108 Jess K. Zimmermanetal.
The recent work of Willset al. (2006) makes it
clear that a rare species advantage may oper-
ate in many tropical forests, strongly suggesting
that neutral theory (Hubbell 2001, Volkovet al.
2003) may not explain the underlying patterns of
relative species abundance.
We reiterate that the evidence provided by
FDPs is phenomenological, and cannot determine
whether pathogens, predators, or competition
for resources are responsible for the observed
patterns, so these analyses do not directly test the
Janzen–Connell hypothesis. In fact, the results are
also consistent with greater intra- versus interspe-
cific competition and facilitation, or the “species
herd” effect (positive density dependence; Peters
2003, Willset al. 2006). Peters (2003) showed
a “species herd” effect at Pasoh (but not at BCI),
a pattern in which overall survivalincreasedwith
increasingnumbersof heterospecifics.Overall,the
results described in this section provide strong evi-
dence that species differences are important for
maintaining species diversity.
Most FDPs have not yet satisfactorily investi-
gated the critical seed-to-seedling and seedling-
to-sapling stages of forest dynamics. However,
information on the seed-to-seedling stage was
provided by Harmset al. (2000; see also Wright
et al. 2005) in the BCI FDP when they com-
pared seed arrival with the adjacent abundance
of seedlings and found evidence of density-
dependent seedling recruitment for all 53 species
studied. In the Luquillo FDP, Uriarteet al. (2005b)
studied density dependence in seedling survival
relative to the location of conspecific adults
using seedling census data recorded shortly after
Hurricane Georges and 2 years later. They used
different individual-based models that included
terms for dependent mortality of seedlings and
compared these with models that did not. They
found that including the density-dependence term
made the model a better fit to the data and was
significant for all nine species tested, very dra-
matically so for some species. Incorporating the
density-dependence term in the model increased
the distanceawayfrom the parent tree for peak
seedling survival, and the apparent clumping
(high local density) of seedlings was less pro-
nounced (solid lines in Figure 7.3) compared
with when it was not included. We note that the
hurricane may have enhanced the degree of NDD
due to the large numbers of seedlings recruiting
into open areas created by the disturbance.Gap specializationGap specialization and colonization–competition
trade-offs are two related explanations for tropi-
cal species diversity, sometimes lumped together
under the intermediate disturbance hypothesis
(Connell 1978, Hubbellet al. 1999). Ga ps pe-
cialization and colonization–competition trade-
offs focus on the reliance of some species on
canopy gaps for their establishment and growth,
emphasizing (1) specialization of pioneers for
gaps (i.e., niche differentiation), and (2) the
ability of species to arrive at and colonize a
ga pbefore more effective com petitors usur pthe
space – the competition–colonization trade-off.
These two processes are theoretically distinct
(Pacala and Rees 1998), though they are often
treated as related issues. Both are stabilizing effects
(Chesson 2000) that, when operating, counter
neutral theory.
The competition–colonization trade-off is
reviewed in this volume by Muller-Landau
(Chapter 11) with regard to trade-offs in seed size
and various competitive traits in tropical forests.
Here we focus on ga ps pecialization because
competition–colonization trade-offs are expected
to be less important for maintaining species
diversity in forested ecosystems with small dis-
turbances (Pacala and Rees 1998). Exploring
patterns of growth and mortality in saplings and
adult trees and shrubs in large FDPs, we con-
sider the evidence for (1) partitioning of gap and
non-ga pforest areas, and (2) life-history trade-
offs in community dynamics and demographic
variability.
The role of ga pde pendence in community
dynamics has been a consistent theme in forest
ecology for many years (Grubb 1977, Hartshorn
1978, Whitmore 1978, Brokaw 1987, Denslow
1987, Pacalaet al. 1996, Hubbellet al. 1999).
One approach has been to directly assess the
degree of gap dependence by comparing the
growth, mortality, or diversity of species in gap
and non-ga pareas. Although there is evidence