4 Stefan A. Schnitzer and Walter P. Carson
contrasting patterns of beta-diversity among
regions. For example, Chave points out how
new approaches can “partition beta-diversity into
deterministic and stochastic processes.” Chave’s
final conclusion is bold: “The debate over the
validity of the neutral theory is now behind us”
because there is now little doubt that unpre-
dictability and dispersal limitation play crucial
roles in structuring plant communities, as does
environmental determinism.
Fine, Ree, and Burnham (Chapter 3) revive
and expand the geographic area hypothesis,
which predicts that tropical latitudes will have
more species because of greater total land area.
Although the hypothesis was recently dismissed
in the literature (Schemske 2002), Fineet al.
demonstrate convincingly that it explains sig-
nificant variation in latitudinal patterns of tree
species richness but only when biome area is
integrated over time to include land area fluctu-
ations over millions of years. Future tests of this
hypothesis with other growth forms will now have
to account for historical shifts in land area.
Paul and Tonsor (Chapter 4) explore the sticky
issue regarding the relationshi pbetween a s pecies’
age and its range size. This idea can be traced
back to Willis (1922) and his studies in Sri Lanka.
In the first test using tropical plants (Piperspp.),
Paul and Tonsor found a significant positive linear
relationship, where species age explained 25% of
the variation in geographic range. They call for
further research that fully evaluates the shape of
the age–area relationshi p(linear, unimodal, etc.)
among many taxa so that its underlying causes
can be fully elucidated.
Dirzo and Boege (Chapter 5) develop a new con-
ceptual model of plant defense allocation. This
model uses contrasting patterns of foliage avail-
ability in strongly seasonal tropical dry forests
versus tropical rainforests to predict variable
patterns of herbivory and defense. Dirzo and
Boege point out that past theoretical frameworks
failed to consider how seasonality in rainfall
creates highly episodic resource availability. This
wateravailability–phenologyhypothesiswilllikely
promote the development of additional models
designed to predict patterns of herbivory and dam-
age in other community types that have sharply
contrasting patterns of resource availability.
Webb, Cannon, and Davies (Chapter 6) use a
phylogenetic approach to explain the taxonomic
and ecological composition of tropical trees at
multiple scales. By examining the evolution of
ecological characters among species within a
community, Webbet al. provide the tools to eval-
uate the relative roles of the biotic and abiotic
mechanisms that together act to filter local species
composition. Ultimately they seek models that will
predict the taxonomic and ecological composition
of tropical forest communities; if successful this
will be a huge ste pforward.
Zimmerman, Thompson, and Brokaw
(Chapter 7) bring to bear the power of large forest
dynamic plots (while confessing their limitations)
to tackle the issue of the relative role of neutral
dynamics, negative density dependence (NDD),
and gap dynamics in explaining high species
diversity in tropical forests around the world.They
argue persuasively that NDD is pervasive at these
sites, thereby weakening the “value of neutral
theory as a general explanation” for high species
diversity. They reject a major role for ga pdynam-
ics for tall-statured tree species, a view that is now
well supported, but which contrasts strongly with
views from the 1970s and 1980s.TESTING THEORIES OF FOREST
REGENERATION AND THE
MAINTENANCE OF SPECIES
DIVERSITY
Inanengaging,unique,andwide-rangingchapter
in the second major section, Leigh (Chapter 8)
addresses the relationshi pbetween theory and
what we really need to know to understand tropi-
cal forests. Leigh weighs in on issues ranging from
the limitations of neutral theory to how theory
addresses the limits on gross primary produc-
tion. He tells us “what mathematical theory has
done” for tropical ecology and why it remains in
relatively “crude” form.
Hubbell (Chapter 9) argues that understand-
ing complex ecological systems, such as tropical
forests, can best be accomplished using empiri-
cal studies to test simple theoretical models that
use few free parameters. Hubbell uses the Neutral
Theory to illustrate how simple theoretical models