Community Ecology Processes, Models, and Applications

feedbacks (DeAngeliset al.1986), regulatory feed-
backs (Bagdassarianet al.2007), trophic cascades
(Carpenteret al.2008) and multiple stable state
dynamics (Scheffer and Carpenter 2003). These
may all be considered organizational forces that
structure food webs, but they may not all be of
equal importance. For example, Ulanowicz (1997)
makes a strong case for the special importance of
indirect mutualism as an organizational force in
food webs, as the resulting feedback loops ‘attract’
resources towards them. Other authors, such as Til-
man (1982), have emphasized the importance of
competition as a key organizational force in ecolog-
ical communities. Again others, such as Krebset al.
(1999) emphasize the importance of predator–prey
interactions in structuring communities. Despite
the insights gained into such specific processes,
the question remains how such modules together
organize into complex interaction webs, and how to
address their relative importance.
Food web modules are characterized by the fact
that they are small systems (two or three trophic

levels) that possess explicit dynamics (Fig. 2.1a–e).

With these simple ‘building blocks’, more realistic

food webs can be ‘built’ (Fig. 2.1f), which in turn are

subsets of the true complexity in trophic interac-

tions found in real ecosystems. For example,

Fig. 2.2 shows the network of trophic interactions

as found on intertidal sand flats in the Wadden

Sea, a soft-bottom intertidal ecosystem with com-

plex trophic structure. This example shows how

exploitative competition, food chains, apparent

competition and intra-guild predation can operate

simultaneously within the same ecosystem. In this,

it should be realized that food web descriptions in

terms of interaction topology and flows (as in

Fig. 2.2) generally capture the long-term averages

of organism densities and fluxes. The actual abun-

dances may vary due to external drivers (such as

varying weather conditions) and internal dynamics

(e.g. limit cycles). To illustrate this point, Fig. 2.3

shows observations of the long-term population

dynamics of some of the bivalve species shown

in the food web of Fig. 2.2. In this case, winter

(a) Food chain (b) Omnivory

Resource

Prey–
consumer

Predator

(c) Apparent

competition

(d) Exploitative

competition

(d) Predation

on competing

prey

(e) Intraguild

predation

(f) A fraction of a food web with

several interrelated modules

Figure 2.1(a–f) Examples of trophic modules that are found in ecological communities. After Holt (1997) and
Bascompte and Melian (2005).

TROPHIC DYNAMICS OF COMMUNITIES 27