This interaction between this grazer species and
its prey can be thought of as an ‘elemental oscilla-
tor’, the basic building block for ecological commu-
nities (Vandermeer 1994). Leibold et al. (2005)
working with a more complex food web, consisting
of three grazer species (Daphnia,Ceriodaphniaand
Chydorus) and edible algae, hypothesized that the
dynamics in such complex food webs can be under-
stood in terms of the simple subsets used by
McCauleyet al. (1999). The behaviour of these
more complex food webs might be understood by
thinking of coupled oscillators consisting of many
such oscillators with interacting damping and am-
plifying harmonics (Hastings and Powell 1991).
They also found both consumer–resource and co-
hort cycles. This indicates that interactions of zoo-
plankton and algae in complex systems still consist
of the same basic elements – in this case consumer–
resource cycles and cohort cycles – and their dy-
namics can be understood from the dynamics of
their component parts. In the study with a more
complex marine web consisting of phytoplankton
and zooplankton, Benincaet al. (2008) found strong
chaotic fluctuations (Fig. 2.6). Species interactions
in this food web are indicated as the driving forces.
The persistence of this food web despite the great
density fluctuations and unpredictability of the
abundances is a rarely demonstrated phenomenon.
It may also be more common than we think. The
constant external conditions used in this study may
be an artefact in itself, causing high productivity,
leading to a situation that has been called the para-
dox of enrichment (Rosenzweig 1971). Many other
important aspects of this study that distinguish it
from real food webs under natural conditions are
the absence of higher trophic levels and the exclu-
sion of interactions between organisms and their
abiotic environment, e.g. through local nutrient de-
pletion or organism–sediment feedback. Even
though isolated modules of species may exhibit cha-
otic dynamics, this may be highly dampened or even
excluded by these effects in natural systems.
2.4 Dynamics enforced by external conditions
In addition to dynamics that arise internally within
communities, species populations are also often
subject to strong external forcing, e.g. where region-
al climatic conditions affect local air, water or soil
temperature. Ecophysiological differences among
species in ability to cope with these changes may
result in species-specific responses (Karasov and
Martinez del Rio 2007), thus leading to community
dynamics under varying external conditions. This
external forcing is the key ‘point of entry’ in study-
ing effects of climate change on food webs, but also
how toxic pollutants will affect trophic structure
and ecosystem functioning. For example, ecto-
therms (at lower trophic levels) and endotherms
(at higher trophic levels) are expected to respond
very differently to short- or long-term temperature
changes. Surprisingly, although there are good rea-
sons to suspect its importance in natural popula-
tions, e.g. in the level of synchrony between species
in long-term ecological monitoring (Bakkeret al.
1996), environmental forcing has hardly received
any attention in the study of consumer–resource
interactions, food webs or other interaction webs.
There is, however, some relevant theoretical work:
a mechanistic-neutral model describing the dynam-
ics of a community of equivalent species influenced
by density dependence, environmental forcing and
demographic stochasticity. The model shows that
demographic stochasticity alone cannot oppose the
synchronizing effect of density dependence and
environmental forces (Loreau and de Mazancourt
2008). Vasseur and Fox (2007) have shown in their
model food web study that the synchronization of
dynamics is the result of environmental fluctua-
tions. This synchrony promotes stability, because
the maximum abundance of top predators is re-
duced by the synchronous decline in the density
of consumers and synchronous increase in consum-
er density is changed by resource competition into
synchronous decline. These authors conclude that
future studies on food web dynamics should take
into account the joint action of internal feedbacks
and external forcing.
Another example concerns recovery after pertur-
bation. The reaction of a system to a perturbation
depends on the size of the ‘basin of attraction’. The
basin of attraction is a theoretical measure of the
maximal perturbation that the system can absorb
without shifting to another state and is often re-
ferred to as ‘ecological resilience’ (Petersonet al.32 DYNAMICS