I investigated the consequences of avian seed dis-
persal for Ocotea tenera, an understory lauraceous
tree that colonizes disturbed sites (gaps) in the forest.
Dependence on disturbance for establishment leads
to a patchy distribution of clusters (subpopulations)
of trees in the forest. Ocotea tenera fruits are eaten by
Emerald Toucanets, Keel-billed Toucans, Resplen-
dent Quetzals, and Three-wattled Bellbirds. Birds
typically swallow one to three fruits from a tree and
fly to another site to digest the fruits and regurgitate
the seeds (Wheelwright 1991). Undoubtedly, birds
move many seeds throughout the forest during a fruit-
ing season, but because suitable sites for seedling es-
tablishment are uncommon and because seeds may
experience postdispersal predation, only a small por-
tion of dispersed seeds successfully germinate. It is
impossible to observe seed dispersal in sufficient de-
tail to follow where seeds originate and where they
eventually settle. However, because existing subpopu-
lations are the products of previous seed dispersal,
genetic data can be collected from these subpopula-
tions for evaluation of seed dispersal and seedling
recruitment patterns.
I conducted a genetic analysis of the Ocotea tenera
population in Monteverde to test two alternative seed
dispersal and recruitment hypotheses (Gibson 1995).
One hypothesis was that if gaps were colonized with
seeds from many maternal trees or if seedlings were
recruited from the progeny of many widely distrib-
uted trees, there should be low genetic relatedness
among trees within subpopulations and low genetic
differentiation among subpopulations. Conversely, if
gaps were colonized with seeds from few maternal
trees or if there were little migration, there should be
high genetic relatedness among trees within subpopu-
lations and high genetic differentiation among sub-
populations. I surveyed 112 O. tenera trees among six
subpopulations for 18 allozyme loci (which served as
genetic markers) to measure levels and structuring of
genetic diversity within and among naturally occur-
ring O. tenera subpopulations.
Levels and structure of genetic diversity within and
among subpopulations (Table 8.7) were consisten
with observations of other tropical trees. However,
measures of genetic differentiation among subpopu-
lations were atypically high (mean GST = 0.13), which
indicated subpopulations were genetically distinct
from one another. Genetic relatedness among trees
within subpopulations was estimated by R, which can
range from 0.00 in groups of unrelated individuals to
0.50 in groups of full siblings (Queller and Goodnight
1989). Relatedness within subpopulations tended
to be high and indicated subpopulations contained
groups of half-siblings (Table 8.8). Comparisons o
sapling and adult genotypes indicated that although
Table 8.7. Estimates of genetic diversity and
genetic structure for individual polymorphic loci
and means calculated across polymorphic loci for
six naturally occurring subpopulations of Ocotea
tenera in Monteverde (Gibson and Wheelwright
1995).
Locus
Pel
Fe2
Gdh
Mdh2
Mnr
Perl
Per2
Per3
MeanHT
0.655
0.467
0.621
0.479
0.480
0.737
0.457
0.662
0.565HS
0.610
0.387
0.551
0.405
0.357
0.629
0.418
0.604
0.495GST
0.082*
0.135*
0.134*
0.180*
0.292*
0.167*
0.100*
0.104*
0.128*
*HT, total genetic diversity; HS, genetic diversity within subpopu-
lations; GST, proportion of total diversity due to genetic differentia-
tion among subpopulations.
*X^2 tests comparing genetic differentiation among subpopulations,
p < .001.relatedness within subpopulations was fairly high,
most saplings within subpopulations were unques-
tionably not the progeny of adult trees at that site.
Further analyses of relatedness showed that saplings
tended to show greater relatedness to one another than
adult trees did to one another. I could not determine
the number of seeds from different maternal trees that
were present within a given subpopulation. However,Table 8.8. Relatedness coefficient (R) estimates for
six natural subpopulations (A-F) and two experi-
mental plots (established in 1981 and 1984) of
Ocotea tenera (Gibson and Wheelwright 1995).
Subpopulation
(cohort)
Natural
A
B
(sapling)
(adult)
C
(sapling)
(adult)
D
E
F
(sapling)
(adult)
Mean
Experimental
1981
1984N26
22
16
6
27
21
6
14
12
11
7
428
25R0.225
0.091
0.060
0.090
0.014
0.147
0.145
0.172
0.185
0.265
0.213
0.157
0.1790.007
0.129
When applicable, R values for sapling and adult cohorts within sub-
populations are also given. N= sample size290 Plant-Animal Interactions