4.2 DYNAMIC FRAGILITY OF MODEL FOOD WEBSANOTHER POPULAR IDEA HAS BE...

20.4.2 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

Figure 20.16 Sets of model food webs, the dynamics of which

most stable food chains could persist. In particular, when Pimm

were examined to determine the effect of food chain length on

and Lawton (1977) examined variously structured four-species

stability having accounted for variations in the number of species

Lotka–Volterra models (Figure 20.16a), webs with more trophic

and the number with self-limitation ( ). (a) The original set

levels had return times after a perturbation that were substantially

examined by Pimm and Lawton (1997). (b) Six-species, four-level

longer than those with fewer levels. Because less resilient sys-

webs with varying degrees of self-limitation. (c) Six-species webs

tems are unlikely to persist in an inconstant environment, it was

of self-limited species with varying numbers of trophic levels and

argued that only systems with few trophic levels will commonly

species concentrated in the basal level. (d) Eight-species webs of

be found in nature. However, these models had self-limitation

self-limited species with varying numbers of trophic levels and

(effectively, intraspecific competition) only at the lowest trophic

species dispersed among the levels. (e) Eight-species webs of self-

level, and food chain length and the proportion of self-limited

limited species with varying numbers of trophic levels and species

species was therefore confounded (Figure 20.16a). When a wider

concentrated in the basal level. (After Sterner et al., 1997a.)

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

carnivores already there? The herbivores are more abundant

chains when the number of species and the number of self-limited

and less well protected. The advantage to feeding low down in

the food chain can readily be seen. Of course, if all species did

species were held constant. Overall, there is no convincing case for

dynamic fragility affecting the length of food chains significantly.

this, competition would intensify, and feeding higher in the food

chain could reduce competition. But it is difficult to imagine

a top predator sticking religiously to a rule that it should prey