Community Ecology Processes, Models, and Applications

in an autocatalytic loop, in which species A pro-
motes B, B promotes C, and C promotes A, result-
ing in indirect mutualism (A helps C through B).
This structure can also be seen as positive feedback
of species A on its own growth, through species B
and C. For example, Ulanowicz (1995) describes
and models how the carnivorous waterplantUtri-
culariaexcretes sugars towards its leaf surface,
which promotes the growth of microepiphyto-
benthos (algae, diatoms and bacteria) on the leaves,
which then attract zooplankton grazers (as cope-
pods), which are in turn caught by theUtricularia
plant for food. Such hypercycles may have played a
role in early prebiotic evolution, in which space and
limited dispersal again seem to stabilize the inter-
action, e.g. against the invasion of parasites in the
mutualistic network (Boerlijst and Hogeweg 1991).

1.3.2 Mechanisms

Information about the drivers of the structure of
mutualistic networks is beginning to emerge. The
clearest example of this is a recent study about
relationships between the shared evolutionary his-
tory of the species in mutualistic networks (Re-
zendeet al. 2007). Across a compilation of 36

plant–pollinator and 23 plant–frugivore networks,

there was a significant effect of phylogeny on the

number of links of a species in 25–39% of the net-

works. That is, more closely related species tended

to both be specialists (or generalists) than more

distant ones. The amount of phylogenetic similarity

of two species was also found to predict the ecolog-

ical similarity (measured as the standardized num-

ber of interactions in common) in nearly 50% of the

networks. These analyses provide good evidence

that evolutionary history at least partly explains

some of the structural regularities of mutualistic

networks.

1.3.3 Unresolved issues

The patterns described above raise additional ques-

tions. Why does phylogenetic history apparently

influence the structure of mutualism networks?

Perhaps evolution, and specifically coevolutionary

processes, have a stronger role in structuring net-

works of mutualists. There is considerable variation

unexplained by phylogeny in the webs considered

above, so obviously phylogenetic history is not the

only factor contributing to structure in these net-

works (Rezendeet al.2007). It is also interesting to

INO1 ABIS
Plants Animals

MONT CORR

Figure 1.5Examples of networks of positive interactions (mutualisms) between plants and their pollinators or dispersers
in four different kinds of communities. In each graph, nodes on the left correspond to different plant species, and
nodes on the right correspond to animals that pollinate the plants or disperse their seeds. Lines connect positively
interacting species. Reprinted from Jordanoet al.(2003), with permission from Wiley-Blackwell.

16 SHAPE AND STRUCTURE