106 Jess K. Zimmermanetal.
theory, as seen in the example for the Pasoh plot
(Figure 7.2b). The ability of neutral theory to
resolve some long-standing issues in community
ecology and to generate realistic relative species
abundance curves made the theory expounded
in Hubbell’s (2001) book appear compelling to
many scientists. However, because of some of
the assumptions used in Hubbell’s (2001) treat-
ment of neutral theory, and because it promoted
the idea that biological identity is unimportant in
community dynamics, it has drawn much contro-
versy and criticism (summarized by Chave 2004
and Missa 2005).
The key problem with Hubbell’s approach to
testing neutral theory is that, however compelling
it is at explaining the relative species abundances
among FDPs (Figure 7.1), other explanations,
including niche-based ones (Chave 2004, Purves
and Pacala 2005), can also generate the same
patterns. A recent modification of a mathematical
formulation of neutral theory (Volkovet al. 2003)
by Hubbell and his colleagues, employing density
dependence (rare species advantage), makes this
precise point (Volkovet al. 2005). The pattern of
relative species abundances reveals nothing about
the processes that generate them, and one must
delve further into the dynamics of tropical forests
to distinguish neutral theory from other potential
explanations.
How can we use data from large FDPs to
hel presolve this issue? Taking the view point that
neutral theory constitutes a null model for com-
munity dynamics, we consider two departures
from the neutral theory as potential explana-
tions for local species diversity: NDD and gap
specialization. Where appropriate in the follow-
ing discussion, we make the distinction between
equalizing versus stabilizing effects, and their con-
tribution to the maintenance of species diversity
(Chesson 2000). Equalizing effects minimize the
impact of the differences in species fitness that
lead to competitive exclusion, and thereby pro-
mote the community dynamics envisioned by
neutral theory. Stabilizing effects, on the other
hand, actively contribute to the maintenance of
diversity by increasing the impact of negative
interactions between conspecifics relative to inter-
actions among species. Discovery of stabilizing
effects and the implication that species’ identity
and their particular life-history attributes con-
tribute to community structure would, therefore,
refute neutral theory.Negative density dependenceOne important assumption of neutral theory, as
formulated by Hubbell and colleagues (Hubbell
2001, Volkovet al. 2003) is that species are
ecologically equivalent and competitively neutral,
that is, competition between neighboring con-
specifics is no different than that between
heterospecific neighbors. Negative density depen-
dence, on the other hand, allows for species-
specific differences and the likelihood that an
individual occurring in a high density of con-
specifics (that compete for the same resources
or share pathogens/herbivores) is less likely to
survive and reproduce than the same indi-
vidual occurring among heterospecifics at the
same density (which suffer less competition for
resources or slower pathogen/herbivore trans-
mission). The effect is often termed “negative
density dependence” although it really refers to
the importance of conspecific versus heterospe-
cific density. A number of studies have utilized
data from large tropical FDPs to address this
issue (Hubbellet al. 1990, 2001, Conditet al.
1992, 1994, Gilbert et al. 1994, Willset al.
1997, 2006, Wills and Condit 1999, Peters
2003, Losos and Leigh 2004, Uriarte et al.
2004a,b, 2005a,b), often couching their stud-
ies as tests of the Janzen–Connell hypothesis. In
simple terms, the Janzen–Connell hypothesis con-
siders density- or distance-dependent recruitment
that is driven by specialized seed or seedling preda-
tors or pathogens, which make the areas around
parents inhospitable for the establishment of con-
specifics (Janzen 1970, Connell 1971). Taking
the case of seedlings, for example, the Janzen–
Connell hypothesis predicts that seedlings of a
species that establishawayfrom their parent and
near to adults of a different species gain a growth
and survival advantage, because these seedlings
are less likely to be attacked by the species-specific
pathogens and predators that are associated with
an adult of another species. In addition, disper-
sal limitation will initially cause higher densities