional fauna between above- and belowground such as ants, dipteran larvae, and ground beetles, or cryp-tozoans such as the isopods, centipedes, and millipedes, linking them to the truly belowground fauna and microbes.
Various techniques noted in several papers in this volume should be extended as well. Stable isotopes, introduced in an initially enriched substrate such as labeled glucose or acetate, will be useful in delineating food webs. The effective use of carbon-13 and nitrogen-15 (15N) was reviewed extensively by Scheu (2002). An innovative use of 15N tagging in a microcosm study detected significant predation on springtails by an ectomycorrhizal fungus, Laccaria bicolor (Klironomos and Hart, 2001). The ectomycorrhizal fungus immobilized the animals before infecting them. Springtails (Folsomia Candida), alive or already dead and labeled with 15N, were added to the microcosms containing mycor-rhizal or nonmycorrhizal Pinus strobus plants. Only the fungus and not the roots made contact with the animals. Amounts of nitrogen were determined in plant tissues and extraradical fungal hyphae over a 2-month period. Up to 25% of plant nitrogen was derived from springtails when they were in the presence of L. bicolor. At the end of the experiment, less than 10% of the number of animals were present compared to at the start. Using the same system, growing Pinus strobus seedlings with a different ectomycorrhizal fungus, Klironomos and Hart (2001) measured less than 5% of plant nitrogen acquired from the springtails. This experiment demonstrates a much greater range of possible interactions between mycorrhiza and fungal-grazing animals, and is yet another example of the tight linkages existing in forest nutrient cycling.
Opportunities for use of radiotracer carbon-14 (14C) must also be kept in mind. For example, Kisselle et al. (2001), Garrett et al. (2001), Fu et al. (2001), and Coleman et al. (2002) described the detrital food web and its dynamics in an agroecosystem as a function of the impacts of above-ground experimentally induced herbivory. They measured increased microbial biomass production in no-tillage treatments that experienced moderate levels of aboveground herbivory (grasshoppers grazing on corn leaves). This was transmitted up the food chain to bacterial-feeding nematodes, with significantly more 14C activity being taken up in the low grazing-intensity treatments, similar to the findings of Holland and Cheng (1996) (Fig. 6.7). Another notable finding was the higher 14C activity in microarthropods extracted from rhizospheres of weed plants, compared to that of corn (Fig. 6.8). Garrett et al. (2001) suggest that weed rhizospheres may be more important than crop rhizospheres in supporting soil food webs. This might be expected, because crop plants are selected to maximize their aboveground net primary production (NPP), unlike weeds. If this pattern is general, weeds may be a significant factor for the protection of soil biodiversity, especially in conventionally tilled agroecosystems. The linkages between above- and belowground food webs is an exciting new topic for the first decade of the 21st century (Hooper et al., 2000; Wolters et al., 2000).
Hall and Raffaelli (1993) suggest two major areas of food web research that would be most beneficial to follow: (1) focusing on community assembly and (2) documenting the strength of trophic interactions between elements in webs. Examining the latter objective, Neutel et al. (2002) studied interaction strengths organized in trophic loops (defined as the product of interaction strengths in a food web). Using seven documented soil food webs, Neutel et al. (2002) introduced the term "loop weight," which is the geometric mean of the absolute values of the interaction strengths in the loop (Fig. 6.9). This enables one to compare loops of different lengths and to use the maximum of all loop weights as an indicator for matrix stability. They used the conservative figure of 0.1 for
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