Community Ecology Processes, Models, and Applications

(1988) predicts that, if two plant species compete
for resources, the one with the lowest zero popula-
tion growth isocline, i.e. the one with lowest re-
source requirements, will be competitively
superior. Furthermore, the model predicts that
two plant species can coexist if their zero popula-
tion growth isoclines cross. Coexistence could be
possible because the growth of each plant species
is limited by a different resource. As a consequence
intraspecific competition is greater than interspecif-
ic competition (Tilman 1988). The reduction of the
zero population growth isocline of mycorrhizal-de-
pendent plant species by AM fungi, as explained
above, can alter their competitive ability with other
plant species. For example, plant B will be outcom-
peted and replaced by plant A in the absence of
AM fungi because the zero growth isocline of plant
B is always inside that of plant A (Fig. 13.5). How-
ever, if AM fungi are present, the zero growth iso-
cline of plant A is unaltered, but that of plant B
is reduced and it crosses the zero growth isocline
of plant A, so that coexistence occurs (Fig. 13.5).

Several studies have indeed shown that AM fungi

alter plant competition (Fitter 1977; Hetricket al.

1989; Allen and Allen 1990; West 1996; Marleret

al. 1999). Several plant species are able to coexist

with other plants only if AM fungi are present,

indicating that AM fungi enhance their competitive

ability (Grimeet al. 1987; Hetricket al. 1989; van der

Heijdenet al. 1998a). The model presented in Fig.

13.5 can also be extended to plant communities

when the response of individual plants to AM

fungi is known, and it can be used to predict the

impact of AM fungi on the composition of plant

communities (see also Urcelay and Diaz 2003).

Moreover, some plant species can also suppress

the abundance of AM fungi (Stinsonet al. 2006). In

that situation, competitive abilities between plants

would be altered in the opposite direction (as

shown in Fig. 13.5).

In many ecosystems mycorrhizal fungi form

large mycelial networks, also called ‘wood wide

webs’ (Helgasonet al. 1998). These networks are

extremely fascinating because many plant

A wins

Without mycorrhiza With mycorrhiza

A wins

(a) (b)

A wins

B wins

A + B coexist

Resource 1 (phosphorus availability)

Resource 2 (nitrogen availability)

Figure 13.5Competition between two hypothetical plant species (plant A (^) and B (*)) without arbuscular
mycorrhizal (AM) fungi (solid line) and with AM fungi (dashed line). The lines represent the resource-dependent zero
growth isoclines, which show the minimum amount of phosphorus (y-axis) and nitrogen (x-axis) that is necessary for
plant growth. The positions of the isoclines determine the outcome of competition between plants A and B. In the
absence of AM fungi (a), plant B is outcompeted by plant A at each resource level because the resource requirement
of plant B is higher than that of plant A. Plant B is able to coexist with plant A when AM fungi are present because
plant A and B are limited by different resources (b). Reproduced from van der Heijden (2002) with permission from
Springer-Verlag.

190 FUTURE DIRECTIONS