Community Ecology Processes, Models, and Applications

and form typical fungus–root structures that are
referred to as mycorrhizas (Smith and Read 1997).
In this section, we largely focus on the role of AM
fungi in community organization.
AM fungi alter plant community composition,
plant productivity and plant diversity. For instance,
AM fungi increase plant diversity in European
grassland by as much as 30% (Fig. 13.4). The fungi
do this by promoting seedling establishment and
enhancing competitive ability of subordinate plant
species relative to dominants (Grimeet al. 1987; van
der Heijdenet al. 2006b). AM fungi may also pre-
vent non-mycorrhizal plants from growing via an-
tagonistic interactions (Francis and Read 1994) and
by enhancing the competitive ability of mycorrhizal
hosts (see below). Such interactions may also con-
tribute to plant succession (Allen 1991), although
this is still poorly understood. In some cases, AM
fungi can reduce plant diversity, especially in eco-
systems where the dominant plants have a high
mycorrhizal dependency and obtain most benefit
from AM fungi, such as in tall grass prairie (Hart-
nett and Wilson 1999) or some annual plant com-
munities in Australia (O’Connor et al. 2002).
Similarly, in tropical rainforests ecto-mycorrhizal
associations may encourage dominance of certain
tree species, at the expense of arbuscular mycorrhi-
zal trees that are less able to acquire nutrients and
tolerate pathogen attack, thereby reducing species
coexistence (Connell and Lowman 1989). Over

20 000 plant species are completely dependent on

microbial symbionts (including mycorrhizal fungi)

for growth and survival, pointing to the importance

of plant–microbe symbiosis as regulators of plant spe-

ciesrichnessonEarth(vanderHeijdenet al. 2008).

Mycorrhiza-enhanced efficiency of nutrient up-

take may be important mechanisms in competition

between plants and may affect plant coexistence

and community organization (van der Heijden

2002). A competition model proposed by Tilman

(1988) has been applied by van der Heijden (2002)

to predict the influence of mycorrhiza in a competi-

tion between host and non-host plants. The model

represents the growth of a species by isoclines that

show the amount of growth in relation to the avail-

ability, or supply, of limiting resources. The re-

source supply levels where plant populations stop

growing define the zero population growth iso-

clines. The supply of additional resources by AM

fungi could reduce the growth isoclines of a mycor-

rhizal-dependent plant, thus broadening its poten-

tial niche. This is shown for a hypothetical plant

species (plant B) in Fig. 13.5. In this case, AM fungi

enhance the supply of phosphorus, thereby lower-

ing the isocline of plant B (Fig. 13.5). The supply of

nitrogen is not affected by AM fungi in this situa-

tion because it is unclear whether AM fungi have a

big impact on nitrogen uptake and contrasting ob-

servations have been made; hence, the growth iso-

cline moves only down and not to the left. Tilman

0.8

0.6

0.4

0.2

0

(a) (b) (c)

MNM

Plant diversity (H)

1.5

1.2

0.9

0.6

0.3

0

MNM

Plant diversity (H)

50

40

30

20

10

0

MNM

Species richness

Figure 13.4The impact of arbuscular mycorrhizal fungi on plant diversity and plant species richness in European
grassland. Studies from (a) calcareous grassland in the UK (modified after Grimeet al. 1987), (b) Swiss calcareous
grassland (modified after van der Heijdenet al. 1998a) and (c) an early successional grassland community in the UK
(modified after Gangeet al. 1993). M, mycorrhizal; NM, non-mycorrhizal.

MUTUALISMS AND COMMUNITY ORGANIZATION 189