Tropical Forest Community Ecology

Response of Seedlings and AMF Spores to Vertebrates 295

INTRODUCTION

How vertebrates affect the diversity of tropical
rainforesttreecommunitiesisof criticalandgrow-
ing importance, given the sensitivity of many
species of vertebrates to human exploitation and
forest fragmentation (e.g., Tabarelliet al. 1999,
Wrightet al. 2000, Peres 2001, Roldan and
Simonetti 2001). Many studies have documented
the important role individual vertebrate species
can play in dispersal and recruitment of selected
tree species by acting as seed dispersers and seed
and seedling predators (e.g., Forget and Sabatier
1997, Wenny 2000, Wyatt and Silman 2004).
The loss of these animals due to anthropogenic
effects like poaching can alter the recruitment
of certain tree species (e.g., Asquithet al. 1999,
Wrightet al. 2000, Roldan and Simonetti 2001).
However, the overall effect of a larger suite of
terrestrial vertebrates on rainforest plant com-
munities has rarely been addressed (Dirzo and
Miranda 1990, 1991). Most studies have focused
on the impacts of vertebrates as seed and seedling
predators or seed dispersers, but vertebrates may
affect seedling regeneration and competitive inter-
actions in other ways as well. For example,
vertebrates could indirectly alter plant commu-
nity dynamics by dispersing mycorrhizal fungal
spores (e.g., McGee and Baczocha 1994, Janos
et al. 1995, Reddellet al. 1997, Mangan and
Adler 2000) or by changing plant–fungi interac-
tions through herbivory (e.g., Franket al. 2003).

In these ways, terrestrial vertebrates could also

impact the abundance and diversity of mycor-

rhizal fungi. Most terrestrial plant species form

associations with mycorrhizal fungi that enhance

the uptake of mineral nutrients, frequently result-

ing in improved plant growth and survival (Smith

and Read 1997). Variation in the dynamics of this

symbiosis can affect community and ecosystem

properties such as plant competition (Allen and

Allen 1984, Hetricketal. 1989, Beveretal. 1997),

plant diversity (Grimeetal. 1987, van der Heijden

etal.1998a,b,HartnettandWilson1999),ecosys-

tem productivity (Klironomoset al. 2000), and

succession (Janos 1980). Terrestrial vertebrates

may therefore affect both plant and mycorrhizal

fungal communities which could in turn feed back

to affect one another’s performance.

Few studies have simultaneously examined the

impacts of terrestrial vertebrates on the diver-

sity of both plants and mycorrhizal fungi. We

did so as part of a larger study examining

the effects of experimentally excluding terres-

trial vertebrates from small areas of Australian

rainforest and found that vertebrate exclusion

had opposite effects for seedlings and arbuscular

mycorrhizal fungal (AMF) spores over the short

term (4.5 years). Exclusion significantly reduced

the abundance and species richness of AMF

spores (Gehringet al. 2002), while it increased

tree seedling abundance and species richness

(Table 17.1). In this chapter, we explore why

these communities responded differently and the

Table 17.1 Seedling community attributes (mean±SE) for 14 exclosure–open pairs before and 4.5 years after

vertebrate exclusion, and fungal spore community attributes for 13 of those pairs after 4.5 years of terrestrial

vertebrate exclusion.

Tree seedling

community before

exclosure

Tree seedling

community 4.5 years

post-exclosure

Fungal spore

community 4.5 years

post-exclosure

Open Exclosure Open Exclosure Open Exclosure

Abundance 104 ± 14 96 ± 14 120 ± 18 ∗ 197 ± 37 ∗ 54 ± 10 ∗a 28 ± 6 ∗

Species richness 22.7±1.5 21.6±1.5 22.8±1.5∗ 28.9±1.4∗ 7.4±0.5∗ 5.5±0.6∗

Evenness 0.71±0.06 0.70±0.05 0.78±0.03 0.74±0.03 0.58±0.06 0.58±0.06

Shannon’s diversity 2.13±0.18 2.04±0.18 2.20±0.17 2.24±0.17 1.13±0.1∗ 0.46±0.06∗

Note :Asterisks indicate that the difference between exclosure and open pairs was significant (P<0.05) using pairedt-tests.

aFungal spore abundance is number of spores per 1 g soil sample.