20.4 Empirical patterns in food webs:
between resilience and energy input per unit standing crop. This
the number of trophic levels
seems to depend in part on the relative importance of hetero-
trophs in the system. The most resilient system, the pond, had a
In the previous section, we examined very general aspects of
biomass of heterotrophs 5.4 times that of autotrophs (reflecting
the short life and rapid turnover of phytoplankton, the dominant
food web structure – richness, complexity – and related them to
the stability of food webs. In this section, we examine some more
plants in this system), whilst the least resilient tundra had a hetero-
specific aspects of structure and ask, first, if there are detectable
troph : autotroph ratio of only 0.004. Thus, the flux of energy
repeated patterns in nature, and second whether we can account
through the system has an important influence on resilience.
for them. We deal first, at greatest length, with the number
The higher this flux, the more quickly will the effects of a per-
of trophic levels, and then turn to omnivory and the extent to
turbation be ‘flushed’ from the system. An exactly analogous
which food webs are compartmentalized.
conclusion has been reached by DeAngelis (1980), but for nutrient
Scotch Broom
Grassland
Ythan 1
Ythan 2
(S = 83, C = 0.06)
(S = 61, C = 0.03)
(S = 124, C = 0.04)
(S = 85, C = 0.03)
0.8
0.6
0.4
0.2
0
El Verde
Canton
Stony
Chesapeake
(S = 155, C = 0.06)
(S = 102, C = 0.07)
(S = 109, C = 0.07)
(S = 31, C = 0.07)
St Marks
Little Rock
Lake Tahoe
St Martin
(S = 48, C = 0.10)
(S = 42, C = 0.12)
(S = 92, C = 0.12)
(S = 172, C = 0.13)
Cumulative secondary extinctions / SSkipwith
Chachella
Bridge Brook
Mirror
(S = 172, C = 1.15)
(S = 25, C = 0.17)
(S = 29, C = 0.31)
(S = 25, C = 0.32)
0 0.2 0.6
Species removed / S
Most connected
Most connected, no basal deletions
Random
Least connected
Figure 20.13 The effect of sequential species removal on the number of consequential (‘secondary’) species extinctions, as a proportion
of the total number of species originally in the web, S, for each of 16 previously described food webs. The four different rules for species
removal are described in the key. Robustness of the webs (the tendency not to suffer secondary extinctions) increased with the connectance
of the webs, C (regression coefficients for the four rules: − 0.62 (NS), 1.16 (P < 0.001), 1.01 (P < 0.001) and 0.47 (P < 0.005)). Overall, though,
robustness was lowest when the most connected species were removed first and highest when the least connected were removed first.
The origins of the webs are described in Dunne et al. (2002). (After Dunne et al., 2002.)
A fundamental feature of any food
from a basal species to a species that feeds on it, to another species
food chain length
web is the number of trophic links in
that feeds on the second, and so on up to a top predator (fed on
by no other species). This does not imply a belief that commun-
the pathways that run from basal species to top predators.
ities are organized as linear chains (as opposed to more diffuse
Variations in the number of links have usually been investigated
webs); rather, individual chains are identified purely as a means
by examining food chains, defined as sequences of species running
a predator that eats the herbivore, and a top predator that eats
the intermediate predator. Assume the top predator is an eagle.
13
12
Even without collecting the data, it is all but certain that the eagle
is attacked by parasites (perhaps fleas), which are themselves
attacked by pathogens. But the convention is to describe the chain
11
as having four trophic levels. Indeed, descriptions of food webs
generally have paid little attention to parasites. There is little doubt
that this neglect will have to be rectified (Thompson et al., 2005).
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