CHAPTER 1
The topology of ecological
interaction networks: the state of
the art
Owen L. Petchey, Peter J. Morin and Han Olff
1.1 Introduction
1.1.1 What do we mean by the ‘topology’ of ecological networks?
Many diverse systems can be described as a network
of linked nodes (Barbarasi 2002); for example, trans-
portation networks of cities linked by roads, net-
works of interacting populations on a landscape,
networks of interacting individuals in a social net-
work, networks of neurones linked by synapses,
networks of interacting genes in the genome or net-
works of biochemical transformations within the
cell. In the paradigm used in this chapter, species
are the ‘nodes’ of ecological networks, and inter-
actions among species are the links.
Ecological communities are immensely complex;
variation exists at every level of organization (in-
dividuals, populations, species) and these entities
interact with each other in any number of ways
(consumption, competition, mutualism, facilitation,
modification). Ecological networks provide a tract-
able simplification of this complexity – they can be
constructed, modelled, experimentally manipu-
lated and analysed with available tools and re-
sources (Proulxet al.2005). This chapter focuses
on the networks of interactions among the species
in ecological communities to highlight the advances
in ecological understanding that research about
these networks can provide.
The links in ecological networks can be charac-
terized through three main aspects: (1) their topol-
ogy, (2) their geometry and (3) the direction and
strength of their interactions (Fig. 1.1). The topology
of an ecological network is a description of the
patterns of interactions (‘who interacts with
whom’). Topology is thus the study of the arrange-
ment of links from an information/organization
perspective. The distances and angles between
nodes have no meaning for the topology; they are
often chosen so that the network can be convenient-
ly graphically represented (Fig. 1.1a). If the geome-
try of the network is included, the Euclidean
distance between nodes has a meaning (Fig. 1.1b);
for example, in the genetic or trait similarity of
species or as the physical distance between species
within a landscape (e.g. Thompsonet al.2001) such
as in a metacommunity context (Leiboldet al.2004).
If the interaction strength is also included (Fig.
1.1c), all interactions are not equivalent – some are
stronger than others.
In this chapter, we focus on the origins and con-
sequences of different network topologies. Chapter
2 will address the origin and consequences of vari-
ation in interaction strength within interaction net-
works. Fundamental questions about the topology
of ecological networks include how many nodes
(species) there are, how many links (interactions)
there are in total and how interactions are
distributed among species pairs (Fig. 1.1a).
The possible interactions that can be represented
in ecological networks theoretically includeallof the
basic pair-wise interactions among species, includ-
ing competition via various mechanisms, predator–
prey and host–parasite interactions, interactions7