A

FIGURE 6.8. Concentrations of 14C (Bq min-1mg-1) in shoots, roots, and microarthropods from rhizospheres of corn and weed plants grown under no-tillage (NT) and conventional tillage (CT) regimes (from Garrett et al., 2001).

trophic transfer efficiencies, from one trophic level to another. They innovatively compared the community matrix, including the patterned interaction strengths ("real matrices") with several randomizations of this matrix ("randomized matrices"). This was done by randomly exchanging predator-prey pairs of interaction strengths, keeping these pairs intact and preserving the sign structure of the matrix. Stability was measured as the minimum degree of relative intraspecific interaction needed for matrix stability (s). Matrices with a smaller s value were considered "more stable."

Loop weights of the longer loops were low in the real matrix and tended to be heavier in the randomized matrices than the shorter loops [Fig. 6.9(a), 6.9(b)]. Interestingly—although absolute values of effects of predators on their prey are generally two orders of magnitude larger than effects of prey on their predators, as shown in the randomized matrices—in the real matrices the long loops with many top-down

Predaceous .mites

Omnivorous nematodes A

Omnivorous nematodes

Predaceous .mites

Omnivorous nematodes

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