Community Ecology Processes, Models, and Applications

could enhance invasion by increasing the likelihood
that a suitable prey species would support the
growth of the invader. Conversely, greater predator
diversity could provide another avenue of biotic resis-
tance by increasing the likelihood that a native preda-
tor would be able to prevent the establishment of the
invader. These possible positive and negative effects
of diversity on different trophic levels are just one
of the many ways that diversity can potentially influ-
ence community structure and functioning. Chapter
8providesmanymoreexamplesofdiversity-
functioning relationships in complex communities.

14.2.2 Interaction networks beyond food webs

The first problem faced in representing the full
range of pairwise ecological interactions in commu-
nity networks is to devise a convention for repre-
senting links that correspond with different kinds of
interactions: predator–prey, competitive and mutu-
alistic ones. One way to do this is to use different
line styles, or colours, to represent different kinds of
interactions. Obviously, the other way to do this in a
non-graphic fashion is simply to have a matrix of
interactions keyed by the signs of the effect of in-

dividuals of each species on the other. While easy to

do, visual inspection of such tables for community

patterns becomes problematic as the tables extend

to more than a small number of species. Figure 14.4

shows in a very general form some of the complex-

ities of community interactions that would have to

be integrated into anything approaching a complete

depiction of an ecological network or a moderately

complex community module. Keep in mind that this

figure simply represents the kinds of interactions

that need to be included, but it in no way completely

represents the aspects of complexity shown by the

full species richness and patterns of interactions

(connectance) in natural systems. For practical

computational reasons, such interactions might be

best represented as a matrix or table showing the

signs and in some cases mechanisms of interactions,

rather than as a graph.

There are two obvious additions to the basic inter-

actions depicted in food webs that need to be

incorporated in any comprehensive ecological net-

work. As suggested in Chapter 1, networks of com-

petitive interactions should be included, since such

interactions can potentially influence the coexistence

of species. Describing the network of interac-

tions (who competes with whom) is relatively

straightforward, but encoding information about

Predator

Producer

Lower diversity web

+

+

Herbivore

Higher diversity web Invader

+ +

Figure 14.3Possible differences in the effects of invader trophic level and diversity within different invaded trophic levels
on invader success. For simplicity, we focus on effects of diversity in different trophic levels on the success of an invader
on the intermediate herbivore level, but similar within and between trophic level effects could influence invaders on
other trophic levels. Line thickness corresponds to the strength of positive and negative effects on invader success,
shown in grey. Trophic interactions are shown in black.

EMERGING FRONTIERS OF COMMUNITY ECOLOGY 199