Chapter 13

ChallengesAssociated

WithTestingAndFalsifying

TheJanzen–Connell

Hypothesis:AReviewand

Critique

Walter P. Carson, Jill T. Anderson, Egbert G. Leigh, Jr, and

Stefan A. Schnitzer

OVERVIEW

The Janzen–Connell hypothesis proposes that density- and distance-dependent natural enemies regulate plant popu-
lations, thereby enhancin galpha-diversity and potentially contributin gto the latitudinal gradient in species richness.
There have been over 50 studies designed to test predictions of this hypothesis, and our review shows that many tree
species exhibit patterns consistent with Janzen–Connell effects. Here, we review studies that were designed to test the
Janzen–Connell hypothesis and raise a number of general issues and challenges with regard to testing it.
First, the Janzen–Connell hypothesis is fundamentally a community-level hypothesis that predicts that enemies
cause higher alpha-diversity; this key prediction remains poorly tested at the appropriate scale. Second, the Janzen–
Connell hypothesis in its most general context is a special case of keystone predation, where specialist enemies keep
species that are superior competitors in check. It remains unknown if the removal of enemies for any woody species
will subsequently cause a reduction in alpha-diversity. Overall, the Janzen–Connell hypothesis is difficult to falsify
because it may promote diversity if enemies act as keystone species by keepin gonly a relatively very small propor-
tion of superior competitors in low abundance. Rare species that have shade-tolerant juveniles and produce large
seeds may be the ones most likely to show Janzen–Connell effects yet least likely to be included in studies due to
low population densities of adults. Third, complex trade-offs underlie Janzen–Connell effects, particularly a trade-off
between competitive or establishment ability and vulnerability to enemies. Many tests of the Janzen–Connell hypoth-
esis assume implicitly that traits that confer high survivorship in the shade are correlated with traits that enhance
survivorship under prolonged pest pressure in the understory. This correlation does not hold for all shade-tolerant
tree species and the tightness of this relationship needs to be directly tested. Consequently an often overlooked but
important trade-off for plant species coexistence may be allocation to those physiological and morphological traits
that confer survivorship at low light versus traits that confer survivorship under varying degrees of pest pressure.
Fourth, diversity may be maintained, at least in part, by episodic outbreaks of specialist pests, which may reduce the
survivorship, growth, and fecundity of adults whenever adults are particularly aggregated. This impact of enemies
on adults, although originally emphasized by Janzen (1970), has received far less attention than the effect of ene-
mies on juveniles even though it is well known to be important outside of the tropics. Challenges notwithstanding,
Janzen–Connell effects are common in tropical systems and thus a likely key mechanism maintainin ghi gh plant
diversity.