et al. (2008). Such studies are expected to mostly
show dynamics that arise from the internal struc-
ture of food webs, and have been performed with
both small modules and more complex webs. Other
studies have explored the dynamics of interacting
species populations under field conditions, which
allows the assessment of the importance of external
forcing factors. However, there is a necessary trade-
off here.
2.3 Internal dynamics in food web modules or simple webs
In theory, consumer–resource interactions consist
of two trophic levels that can fluctuate for a long
time, but can also lead to unstable dynamics, with
the precise type of dynamics depending on model
formulation (Fig. 2.4). A good example of a study
of a simple consumer–resource interaction is that
of the limnetic crustacean zooplankton species
Daphniaand its edible algal prey. McCauleyet al.
(1999) found large- and small-amplitude cycles
in the same global environment, i.e. consumer–
resource (predator–prey) and cohort (stage-
structured) cycles (Fig. 2.5). In cohort cycles, de-
mographic stages (usually thought to be the juve-
nile stage in Daphnia) are capable of strongly
suppressing the other stages (adults) by compet-
ing for food. As the suppressing stage matures or
dies, a pulse of reproduction or growth follows in
theotherstage,causingacyclingstronglyoutof
phase (McCauleyet al.1999).Year YearNumber of individuals. m–2(a) Cerastoderma edule (b)Mya arenaria(c)Mytilis edulis (d) Macoma baltica1970 1975 1980 1985 1990 1995 1970 1975 1980 1985 1990 19951970 1970 1975 1980 1985 1990 19956005004003002001000600500400300200100010008006004002000100012001400800
600
400
200
0
1975 1980 1985 1990 1995Figure 2.3(a–d) Long-term population dynamics of some of the bivalve species shown in Fig. 2.2, as observed on the
tidal flats of the western Wadden Sea in the Netherlands. Reproduced with permission from Beukemaet al.(2001).
TROPHIC DYNAMICS OF COMMUNITIES 29