Dynamic fragility of model food webs

Another popular idea has been that the length of food chains is limited by the lowered stability (especially resilience) of longer chains. In turn, we might then expect food chains to be shorter in environments subject to greater disturbance, where only the most stable food chains could persist. In particular, when Pimm and Lawton (1977) examined variously structured four-species Lotka-Volterra models (Figure 20.16a), webs with more trophic levels had return times after a perturbation that were substantially longer than those with fewer levels. Because less resilient systems are unlikely to persist in an inconstant environment, it was argued that only systems with few trophic levels will commonly be found in nature. However, these models had self-limitation (effectively, intraspecific competition) only at the lowest trophic level, and food chain length and the proportion of self-limited species was therefore confounded (Figure 20.16a). When a wider range of food webs was examined with self-limitation distributed more systematically (Figure 20.16b-e) (Sterner et al., 1997a), there was a weak but significant increase in stability in longer food chains when the number of species and the number of self-limited species were held constant. Overall, there is no convincing case for dynamic fragility affecting the length of food chains significantly.

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