1.2.3 Unresolved issues
Except for a very few well-studied and relatively
simple examples, we know little about the structure
of real competitive networks. This is partly due to
the difficulty of clearly establishing whether species
actually compete, if so to what degree, and of identi-
fying which mechanisms are in play. The necessary
experiments are often difficult to perform, except
under simplified conditions in the laboratory.
How would inclusion of competitive networks
into food webs (networks in which links are con-
sumption) alter our predictions about web stabili-
ty? We can guess, in part, that inclusion of
competitive interactions into a community matrix
based only on predator–prey interactions would
tend to destabilize the network, to the extent that
May’s (1972, 1973) analysis holds true.
How common are subnetworks of intransitive
competitive interactions, and do they explain the
persistence of high species richness? Although they
have long been recognized as a potential scenario for
maintaining diversity in systems (see Connell 1978),
we still have very few well-documented examples of
non-transitive competitive networks of any sort.
Minimally, such networks would require that com-
petitors interact through more than one kind of com-
petitive mechanism (Schoener 1983). The recurring
problem is that, based on traditions, expertise and
practical limitations, almost all studies still focus on
subsets of entire food webs, such as beetle commu-
nities, plant communities, pollinator communities,
soil food webs, soil microbial communities, but
hardly ever study the entire system with the same
level of aggregation. This will require large interdis-
ciplinary efforts and most likely new theoretical ap-
proaches to deal with the resulting data.
1.3 Mutualistic networks
1.3.1 Structural regularities
For the most part, mutualisms do not figure promi-
nently in traditional depictions of ecological net-
works. This reflects, in part, short shrift given to
reciprocal positive interactions in community ecol-
ogy (Boucher 1985), at least until recently (Brooker
et al.2008). Some of the earliest works consider
plant–pollinator systems (Feinsinger 1976; Petan-
diou and Ellis 1993; Fonseca and John 1996; Waser
et al.1996), and recently there has been a growth in
the use of the network perspective to analyse the
greater amounts of data collected on mutualistic
networks (see following references in this section).
For the main part, the two types of mutualistic net-
works examined are plant–pollinator networks and
plant–seed disperser networks. The convention
used to depict these networks is to show interac-
tions in a two-layer network between one group of
species (plants) and their mutualists (pollinators or
dispersers), as shown in Fig. 1.5. The species are not
depicted on different trophic levels, as in food
webs, and only interactions between the two
groups of mutualists are considered (no direct com-
petitive interactions are included).
Mutualistic networks appear to have several
types of structural regularity. First, there are nested
sets of interactions (Bascompteet al.2003; Lewin-
sohnet al.2006). That is, more specialized mutual-
ists tend to interact with a proper subset of the
species that more generalist mutualists interact
with. One consequence is that there is a set of spe-
cies that form a highly connected ‘core’ in mutual-
istic networks. This makes the number of links
across the network required to connect any two
species rather short (Olesenet al.2006). Indeed,
mutualistic networks tend to be more nested than
food webs and to have shorter paths between any
two species than food webs (Bascompteet al.2003).
This ‘core’ of highly connected interactors appears
to have consequences for how mutualistic networks
respond to disturbance, and seems to make them
more robust to potential perturbations (Bascompte
et al.2003).
Another structural feature of mutualistic net-
works is of asymmetric patterns of interactions
(Bascompteet al.2006; Vazquezet al.2007). In gen-
eral, species with high numbers of connections tend
to interact with those that are connected to relative-
ly few species. This asymmetry in connections
within mutualism webs is consistent with the fea-
tures of models that confer greater stability on these
networks (Bascompteet al.2006).
Similar to competitive networks, mutualistic net-
works can also show an intransitive structure, also
called hypercycles. Three species may be arrangedTHE TOPOLOGY OF ECOLOGICAL INTERACTION NETWORKS 15