by loss of dispersers or mates via extirpation or
insularization. Even if plant and pollinator popula-
tions persist in fragments for long periods of time,
insularization may lead to altered mating and genetic
structures, loss of variation, and inbreeding depres-
sion (sensu Bawa 1990). Studies of plant population
structure and gene flow can improve conservation
decision-making, help evaluate the need for corridors,
and motivate local restoration of plant resources for
pollen and seed dispersers (Murawski 1995).
8.2. Plant-Frugivore Interactions
8.2.1. Why Disperse Seeds?
The ubiquity of morphological characteristics asso-
ciated with seed dispersal begs for an explanation of
the advantages of moving seeds away from their source.
Howe and Smallwood's (1982) "escape hypothesis"
proposes that dispersal, even if directionally random,
benefits plants by reducing density-dependent pre-
dation on seeds and seedlings and/or competition
among seedlings (or between them and their parents)
near the parent plant. The "colonization hypothesis"
proposes that dispersal benefits plants by increasing
the likelihood that some of their offspring will land
in patches of suitable habitat, which occur unpredict-
ably in space and time (e.g., treefall gaps). The "di-
rected dispersal" hypothesis suggests that dispersal
by certain animals results in nonrandom movement
of seeds into particular sites where the probability of
germination and establishment is high (e.g., dispersal
by ants to refuse piles on rotting logs). These ideas are
a useful framework for understanding plant-frugivore
coevolution.
8.2.2. Who Eats What, Where,
How, and Why
Just as plants can be categorized on the basis of their
likely pollinators, ecologists group plants into broad
"dispersal syndromes"—suites of characteristics (e.g.,
color, size, number and sizes of seeds, nutrient con-
tent, presence of a surrounding husk) associated with
dispersal agents (van der Fiji 1969).Dispersal syndromes: abiotic. Wind dispersal Suc-
cessful dispersal by wind requires mechanisms to
reduce the velocity with which a seed falls, so that
lateral air movements will carry it farther from the
source. One means is to greatly reduce seed size; the
best examples of this reduction in Monteverde are
among orchids, bromeliads, and begonias. Highly re-
duced seeds also occur in families that are typicallyassociated with animal dispersal, for example, Cen-
tronia phlomoides and Monochaetum spp. and other
members of the Melastomataceae, the majority of
which are dispersed by birds. Another way to reduce
downward velocity is to enlarge some part of the seed
coat or fruit to form a wing or plume that increases
the surface area of the dispersal unit. In Monteverde,
the best examples are herbs or epiphytes in the sun-
flower family (Asteraceae), for example, Senecio and
Neomirandea. Some Monteverde trees also produce
wind-dispersed seeds (e.g., Roupala glaberrima [Pro-
teaceae], Lonchocarpus sp. [Fabaceae], and Wein-
mannia sp. [Cunoniaceae]).
Water dispersal Water-dispersed seeds generally
have the ability to float for an extended period, due
either to the high surface area-to-volume ratio of
minute seeds or to a flotation device, for example, coco-
nuts and mangroves. The only known native Monte-
verde example is Heteranthera reniformis (Pontederi-
aceae). Secondary dispersal (the further movement of
seeds that have already been dispersed by other means)
may occur via water for some species.
Ballistic dispersal Some plants produce fruits (of-
ten capsules) that "explode" when mature, throwing
the seeds a short distance away. When mature, the
fruit wall splits along one or more sutures, allowing
part of the fruit wall to change shape rapidly and
throw the seeds. Monteverde examples include the
vine Cyclanthera explodens (Cucurbitaceae), the
Pacific-slope tree Hura crepitans (Euphorbiaceae),
the exotic herb Impatiens walleriana (Balsaminaceae),
and most members of the family Acanthaceae.
Gravity dispersal or dispersal mechanism unknown
In a small number of species, no apparent specialized
structures exist for moving seeds away from the par-
ent plant by either physical factors or animals. The
seeds seem merely to fall from the plant. Further study
of these plants may reveal a more effective means of
dispersal, or the dispersal agent with which the plant
evolved may be extinct, as with some common trees
in Guanacaste Province (Janzen and Martin 1982, but
see Howe 1985).Dispersal syndromes: biotic Ants Seeds adapted for
dispersal by ants typically are small, and have an at-
tached lipid-rich (sometimes starch-rich) body ("elaio-
some") as a food reward to the ant. Such seeds are usu-
ally carried back to the nest where the elaiosome is
removed and consumed. The seed itself is usually
discarded on a refuse pile near the nest. Refuse piles
are often prime seedling establishment sites, being
free of other plants and rich in decaying organic mate-
rial. Langsdorffia hypogaea (Balanophoraceae) is a
Monteverde species that fits the ant dispersal syndrome.
Seed-eating ants may sometimes act as secondary dis-256 Plant-Animal Interactions