134 Egbert G. Leigh, Jr
rat edNi/dtof tree speciesiby th eamountaiNiCi,
let total tree biomassNdiminish per capita growth
dlnNi/dtof every speciesiby an amountbN,let
mbe the per capita mortality for each consumer
species, and letλiaiNib eth ep er capita birth rat e
of consumer speciesi. Then,
dNi/dt=riNi−aiNiCi−bNiN (8.1a)
dCi/dt=−mCi+λiaiNiCi
=λiaiCi(Ni−m/λiai) (8.1b)As postulated, consumer speciesideclines if its
host tree’s biomass falls belowm/λiaiand the
consumers no longer encounter their hosts often
enough to maintain themselves, whereas ifaiCi>
r−bN, overabundant consumers cause the popu-
lation of tree speciesito decline. Experiments with
microorganisms show that a consumer popula-
tion declines when the abundance of prey is below
a critical thr eshold, whil eth eabundanc eof pr ey
declines when their consumers are too abundant
(Maly 1969, 1978).
At equilibrium, when dCi/dt=dNi/dt=0,
Ni=m/λiai, the tree’s population is sufficient to
support its consumer. Here, lowerλiai– improved
defense in tree speciesi– increasesNiand reduces
the proportion of these trees killed by consumers.
To see how pest pressure influences tree diversity,
setri=r,ai=a, andλi=λfor all tree species
and their consumers. ThenCi>0ifr−bN=
r−bnm/λa>0 for all consumer speciesi. The
mor eint ens eth epr essur eλaon each tree species
from specialized pests, the more tree species can
coexist.
In fact, pests are far smaller than their host
trees. The population of tree speciesishould be
modeled as a set of islands in a sea of trees
of other species, with satellite islets representing
young appearing and disappearing among larger
islands representing adults. Consumers finding a
tree of their host species colonize it, and their
descendants disperse in search of other members
of this species, young and adult. Three factors
influence a pest’s pressure on its hosts (Webb and
Peart 1999). The proportion of infested adults
is lower where a species is rarer; pests in an
infested adult are more likely to find and dam-
age nearby young of this species; and a seedling’s
pests spread more readily to conspecific seedlings
if they are nearer. Modeling these processes, how-
ever, requires a dynamics of spatial arrangement,
which we have not yet got. Therefore, we can-
not predict the precise relationship between the
strength of these influences and the tree diversity
they support.
The pest pressure hypothesis has been tested pri-
marily by asking whether trees recruit or grow
more slowly, or die faster, when closer to con-
specifics (Hubbellet al. 2001, Peters 2003, Wills
etal. 2006), and whether seeds germinate less fre-
quently, and seedlings die faster, where they have
more conspecific neighbors (Harmset al. 2000).
Pest pressure, however, is not the only explanation
for these patterns (Willsetal. 2006).Theory, pest pressure, and the ecology
and evolution of tropical forestPests and pathogens exert a pervasive influence
on tropical forest. Herbivores influence the tim-
ing of leaf flush and fruit fall. In south India,
where the dry season lasts 6 months, canopy trees
flush leaves before the rains come, while insects
ar estill rar e, r educing h erbivor edamag e(Murali
and Sukumar 1993). Many tree species in the
great dipterocarp forests of Southeast Asia fruit
in synchrony every few years: the long interven-
ing periods of fruit scarcity depress populations of
seed predators, allowing many of the seeds pro-
duced during fruiting peaks to escape being eaten
(Janzen 1974, Chan 1980).
Pests and pathogens also drive the trade-offs
trees face between growing fast on clay soil ver-
sus surviving on whit esand (Fin eet al. 2004),
and growing fast in bright light versus surviving in
shad e(Kitajima 1994). Slow-growing plants must
be well defended, whereas if rapid growth is pos-
sible, plants can “outrun” herbivores rather than
deploy costly defenses (Coleyetal. 1985).
The central importance of herbivores is
revealed by their role in driving the evolution
and spread of flowering plants. Unlike grasslands,
which depend on herbivores to exclude woody
competitors, trees are adapted to reduce herbivory
(McNaughton 1985). The last section’s theory
suggests that effective anti-herbivore defense and