CHAPTER 2
Trophic dynamics of communities
Herman A. Verhoef and Han Olff
2.1 What types of dynamics can be
distinguished?
Having examined the geometry and structure of
the ecological networks in Chapter 1 we will move
on to consider the dynamics of communities, and
concentrate on the analysis of changes in the abun-
dance of species in a multitrophic context. In 1927
Charles Elton stated that the structure of a com-
munity is determined by the net of feeding rela-
tions between trophic units, thefood web. The
topology of these feeding links (Chapter 1) natu-
rally emerges from thedynamicsof populations
within ecological communities (May 1973; Neutel
et al.2002, 2007; Rooneyet al.2006), while the
topology of the network in turn will affect the
dynamics of the populations it contains (DeAnge-
lis 1992).
One of the central goals in ecology is to discover
why populations change over time. Much of the
attention to this important subject is theoretical
and the findings and consequent discussions are
based on models outcome. However, empirical
data on the different types of dynamics and the
underlying mechanisms are increasingly reported.
Four main patterns of population dynamics leading
to coexistence (or long-term co-occurrence) of dif-
ferent species can be identified: coexistence at equi-
librium, coexistence at alternate equilibria (with
critical ‘tipping points’), coexistence at stable limit
cycles and coexistence at chaos.
2.1.1 Stable equilibria
Only a few studies address whether collections
of multiple species show stable compositions
corresponding to stable equilibria in mathematical
models. Resource-based competition theory that
makes such predictions for multiple species
(Tilman 1982) seems not to hold for more than
two species competing for two resources (Huis-
man and Weissing 1999, 2001). Also, the life span
of the organisms is often too long compared with
the length of the study, making the judgement of
true coexistence across multiple generations prob-
lematic (Morin 1999). A good example is presented
by Lawton and Gaston (1989). Despite natural per-
turbations affecting the community of about
20 herbivorous insects living on bracken fern, the
relative abundances of the species and the taxo-
nomic composition remained the same over a
period of 7 years. The generation time of the
respective populations is about 1 year. Similarly,
in a 25-year-long study of large herbivore coexis-
tence in a tropical savanna, Prins and Douglas-
Hamilton (1990) found high community-level
stability in species composition, despite fluctua-
tions in abundance of individual species. A more
recent study deals with a small microbial food
web. Changes in the dynamics of a defined preda-
tor–prey system, consisting of a bacterivorous cili-
ate (Tetrahymena pyriformis) and two bacterial prey
species, were triggered by changes in the dilution
rates of a one-stage chemostat. The bacterial spe-
cies preferred by the ciliate (Pedobacter)outcom-
petedBrevundimonas, the second bacterial species.
At relatively high dilution ratesBrevundimonas
died off by the sixth day, whereas the remaining
species existed in stable coexistence at equilibrium
(Beckset al.2005).Alsointhisexperimentthe
generation time of the organisms involved was
much shorter than the duration of the experiment.25