evidence for gap preference among understory fruit-
eating birds exists (Murray 1988). The Black-faced
Solitaire shows a significant preference for forest
understory, not gaps. Even though the solitaire is the
primary dispersal agent for many pioneers (based on
mist-netting records and analysis of several hundred
fecal specimens; Murray 1988 and unpubl. data; see
Murray, "Importance," pp. 294-295), its diet is so
broad that its most important food sources (subcanopy
trees and epiphytes) are most dense outside of gaps.
As a result, most of the seeds dispersed by solitaires
are probably deposited beneath closed canopy, rather
than in gaps. Clearly, the degree to which fine-scale
habitat preferences influence seed dispersal patterns
remains unresolved.
8.2.4. Population Genetic Consequences
of Seed Dispersal
Seed dispersal also accomplishes gene flow. A single
seed successfully dispersed several hundreds of
meters may result in significant gene flow between
semi-isolated subpopulations. The only direct mea-
surements of genetic population structure among
plants at Monteverde are for Ocotea tenera (Laura-
ceae; Gibson 1995, Gibson and Wheelwright 1995,
1996; see Gibson, "Seed Dispersal," pp. 289-291). Sig-
nificant genetic differences exist among subpopula-
tions in remnant and second growth forest patches.
Because most saplings within subpopulations were
not offspring of trees within the same subpopulation,
and because subpopulations always contained more
than one sibling group, seed dispersal in this species
was considered extensive (Gibson and Wheelwright
1995). These findings are consistent with large fruit-
eating birds regurgitating several seeds from plants of
one subpopulation while visiting fruiting plants in
a different subpopulation. These studies illustrate
the potential for linking data on seed dispersal to
genetic population structure. Work on O. tenera was
conducted in a landscape dominated by human-al-
tered habitats. Understanding how animal foraging be-
havior, seed dispersal, and plant genetic structure are
influenced by the patchiness of a pasture-forest ma-
trix awaits comparative studies in the pristine forests
of the area.
8.2.5. Insights into Plant-Frugivore
Coevolution Gained via Studies
at Monteverde: What Next?
Studies in Monteverde have added to our knowledge
of plant-frugivore interactions, but much remains to
be done. One of the most critical needs is to clarify
the nature of the coevolutionary relationships be-
tween plants and frugivorous animals (Howe 1993).
Because the fitness consequences to plants of fruit
consumption by different animals lie at the heart of
those evolutionary relationships, studies that focus on
both animal foraging ecology (where seeds go) and
plant demography (what happens to them as a result
of going there) are needed. Quantitative estimates of
vertebrate-generated seed shadows are needed to dis-
cern systematic differences among dispersers and to
understand the spatial scale on which to conduct stud-
ies of the demographic consequences of dispersal. Stud-
ies of density- and distance-related effects on seed pre-
dation (virtually unstudied at Monteverde for small
seeds) and pathogen attack rates (unstudied at Monte-
verde, but see Augspurger 1983a, b) are also needed.
Research on dispersal consequences should also
include the ecophysiology of seedlings, especially in
pioneer species and mistletoes with restrictive habi-
tat requirements. For some species, seedling physi-
ology may explain the evolution of fruit ripening
phenology (see Wheelwright, "A Hypothesis," pp. 281—
283, and Murray, "Fruiting Phenologies," pp. 283-286).
Studies on the demographic consequences of different
seed dispersal patterns will help develop effective con-
servation strategies because some dispersers are more
effective than others. We are currently restricted to sim-
plistic criteria (e.g., counts of fruit species eaten, pro-
portion of fruits removed) by which to assess the rela-
tive importance of different dispersers.
Research on the population biology of important
Monteverde frugivores is needed. Detailed studies of
Resplendent Quetzals (Wheelwright 1983, Powell and
Bjork 1995) have been valuable in documenting how
an important frugivore interacts with its food plants
and the complexities of preserving it. Many other
frugivores are as crucial to the Monteverde ecosystem
as quetzals, yet we know virtually nothing of their
population biology. No one has even attempted to
assess population densities or home range sizes; for
some species, we are ignorant about basic breeding
biology. For example, the first active nest of the Three-
wattled Bellbird—one of Monteverde's most impor-
tant large frugivorous birds—was found as recently
as 1992 (G. Powell, pers. comm.).
Understanding Monteverde's frugivores and their
food plants at the community level is also needed.
Current data are limited to qualitative feeding records
(see Murray, "Importance," pp. 294-295). Quantita-
tive estimates of the proportions of fruit crops dis-
persed by each frugivore species would provide more
informed estimates of the relative importance of each
frugivore at the community level, which would en-
hance conservation efforts. The roles of nonnutritional
fruit chemicals and their roles in the plant-disperser
interaction need research. Chemical manipulation of
266 Plant-Animal Interactions