Scope of the Book and Key Contributions 5make predictions that are consistent with patterns
of tree species abundance and diversity that have
been observed in tropical forests worldwide. The
chapter begins with an entertaining and candid
account of the history and development of the
Neutral Theory. Hubbell then provides an excel-
lent overview of the theory, beginning with the
model in its most simple terms and subsequently
adding complexity. Along the way, he explains
the key components of the model and emphasizes
their unique attributes and importance. Hubbell
concludes with some recommendations for the
advancement of ecology, including the value of
simple, approximate theoretical models, as well
as the need for honesty, not advocacy, in testing
theory in ecology.
In Chapter 10, Kitajima and Poorter tackle the
concept of “the niche” by evaluating the func-
tional basis of resource specialization of tropical
trees. They demonstrate that light is partitioned
among tree species at all developmental stages
and trade-offs exist between growth, survival, and
reproduction for many species. The next logical
question then becomes, are there similar trade-
offs along other niche axes? If so then there
will likely be far more niche opportunities when
additional environmental gradients (e.g., fertility
and soil moisture) are considered. These findings
would appear to challenge the viability of neutral
theory.
Muller-Landau (Chapter 11) addresses impor-
tant trade-offs that are putatively responsible for
the maintenance of species diversity. She focuses
mostly on the trade-off between competition and
colonization, which has garnered much theoreti-
cal and empirical attention over the last 50 years.
Muller-Landau argues that the available empiri-
cal evidence does not support this trade-off for
tropical trees, and that other important trade-offs
(dispersal–fecundity and tolerance–fecundity) are
much more likely candidates to explain plant
species coexistence.
Schnitzer, Mascaro, and Carson (Chapter 12)
revisit the long-held belief that gaps promote the
maintenance of plant species diversity in tropi-
cal forests. They concur with Zimmermanet al.
(Chapter 7) that gaps do not maintain the diversity
of tall-statured shade-tolerant trees. Nonetheless,
they argue that gaps may be critical for the
persistence of lianas, pioneer trees, and small-
statured species trapped in the understory, groups
that comprise more than 50% of most tropical
floras. In addition, nearly all studies have failed
to evaluate the degree to which gaps enhance
the fecundity of any life-form, including trees –
a potentially important oversight.
The Janzen–Connell hypothesis is one of
the most widely accepted explanations for the
maintenance of species diversity in tropical
forests. Carsonet al. (Chapter 13) compile the
available literature to evaluate Janzen–Connell.
They conclude that there are many examples
of distance- and density-dependent effects on
survival,growth,andrecruitment.Thereremains,
however, a paucity of evidence that these
effects maintain diversity at the community level.
Additionally Carson et al. argue that falsify-
ing Janzen–Connell is extremely challenging and
suggest that the Janzen–Connell effect could be
strongest in the least common species.
Dalling and John (Chapter 14) examine the
critical role that seed, dispersal, and recruit-
ment limitations play in structuring plant com-
munities. Using simulations based on seed and
dispersal traits of pioneers on Barro Colorado
Island, Panama, they evaluate whether these
limitations minimize competitive interactions,
thereby reducing the probability of competitive
exclusion. They find that pioneer species appear
to be strongly seed limited. However, even the
most seed-limited species can become relatively
abundant, suggesting that other processes also
structure pioneer tree communities.
In Chapter 15, Arnold describes the nascent
but increasingly important study of endophytic
fungi. To date, endophytes have been found in the
photosynthetic tissue of every tropical plant
ever examined, and a single tree may har-
bor thousands of species. The ecological role
of endophytes in tropical forests is substantial
and complex. Endophytes may act as “environ-
mental acquired immune systems” for plants or
bolster a plant’s own defense system against
pathogens and herbivores. Arnold points out
that elucidating the role of natural enemies
in structuring plant communities may rest
on understanding plant–endophyte–pathogen
interactions.