Community Ecology Processes, Models, and Applications

1998; Folkeet al.2004). Systems with a high ecolog-
ical resilience can be seen as particularly stable
systems, whereas systems with a low ecological
resilience can be seen as unstable systems close to
the tipping point into an alternate state. Recent
studies have explored whether there are early
warning signals for this shift into another state.
The slow recovery from perturbations may be a
possible indicator of an impending state shift (van
Nes and Scheffer 2007).

2.5 Equilibrium biomass at different productivities

Important lessons can be learned from comparing
the configuration of food webs (abundances of dif-
ferent species on different trophic levels) under
different external conditions. A simple approach
that has been used is to compare systems subject
to different boundary conditions that impose con-
straints on their level of primary production. Such
differences can be caused by variation in tempera-
ture or nutrient input into the system.
This approach can be used to study simplefood
chainsconsisting of a predator, a consumer and a
resource species. This approach can also be used for
simple systems with three levels, or for systems

where all species at one trophic level are lumped

into ‘trophic species’, under the assumption that the

‘food chain module’ in the system (see Fig. 2.1)

strongly overrules the dynamics of the system

with respect to other modules (whether this is true

is, however, an open question). This approach to

simplify systems leads to the concept oftrophic

cascades. Hairstonet al.(1960) and Fretwell (1977)

mention this trophic cascade phenomenon in their

writings about population regulation, although

they did not call the process by this particular

name (Morin 1999). Paine (1980) used the term

trophic cascadeto describe how the top-down effects

of predators could influence the abundances of

species in lower trophic levels, a concept that was

developed further by Oksanen et al. (1981)

(Fig. 2.7a). If we increase the length of thistrophic

food chainby adding one additional trophic level

(top carnivores) (Fig. 2.7b), indirect mutualism

between non-adjacent trophic levels and a decrease

or constancy in the abundance of ‘odd’ levels with

increasing productivity appear.

Evidence for top-down trophic cascades is sur-

prisingly scarce, and comes primarily from aquatic

systems: stream communities (Poweret al.1985)

and lakes (Carpenter and Kitchell 1993). There are

few terrestrial examples: Emmons (1987), Terborgh

Primary production

Primary producers

Primary consumers

Secondary consumers

Abundance

Primary production

0

0

0

max

max

max

0

0

0

max

max

max

Primary producers

Primary consumers

Secondary consumers

Tertiary consumers

(a) Three trophic levels (b) Four trophic levels

P

C1

C2

C3

Figure 2.7Equilibrium of three (a) and four (b) trophic levels along a gradient of primary productivity according to the
ecosystem exploitation theory of Oksanenet al. (1981).

TROPHIC DYNAMICS OF COMMUNITIES 33