Resource Niche and Trade-offs 175biomes, fire-related adaptations explain species
distribution in relation to topography better than
adaptations to nutrient and water availability
(Hoffmannet al.2004). Topography at small spa-
tial scale also creates rare niches, modulating
resource availability regimes in time and space
(Grubb 1977, 1996). Steep ravines, for example,
offer moist and shady microhabitat, but its unsta-
ble substrate leads to frequent disturbance that
benefits light demanders and resprouters. Abiotic
microsite characteristics, such as tip-up mounds
in gaps (Putz 1983) and litter-free slopes in shade
(Metcalfe and Grubb 1995), benefit small-seeded
species that would not be able to tolerate burial by
litterfall otherwise.
Biotic factors are as important as abiotic fac-
tors in structurin gniche hyperspace (Colwell and
Fuente 1975, McPeek 1996, Chase and Leibold
2003). Growth–survival trade-offs involvin gher-
bivore defense are important in explainin gniche
position not only alon gli ght gradients (Coley
1988, Kitajima 1994) but also alon gsoil fertility
gradients (Fineet al.2004). Positive cross-species
correlations for growth between high and low
fertilities amon g34 species in Mexico (Huante
et al.1995) and nine species in French Guyana
(Baralotoet al.2005) suggest that specialization
to rich versus poor soil cannot be explained by
rank reversal of growth rates across the soil gradi-
ent. Indeed, species specialized to fertile sites tend
to grow faster across soil gradients in both tem-
perate (Schreeget al.2005) and tropical forests
(Huanteet al.1995, Fineet al.2004). Species that
specialize in infertile white sand allocate more to
defenseandgrowslowlyregardlessof soiltype,but
they survive significantly better than clay special-
ists when seedlings are grown in the white sand
and exposed to herbivores (Fineet al.2004).
Natural enemies are important for species coex-
istence not only via density-dependent predator
control accordin gto the Janzen–Connell mecha-
nism (see Carson and Schnitzer Chapter 13), but
also in changing the realized niche position and
breadth of a species from its fundamental niche
(Gilbertet al.2001, Ahumadaet al.2004). Cases
of biological invasions, where species are intro-
duced to environments free of natural enemies,
can offer excellent natural experiments to test
this idea.Clidemia hirtainvades the understory
of rainforest in Hawaii, where it persists in much
shadier environment than in its native habitats in
Costa Rica (DeWaltet al.2004). Herbivory rates,
as well as plant response to experimental fumi-
gation, are lower in Hawaii than in Costa Rica.
These results demonstrate that the absence of
natural enemies enhances the carbon balance of
the plant, reducin gthe minimum li ght require-
ment and expandin gthe realized li ght niche
(DeWaltet al.2004). Such truncatin geffects of
natural enemies can be treated in a theoretical
model as a second niche axis (Chase and Leibold
2003). The presence of mutualists would have an
opposite effect to those of natural enemies and
expand the realized niche breadth. We hope to
see many more experimental studies to reveal the
interactions between biotic and abiotic factors in
shapin gniche hyperspace in species-rich tropical
forests.CONCLUDING REMARKS
The role of niches in explainin gspecies coexis-
tence has been debated as lon gas the history of
ecologyasascientificdiscipline(ChaseandLeibold
2003). The classical and strictly deterministic
view of niche is no longer favored by many ecol-
ogists. The niche theory is most strongly objected
by the neutral theory proposed by Hubbell (2001)
who uses tropical tree communities for empirical
support of his theory. Yet, species are not equal as
postulated by the neutral theory, but exhibit wide
variations in multiple functional traits and life-
historystrategiesinrelationtotheirpreferredlight
environment. Hence, the contemporary views of
ecological niches incorporate a variety of eco-
logical factors in relation to life-history trade-offs
(Chase and Leibold 2003, Leighet al.2004). At a
first glance, growth–survival trade-offs associated
with contrastin gallocation patterns may simply
provideanequalizingforcethatcontributestoeco-
logical similarity of the species as postulated by
the neutral theory, instead of a stabilizin gforce
to allow each species to persist via its competi-
tive superiority in its own niche in the community
(Chesson 2000). Yet, there is an empirical link
between growth–survival trade-offs and appar-
ent light niches of tree species. Species whose