Community Ecology Processes, Models, and Applications

prey species differing in competitive ability and
edibility (Leibold 1996).

8.5 Interaction strengths and dynamic stability in marine food webs

8.5.1 Conceptual background

Predicting the cascading indirect effects of predator
depletion in food webs depends both on the topo-
logical structure of webs, which the previous section
showed are changing systematically, and on the
distribution of interaction strengths. One approach
to the challenge of scaling up ecological processes to
real ecosystems uses empirical data on topology
and interaction strengths to explore dynamic re-
sponses to perturbations through simulation and

modelling (de Ruiteret al. 2005). A principal chal-

lenge is the paucity of data on interaction strengths

to parameterize models realistically (Wootton and

Emmerson 2005). Most existing food webs are ‘con-

nectance webs’ or ‘energy webs’, based on qualita-

tive trophic links between species or on patterns of

energy flow, respectively. Paine (1980; see also Raf-

faelli and Hall 1996) showed that links in the food

web that are important energetically are often not

the same links that are functionally (dynamically)

important, i.e. that have strong impacts on the struc-

ture and organization of the community and, by

inference, on ecosystem processes. It is the latter,

functional food web structure that is of primary

interest in understanding a system’s stability and

response to perturbations (Berlowet al. 2004; Woot-

ton and Emmerson 2005).

25

20

15

10

5

0

0.000

1.0

0123

0.8

0.6

0.4

0.2

–0.005–0.010

Interaction strength

ac

b

Number of species

Interaction strength

% of interactions

–0.015

Figure 8.7Skewed distributions of interaction strengths in (a) 45 herbivore species feeding on sporelings of giant kelp in
California (reproduced with permission from Sala and Graham 2002), (b) three bird species feeding on 23 species of
intertidal animals in Washington, USA (reproduced with permission from Wootton 1997) and (c) fishes feeding on
diverse prey in a large Caribbean coral reef food web (reproduced with permission from Bascompteet al. 2005). Note
that, in all food webs, only a small percentage of interactions have strong effects (thickness of arrows is proportional to
interaction strength in (c)).

STRUCTURE AND FUNCTIONING OF EMERGING MARINE COMMUNITIES 109