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
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