through behavioural interference and positive inter-
actions including mutualisms and commensalisms.
In practice, few representations of ecological net-
works include all of these kinds of interac-
tions, partly for historical reasons and partly
because, for reasons of pragmatism and feasibility,
the ecologists who describe these networks often
focus on only a subset of possible interactions, e.g.
when they study the importance of a particular type
of interaction in structuring the ecological commu-
nity of interest.
1.1.2 Different types of ecological networks
What might constitute the links among species in
an ecological network? Very broadly, one can think
of two types of information represented by a link.
The first is when a link represents one specific
biological mechanism or process, the second is
when a link represents the net effect of a variety of
mechanisms or processes. Links that represent
biological mechanisms or processes can be recog-
nized by the transfer of something tangible (such as
biomass, energy, nutrients, information or combi-
nations of these) between the linked entities, where
this usually requires close physical proximity be-
tween the organisms involved in time and space.
Food webs are an example of an ecological
network with this type of link, in which the
consumer–resource interactions represent transfer
of energy and material and require a physical inter-
action between individuals. Food webs are perhaps
the most commonly encountered type of ecological
network (Elton 1927; Cohen 1978; Pimm 1982; Polis
and Winemiller 1996; de Ruiteret al. 2005). An
example of a food web, the Silwood Park, UK,
grassland food web (Dawahet al.1995), is shown
in Fig. 1.1a and depicts species as ‘balls’ and feed-
ing links as ‘sticks’. In this particular food web,
there are plants, herbivores, primary parasitoids of
the herbivores, and hyperparasitoids. Trophic posi-
tion increases with height and is also coded by
colour (from http://www.foodwebs.org)..) This is a rela-
tively simple food web (87 species) in which the
identity of each resource and consumer is resolved
to the species level and sufficient sampling effort
ensured that virtually all species and feeding links
were recorded (Dawahet al.1995). However, many
food webs are more complex and less complete,
with less well-resolved taxa, especially in the smal-
ler, relatively inconspicuous and taxonomically dif-
ficult groups such as microbes and micro-
invertebrates.
One should note the very important assumption
in the kind of the topological approach shown in
Fig. 1.1: every species is treated equally in the net-
work from an interaction perspective. For example,
species that are very abundant in biomass and spe-
cies that are very rare are treated exactly the same.
Only if these differences in abundance result in a
difference in their number of interactions with
other species will those differences show up in the
topology of the web. In other words, the central
assumption of the approach is that the key point
for understanding the food web isthatspecies in-
teract, while information on how abundant differ-
ent species are and how strong these interactions
are (e.g. expressed in per capita effects of predators
on prey, and vice versa) can be disregarded. This is
of course a strongly simplifying and yet poorly
tested assumption. Efforts to attach more informa-
tion to nodes have led to significant advances in
understanding (Cohenet al.2003). Nevertheless, a
huge body of research concerns the search for pat-
terns in food web topology (Cohen 1978).
Plant–pollinator networks also use links to repre-
sent transfers of something tangible among organ-
isms that share close proximity in space and time.
Pollen moves from plant to pollinator, and again to
the plant, which requires repeated physical contact.
These interactions involve different currencies.
From the plant perspective, the exchange of genetic
information through cross-fertilization is the most
important aspect. From the pollinator perspective,
the resource rewards of visiting the flower are the
driving force. Interest in these so-called two-layer
networks is a rather recent development, at least
compared with the long study of food webs, and
the networks typically describe which plants are
visited by which pollinators or seed dispersers
(Bascompteet al.2003; Jordanoet al.2003; Rezende
et al.2007).
Networks of energy and material flow also docu-
ment this first broad type of link, and they have a
venerable place in ecology, dating back at least toTHE TOPOLOGY OF ECOLOGICAL INTERACTION NETWORKS 9