Tropical Forest Community Ecology

Testing and Falsifying the Janzen–Connell Hypothesis 235

importance for the general maintenance of diver-
sity in plant communities is underappreciated.
Thus, insect herbivores and pathogens may act
as keystone species by reducin gthe vi gor, abun-
dance, and fecundity of aggregated stands of adult
conspecifics, thereby increasin gthe diversity of
coexistin gspecies (Carson and Root 2000, Lon g
etal. 2003, Carsonetal. 2004). In many instances
these taxa are some of the most abundant woody
species in the community (Carsonet al. 2004).
Thus, aggregations of conspecifics are in double
jeopardy because both their adults and their juve-
niles are likely to be more vulnerable to enemies
and suffer greater per capita rates of attack or
damage.
In tropical forests far less attention has been
given to how aggregations of adults make them
more vulnerable to enemies. Janzen (1970) rec-
ognized that pre-dispersal seed predation would
have a stron geffect on the “intensity and pat-
terns of seed shadows cast by parent trees”
and considered this a key part of his hypothe-
sis. Indeed, Janzen (1970) presented a graphical
model whereby intense pre-dispersal seed preda-
tion would lead to peaks in seedlin gabundance
that would be smaller andcloserto the parent
tree compared with light pre-dispersal seed pre-
dation. Thus, enemies are likely to have a greater
impact on both juveniles and adults of any given
plant species whenever adults are aggregated. Fur-
thermore, these aggregations may lead to periods
when insects become abundant or to episodic
outbreaks of specialist enemies that function as
keystone species that defoliate and sometimes kill
adults over large areas, thereby increasing plant
species diversity in the habitat (Carson and Root
2000, Carsonet al. 2004).
In the tropics these outbreaks or periods of high
insect abundance may be frequent when viewed
from a phytocentric perspective. An outbreak of
a specialist that occurs only once every 50 years
means that it occurs multiple times in the life of a
long-lived tree species.There are now a number of
examples from tropical forests where outbreaks of
specialists defoliate common or abundant woody
species (Wolda and Foster 1978, Janzen 1981,
Wonget al. 1990, Torres 1992, Nascimento and
Proctor 1994). In Indonesian forests, Nair (2000,
2001) concluded that outbreaks often occurred

when tree species grew in aggregation and that

low-diversity stands precipitated such outbreaks.

It is likely that outbreaks are more common than

previously thought because they are spatially and

temporally patchy and occur high in the canopy

(Wolda and Foster 1978, Lowman 1987). In

addition, if relatively infrequent periods of high

herbivore abundance regulate populations, then

short-term studies of insect abundance and dam-

age or incidence of specialists versus generalists

will have little relevance for the Janzen–Connell

hypothesis. Research is needed that links how

insect abundance and damage on adults varies

with host abundance over lon gtime periods, and

how this damage affects lifetime fecundity and

juvenile mortality. The fact that insects and insect

outbreaks have a stron gtop-down impact on plant

communitiesiswellrecognizedoutsideof thetrop-

ics (but see Janzen 1981 for tropical examples).

Although conventional wisdom suggests other-

wise, there is little solid empirical evidence at the

appropriate temporal and spatial scales to suggest

that outbreaks are either less common or less pro-

found in their impact in tropical forests relative to

temperate forests.

There are few studies of

Janzen–Connell effects both among

and within latitudes

Janzen–Connell effects may be stronger in tropi-

cal than temperate forests because of the higher

abundances of natural enemies, a greater degree

of specialization in aseasonal tropical habitats,

and greater rates of damage even when leaves

are better defended (Coley and Barone 1996,

Novotnyet al. 2002, Dyer 2007). Both Janzen

(1970) and Connell (1971) proposed that these

differences may help explain the latitudinal gra-

dient in species richness. Nonetheless, very little

effort has been expended in comparin gJanzen–

Connell effects across latitude. We found that the

great majority of studies that have investigated

the Janzen–Connell hypothesis have been con-

ducted in tropical habitats (81% of 53 studies;

Table 13.1). The few temperate studies that exist,

however, have found evidence for density- and

distance-dependentmortalityintemperateforests.