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o246 Maximum loop weight (yr-1)

FIGURE 6.9. Loop length, loop weight, and stability in the Central Plains Experimental Range (CPER) food web and randomizations of this matrix. (a) Loop weight versus loop length in the real matrix. (b) Loop weight versus loop length in a randomized matrix (a typical example). Long loops with a relatively small weight—those with many bottom-up effects—are not shown because they are not relevant for maximum loop weight. (c) Maximum loop weight and stability of the real matrix (solid diamond) and of 10 randomized matrices (open diamonds). Stability was measured as the value s that leads to a minimum level of intraspecific interaction strength needed for matrix stability. In a sensitivity analysis, variation in the parameter values within intervals between half and twice the observed value led to only a small variation in stability (from Neutel et al., 2002).

o effects had a relatively low weight. This revealed that not all top-down effects were equal. With the maximum loop weight in the real matrix being markedly lower, the real matrix was much more stable [Fig. 6.9(c)]. Neutel et al. (2002) explored the ramifications of omnivory. For a three-species omnivorous interaction (Fig. 6.10), the omnivore feeds on two prey types, which are at different trophic levels. Assuming that it feeds according to prey abundance, and that the biomass of the prey on

(b)

FIGURE 6.10. Interaction strengths and loop weights in an omnivorous food web. (a) Equilibrium feeding rates and population sizes. Feeding rates were assumed to be proportional to the population sizes of the prey. (b) Interaction strengths. In the example, efficiencies were assumed to be 0.1 for all species. The loop weights of the two loops of length 3 are ([-0.5] x [-5] x 0.05)1/3 = 0.5 (anticlockwise loop, starting with the omnivores) and ([-5] x 0.05 x 0.05)1/3 = 0.23 (clockwise loop). The relatively small top-down effect (-0.5) keeps the weight of the loop with two top-down effects relatively low (from Neutel et al., 2002).

the lower trophic level is significantly larger than that of the prey on the higher trophic level, then the omnivore feeds largely on the lowest trophic level. Consequently, it exerts a relatively large top-down effect on its lowest prey and relatively small top-down effect on its higher prey, because the top-down effect is the feeding rate per unit of predator biomass. This approach was extended further to a wide range of published food webs, and their findings held true even for aboveground-oriented food webs.

In a seminal review paper, Moore et al. (2003) noted that predators within the rhizosphere alter the interactions between microbes and plants in two contrasting but probably equally important ways. Predators regulate their prey in a traditional "top-down" fashion but in doing so, they alter the release of nutrients that may limit plant productivity and thereby affect plant growth in a "bottom-up" fashion as well. They note that the interdependence between the aboveground and below-ground realms can be explained in terms of the patterning of trophic interactions within the rhizosphere and the influence of these interactions on the supply of nutrients and rates of nutrient uptake by plants.

We suggest that a useful approach will include a melding of the two objectives named earlier, in terms of documenting the extent of soil food webs, the relative impacts of the trophic interactions at the various hier archical levels of organization, and the location in the landscape (Coleman and Schoute, 1993; Hooper et al., 2000). For example, does the soil system in the absence of earthworm or termite activity operate at more or less of a background or maintenance level? When the macrofauna move through the soil matrix, literally consuming and chaotically reassembling it, does this represent a more intensive level of activity? Certainly it is at a different level of resolution, but one dependent upon the myriad interactions of the microbes and the micro- and mesofauna.

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