Chapter 5

PATTERNSOFHERBIVORYAND

DEFENSEINTROPICALdryAND

RAINFORESTS

Rodolfo Dirzo and Karina Boege

OVERVIEW

Studies on tropical herbivory are largely based on information from tropical rain forests (TRFs) but we argue that

our understanding of this and other phenomena of tropical community ecology and evolutionary biology will benefit

from comparisons with tropical dry forests (TDFs). In particular we analyze the possible consequences of rainfall

seasonality. We develop a line of reasonin gas to how rainfall seasonality and phenolo gy of TRF and TDF plants brin g

about fundamental differences in the availability of foliage for folivores and how these differences can in turn lead

to predictable and contrastin gpatterns of herbivory and defense. We then compare available information, from the

literature and from our own ongoing work, on herbivory and defense in plants from both forest types. We found strong

evidence that higher constancy of foliage, implying greater risk and impact of herbivory in TRF plants in ecological

time, may lead to a greater evolutionary history of herbivory, favoring greater selection for increased defense and

lower herbivory. The predicted patterns were evident when we controlled for interspecific heterogeneity in herbivory

and defense within both TDF (due to contrasts in life history and growth) and TRF (due to contrasts in phenology).

In addition, the expected patterns were mirrored usin gmore controlled intra-site (dry forest) comparisons lookin g

at plants of contrasting phenologies. Moreover, preliminary evidence suggests that the observed patterns might hold

independently of, or in addition to, phylogenetic influence.

INTRODUCTION

The term “tropical forest” conceals the existence
of a complex and diverse variety of vegetation
entities and plant associations with distinct phys-
iognomic features and ecological characteristics,
ranging from the savanna-like formations to the
usual rain forests. Amon gthis exuberant variety
of tropical forest formations, two major categories
of lowland (≤1000 m above sea level) tropical
forest can be distinguished (Dirzo 2001): rain
(includin gwet and moist, i.e., ever green) and
seasonally dry (i.e., deciduous) forests.
Although we recognize the existence of a
seasonality gradient, for the purposes of the

arguments we develop in this chapter we dis-

tinguish tropical dry forests (TDFs) from tropical

rain forests (TRFs) as those in which the num-

ber of dry months (rainfall≤100 mm) per year

is five or more, and durin gwhich time the ve g-

etation is almost entirely devoid of foliage. Such

contrast in foliage availability has the poten-

tial to affect the evolutionary ecology of plant–

herbivore interactions, as has been demonstrated

by studies lookin gat the consequences of plant

phenology on herbivory within a single site at

both the community (Janzen 1981, Janzen and

Waterman 1984, Filipet al. 1995) and popula-

tion (Feeny 1970, Aide 1988, Forkneret al. 2004)

levels.