Testing Explanations for Species Diversity using FDPs 111does not suggest two distinct groups of species,
conforming to “pioneer” or “mature” species.
In fact, the large number of species with low
mortality in the shade and relatively low growth
rates in high light was viewed as evidence that
most tree and shrub species are generalists, and
pioneer-like species are infrequent in the com-
munity (Figure 7.4a; Hubbell and Foster 1992).
Uriarteet al. (2004a) demonstrated life-history
variation in the hurricane-disturbed Luquillo FDP
by showing hurricane susceptibility (likelihood
of stem breakage or tip-up during a hurri-
cane) is positively correlated with post-hurricane
sapling mortality (a measure of shade tolerance;
Figure 7.4b). These two studies directly address
the issue of modality in life-history characteris-
tics, showing that there are no modes of variation
corresponding to “pioneer” and “non-pioneer”
species typologies (Swaine and Whitmore 1988).
Rather, variation in life histories is uniform or
unimodal, with species at the extremes repre-
senting pioneer or mature species (Alvarez-Buylla
et al. 1992, Zimmermanet al. 1994, Agyeman
et al. 1999, Wrightet al. 2003), what Pacala
and his colleagues call the “life history mani-
fold” (Moorcroftet al. 2001, Purves and Pacala
2005).
Focusing on the variability in species’ demog-
raphy alone, efforts have been made to show that
the more diverse plots have greater demographic
variability among species (indicative of greater
variation in life-history types) than less diverse
plots (Conditet al. 1999, 2006). Because so many
factors lead to the evolution of a species’ demo-
graphic characteristics, not just gap dependence,
this is only an indirect test of that particular aspect
of a species’ life history. However, if it were to be
shown that more diverse plots have greater demo-
graphic variability, it would be consistent with the
idea that demographic variability facilitates high
speciesdiversity.Conditet al.(1999)foundthatthe
Pasoh plot contains fewer species with extremely
high growth rates (one of many life-history char-
acteristics of “pioneers”) than BCI (Conditet al.
1999) and gaps are smaller and less frequent
than those on BCI (Putz and Appanah 1987,
Leighet al. 2004). Yet, species diversity is much
higher on Pasoh, nearly three times that of BCI,
contrary to the prediction that species diversity
is promoted by greater numbers of demographic
niches. Conditet al. (2006) recently conducted
a much larger comparison of the tree mortality
and relative growth rate among 10 of the CTFS
plots. Again, contrary to the expectation that high
demographic variability among species in a plot
should be related to high species diversity, the most
diverse plots had the least demographic variation.
Those plots with high demographic variability
included the American plots of BCI (Figure 7.4a),
Yasuní and La Planada, where species belonging
to the genusCecropiahad some of the high-
est demographic rates (Conditet al. 2006), so
some degree of species diversity may be explained
by demographic variability. Conditet al. (2006)
thus conclude that the results do not completely
eliminate a role of demographic variability in
explaining species diversity of trees and shrubs,
but suggest this role is very limited.
From the perspective of theory, it is important
to note that the existence of life-history trade-offs
does not provide for the stabilizing effects needed
to disprove neutral theory.These trade-offs, in fact,
constitute equalizing effects (Chesson 2000) that
will not contribute to the maintenance of species
diversity in a way that refutes neutral theory, as
noted by Hubbell (2001). Only distinct differen-
tiation of species’ gap response or any form of
niche differentiation, as described in a theoretical
context by Pacala and Rees (1998), can provide
for the stabilizing effects that maintain species
diversity. Yet, there is no convincing evidence,
within or between FDPs, to support this explana-
tion for the high tree and shrub species diversity
of tropical forests. Schnitzer and Carson (2001)
argued that gaps were likely to be critical for the
maintenance of pioneer trees and lianas in addi-
tion to herbs, shrubs, and herbaceous vines that
together account for more than 60% of the plant
diversity on BCI. This is undoubtedly true for the
many species trapped in the understory. Similarly,
Wright (2002) suggested gaps were necessary
for the small-statured tree species that otherwise
might fail to reproduce if permanently shaded in
the understory. However, for taller-statured tree
species, the species addressed in the FDPs, gap spe-
cialization appears to make a limited contribution
to explaining species diversity. This is in contrast
to the long-standing belief that ga pde pendence