comparisons of genetic diversity and relatedness val-
ues of naturally occurring subpopulations with val-
ues from experimental plots (whose establishment
history was known; Wheelwright and Bruneau 1992)
suggested that naturally occurring subpopulations
contain the offspring of five to seven maternal trees
(Gibson and Wheelwright 1995).
My data provide three conclusions about avian seed
dispersal and seedling recruitment in O, tenera. First,
genetic relatedness values indicate clumped dispersal
of related seeds by birds. Relatedness among groups
of seeds dispersed together persists into sapling and
adult age classes. Second, saplings underneath adult
O. tenera trees are not the progeny of the adults; there
is relatively long-distance seed dispersal of related
seeds. Third, clumped dispersal of related groups of
seeds causes high genetic differentiation among sub-
populations. The random occurrence in the forest of
disturbed sites suitable for germination and the random
chance of a bird depositing any given group of seeds
at that site cause subpopulations to be genetically dis-
tinct from one another. Fruit consumption and seed dis-
persal behaviors of foraging birds have a profound ef-
fect on genetic structure among subpopulations and
among age classes of trees within subpopulations. Fur-
ther genetic analysis of tropical tree populations will
provide a foundation for evolutionary and conserva-
tion studies in Monteverde.•MYRMECOPHYTES.
JohnT, Long]non tropical regions throughout the world, one can
find myrmecophytes, plants that provide special-
ized food and housing for ants, and ant species
specialized for finding and occupying these plants
(Bequaert 1922, Wheeler 1942, Buckley 1982a, b,
Beattie 1985, Davidson and McKey 1993a, b). Myr-
mecophyte-ant interactions are often assumed to be
mutualisms; the plants benefit the ants by providing
food and nest sites, and the ants benefit the plants by
providing a bodyguard service, driving away herbivores
and in some cases trimming encroaching vegetation.
In a few cases, the assumption has been borne out
with field experiments (Janzen 1966, 1967a, b, 1969,
Letourneau 1983, Schupp 1986). Now that myrme-
cophyte-ant interactions are known to be mutualistic,
emphasis has shifted to revealing their evolutionary
history (McKey 1991, Ward 1991, Davidson and McKey
1993b, Ayala et al. 1996) and explaining temporal and
spatial variation in the interactions (Longino 1989a,
1991a, b, 1996, Davidson and Fisher 1991, Davidson
et al. 1991). Myrmecophyte-ant interactions that occur
in Monteverde are Cordia alliodora trees with associ-
ated Azteca and Zacryptocerus ants (Longino 1996),
Triplaris trees with associated Azteca ants (Longino
1996), understory Ocotea and Guarea trees with associ-
ated Myrmelachista ants (Stout 1979, Longino and Han-
son 1995), and Cecropia trees with associated Azteca
ants. The Cecropia-Azteca interaction is the most con-
spicuous, abundant, and accessible of the myrmecophyte-
ant interactions and is the focus of this essay.
Cecropia is a genus of over 100 species of neo-
tropical trees, five of which occur in mainland Costa
Rica (Burger 1977). Most species are myrmecophytes,
with a suite of traits that facilitate ant habitation
(Miiller 1876, 1880-1881; Fig. 8.24). The stems are
hollow with internal partitions, much like bamboo
(Fig. 8.24B), which form ample nesting space for ants.
The stern walls produce viscous sap when damaged
by chewing, but at regular intervals along the stem
there are thin spots through which the ants can safely
chew. At the base of each leaf petiole, a dense pad of
hairs produces small white beads called Miillerian
bodies (Fig. 8.24), which contain glycogen (Rickson
1971) and appear to be the sole food for the ant colo-
nies that inhabit the trees (Fig. 8.24D,E).
Azteca is a genus of ants containing hundreds of
neotropical species (Forel 1899, Carroll 1983), of
which 13 are known to be obligate Cecropia inhabit-
ants (Longino 1991b). New queens fly from their
natal nest and find small Cecropia saplings. They shed
their wings and chew a hole through one of the thin
spots. Once inside, they plug the entrance hole with
scrapings from the inner stem surface. The hole
quickly grows closed, leaving the queen sealed inside.
She rears a few small workers which emerge and for-
age on Miillerian bodies. Many founding queens may
inhabit the stacked chambers of a single sapling, but
only one ultimately succeeds in establishing the
colony that comes to dominate the tree. During this
founding phase, multiple species of Azteca queens
can be found in the same sapling, and multiple queens
may occur in the same chamber (Longino 1989a).
Several lineages of Azteca are obligate inhabitants
of Cecropia (Longino 1989b, 1991b, Ayala et al. 1996),291 Plant-Animal InteractionsOI