et al. (2006), Marquis and Whelan (1994) and
Jefferies (1999). For example, Emmons and Terborgh
et al. describe the disappearance of several common
species of birds from Barro Colorado Island. The
building of the Panama Canal created an island
that was too small to sustain large predators, such
as jaguars and pumas. Their extinction led to popu-
lation increases of their prey species. These meso-
predators fed on the eggs and young of ground-
nesting birds and their increase in numbers was
sufficient to wipe out many bird populations.
Another example concerns a terrestrial trophic
cascade from a study by Marquis and Whelan
(1994). They found strong effects of insectivorous
birds foraging herbivorous insects on white oak
trees. Birds significantly reduced the abundance of
herbivorous insects on the oaks. Netting around
some trees caused exclusion of birds from the in-
sects, while other uncaged trees remained available
to the birds. Oaks with birds and reduced herbivo-
rous insects had less leaf damage from insects and
subsequently had a higher biomass.
A slight modification of simple trophic cascades
leads to intra-guild predation (IGP; Poliset al.1989).
This type of interaction can be seen as an extension
of a simple predator–prey interaction: the predator
also eats some of the consumers, but potentially
competes with uneaten consumers as well for a
common resource. Spiller and Schoener (1989)
studied interactions between predatory Anolis
lizards, predatory web-building spiders and their
arthropod prey on small islands in the Bahamas.
The interaction between lizards and spiders can be
described as an IGP interaction, because lizards eat
some spiders, but lizards also potentially compete
with uneaten spiders for small arthropod prey.
There is even an effect on the level of anti-herbivore
defence of the dominant vegetation on the islands,
Conocarpus erectusor buttonwood (Schoener 1988).
On islands without lizards the leaves have tri-
chomes to discourage insect attack; on islands
with lizards, leaves are without them.
Summarizing we can state that in small food
webs, oscillating consumer–resource interactions
are not only predicted by models but also occur in
natural systems. In chains of three or more levels
trophic cascades are important, but experimental
support is limited.
2.6 Dynamics of complex interactions
Food webs are conceptualized by their basic unit of
interaction, consumption, and this basic process is
oscillatory. When these basic units are connected,
the conceptual framework becomes a system of
coupled oscillators.This concept generated notable
patterns in the theoretical literature as described
by, for example, Vandermeer (2004). The conclu-
sion that weak interactions can have strong effects
on stabilizing ecosystems (McCannet al.1998; Neu-
telet al.2002) derives from this concept of coupled
oscillators. According to May (1973), measures of
interaction strength are the elements in a communi-
ty matrix at equilibrium, which represent the direct
effect of an individual of one species on the total
population of another species at equilibrium. These
results suggest that average interaction strength
should be weak in species-rich, highly connected
systems. The fundamental question is whether
the configuration or distribution of interaction
strengths within food webs is important for ques-
tions of community stability. de Ruiteret al.(1995)
linked the differing approaches by deriving values
of the matrices from empirical observations
(Fig. 2.8). The data come from a terrestrial food
web study, the Lovinkhoeve Experimental Farm
(Integrated Management) in The Netherlands.
This study indicates that the patterning of interac-
tion strengths is essential for system stability. How-
ever, there is no direct correlation between
interaction strength and stability. Weak interactions
may be strong in terms of their stabilizing effects to
the community. Further, it has been shown that
long trophic loops contain relatively many weak
links increasing food web stability because they
reduce maximum loop weight, thus reducing the
amount of intraspecific interaction needed for
system stability (Neutelet al. 2002) (Fig. 2.9).2.7 Conclusions
The types of dynamics leading to long-term
co-occurrence of species in communities can be
indicated as stable equilibria, alternate equilibria,
stable limit cycles and chaotic dynamics. Although
much of the attention to this subject is theoretical,
empirical data are increasingly reported, although34 DYNAMICS