Community Ecology Processes, Models, and Applications

temperatures are thought to control abundances
through their effect on recruitment (Beukemaet al.
2001).
Not all modules will be equally important in
every ecosystem. For example, ecosystems that are
dominated by many species within the same tro-
phic level, such as diverse grasslands, may be
strongly structured by exploitative competition.
On the other hand, ecosystems such as the marine
pelagic zone seem dominated by trophic chains.
Bascompte and Melian (2005) recently compared
the frequency of different types of modules across
natural food webs. They found that apparent com-
petition and intra-guild predation (Fig. 2.1) were
generally overrepresented with respect to a suite
of null models, while the level of omnivory varied
highly across ecosystems.

The relative importance of external forcing ver-

sus internal dynamics as causes of dynamics in

food webs under natural conditions is still hard to

assess, despite a long history of research on the

subject (Pimm 1982, 1991; Loreau and de Mazan-

court 2008). Various research lines can be distin-

guished here, depending on their theoretical

versus experimental nature, the complexity of the

system under study and the level of control of

variation in external conditions (exclusion or inclu-

sion of forcing factors). Some theoretical studies

have investigated the dynamics of species

organized in simple modules (DeAngelis 1992).

Several studies have been performed under experi-

mentally controlled conditions, in which the influ-

ence of external variation on populations has been

mostly eliminated, e.g. as in the study by Beninca

12

Mya

arenaria

12

Cerastoderma

edule

12

Arenicola

marina

12

Lanice

pygospio

12

Macoma

balthica

12

Hydrobia

ulva

12

Scolopus

armiger

12

Meiobenthos

12 small

polychaete

worms

20

Nephtys

hombergii

20

Pomatoschistus

minutus

25

Calidris

alpina

20

Crangon

crangon

25

Phalacrocorax

carbo

20

Pluvialis

apricaria

20

Haematopus

ostralegus

20

Larus

ridibundus

Birds

Fish

Crustaceans Unicells

Bivalves

Snails

Worms

20

Numenius

arquata

20

Limosa

lapponica

20

Somateria

mollissima

20

Calidris

canutus

0.1–1

Carbon flow mgC m–2 d–1

1–10

10–100

>100

20

Larus

canus

8

capitellidae

2

sediment

bacteria

3+5+7

sediment

POC

3+5

suspended

POC

9

micro-

phytobenthos

9

phytoplankton

Figure 2.2Example of a real food web, as observed on intertidal sand flats in Wadden Sea near the German island of
Sylt, showing how the different types of modules from Fig. 2.1 can together form a complex network of interactions.
POC, particulate organic carbon. Data from Bairdet al.(2007).

28 DYNAMICS