Resource Niche and Trade-offs 161more types of resources are required to allow
coexistence in theoretical models that consider
only resource competition. However, often a sin-
gle resource constrains growth rates, as stated
by Liebig’s law of the minimum (Tilman 1982).
Thus, it is more likely that the fundamental niche
of a species alon ga resource axis is defined by
its tolerance of high and low levels of the most
limitin gresource, while interactions with compet-
in gnei ghbors and other biotic factors, includin g
natural enemies, narrow each species distribution
down to its realized niche where it is superior to
its competitors. In theoretical models, the influ-
ence of natural enemies, such as predation rates,
can be treated as a second niche axis (Chase and
Leibold 2003).
Light is generally the most limiting resource in
closed-canopy forests, and light competition is the
major driver of successional change where soil
resources are not limitin g(Grime 1979, Tilman
1988). The total daily light varies more than
1000-fold within a forest from the forest floor
to the canopy top, as well as from the shaded
understory to an open clearin g(Yoda 1974,
Chazdon and Fetcher 1984). Yet, the light pref-
erences of trees may be difficult to distinguish
beyond a few broadly defined guilds (Hubbell and
Foster 1986, Brown and Jennin g1998, Hubbell
et al. 1999, Brokaw and Busin g2000), prompt-
in gdevelopment of a radically different view of
species coexistence based solely on stochastic pro-
cesses, known as the neutral theory (Hubbell
2001, 2005). However, the neutral theory fails to
explain observations that unrelated species occu-
pyin gsimilar habitats conver ge in physiolo gical
and life-history traits (ter Steege and Hammond
2001, Cavender-Bareset al.2004a,b, Zanneet al.
2005). Furthermore, widely separated communi-
ties sharin ga re gional species pool often conver ge
on similar species composition (Tuomisto and
Poulsen2000,ClarkandMcLachlan2003).These
observations support a stron grole of niche-based
mechanisms for distribution and abundance of
species at local and regional scales (Conditet al.
2002). The real world perhaps falls somewhere
between these two extreme theoretical views, such
that stochastic processes interact with niche pref-
erences of species (Svenninget al. 2004). An
example of such compromise is a stochastic-niche
model of Tilman (2004), which assumes that a
new species can invade the space already occu-
pied by an established species only if the former
can survive stochastic mortality while growing
to maturity consumin gthe resources left uncon-
sumed by the established competitor. Indeed,
contemporary views on ecological niches recog-
nize demographic and environmental stochastic-
ity and consider spatial and temporal dimensions
in biotic and abiotic factors (Chase 2005).
Hereafter, we evaluate functional mechanisms
underlyin gresource niche specialization by trees
and evidence for resource niche partitionin gin the
three-dimensional space within tropical forests.
Which resource constitutes a significant niche
axis depends on the temporary and spatial scale of
investigation. Soil nutrient availability and water
regime determined by bedrock and topography are
relatively stable throughout the lifetime of a tree,
creatin ga coarse matrix of niches for tropical
trees and contributin gto species turnover (=beta-
diversity) at regional scales (Schulz 1960, Clark
et al.1998, Harmset al.2001, Conditet al.2002,
Svenninget al.2004). In contrast, the light niche
is important as a potential mechanism to pro-
mote local (alpha) diversity, because light creates
much finer and complex environmental hetero-
geneity in three dimensions within a forest. Light
also exhibits unpredictable changes, such that a
variety of ontogenetic trajectories for light niche
preference could be potentially successful at a spot
where a seed may arrive (Figure 10.1a). Thus,
we focus primarily on light as the key limiting
resource that shapes species-specific traits under-
lyin gtrade-offs essential for niche partitionin g.
However, parallels can be drawn where and when
nutrients or water are the limitin gresource of
species distribution and abundance.
This review consists of four parts. First, we con-
sider the functional basis for light competition.
Second, we review contrastin gtypes of trade-offs
that may contribute to light niche partitioning
within a horizontal plane of the forest. Third, we
review how light niche may be partitioned verti-
cally. Lastly, we briefly review how more niches
may be created through interaction of multiple
resource axes, as well as how pests and mutualists
may influence the realized niche breadth of each
species.