Monteverde : Ecology and Conservation of a Tropical Cloud Forest

SPECIALIZED SEED DISPERSAL:

MISTLETOES AND FRUIT-EATING BIRDS

Sarah Sargent

mall-fruited plants usually have broad, nonspe-
cific mutualistic interactions with the birds
who consume their fruits and disperse their
seeds. In most cases, fruits of a particular species of
plant are eaten by many species of birds, which eat
the fruits of other species of plants. Thus, strong spe-
cializations between fruiting plants and their avian
dispersers are usually absent. In Monteverde, as in
many other parts of the world, mistletoe fruits are
eaten by only a handful of bird species, and several
of the mistletoe-eating birds depend heavily on mis-
tletoe fruits in their diets. This is one of the few ex-
amples of interactions between plants and avian seed
dispersers that is highly specialized and that may
constitute coevolution.
Mistletoes are parasitic plants that have specific
requirements for seed dispersal (see Sargent, "Mistle-
toes," pp. 81—82). Unless their bird-dispersed seeds are
deposited on the smaller branches of suitable host
trees, the seeds die. Mistletoe seeds can be retained on
branches because they are covered with viscin, a thick,
gooey mucilage made up of semisoluble and insoluble
plant fiber. Birds that eat mistletoe fruits cannot digest
the viscin-coated seeds; they must either regurgitate
or defecate them and then dispose of them without
getting the seeds stuck to their feathers. Usually, the
birds wipe the seeds onto the twig where they perch;
the viscin dries and glues the seeds in place. Viscin

is thus a key adaptation of mistletoes, one that greatly

increases the probability that a seed will be dispersed

successfully and be deposited and retained at a suit-

able site.

The three main families of mistletoes, the Viscaceae,

Loranthaceae, and Eremolepidaceae (Table 8.6), have

fruits that are substantially different from one another,

structurally and nutritionally. These differences dic-

tate which birds eat the fruits and how they handle

them. Fruits of all three families at Monteverde are

similar in size (4-10 mm long x 2.5-6.5 mm in diam-

eter) and have only one seed per fruit. Viscaceae fruits

have small seeds relative to fruit size, a tough skin that

separates easily from the pulp layer, a high water con-

tent of the pulp, viscin completely surrounding the

seed, and energy in the form of sugars in the pulp. In

contrast, the Loranthaceae and Eremolepidaceae fruit

have large seeds relative to fruit size, thin skins that

are firmly attached to the thin layer of pulp, viscin

attached at only one end of the seed, pulp that sepa-

rates easily from the seed, and energy in the form of

fats in the pulp (Sargent 1994).

In Monteverde, as in Panama (Davidar 1987) and

Colombia (Restrepo 1987), nonoverlapping sets of birds

eat the two types of fruits. The Monteverde avifauna

includes over 80 species of frugivorous birds (Wheel-

wright et al. 1984), but only four species of tanagers

(Thraupinae) consume fruits of mistletoes in the fam-

Other species occur in the area, especially at lower and higher elevations, Mistletoes are most visible in

trees in pastures, along forest edges and along roadside edges. Some mistletoes, including Antidaphne

and Oryctanthus, can use other mistletoes as hosts.

288 Plant-Animal Interactions

Table 8.6. Common species of mistletoes in Monteverde.

Mistletoe Species

Viscaceae

Phoradendron robustissimum

Phoradendron flavens

Phoradendron robaloense

Phoradendron corynarthron

Loranthaceae

Gaiadendron punctatum

Stru th anthus oerstedii

Struthanthus costaricensis

Struthanthus quercicola

Oryctanthus spicatus

Psittacanthus schiedeanus

Eremolepidaceae

Antidaphne viscoidea

Typical Hosts

Sapium spp. (Euphorbiaceae)

Conostegia spp. (Melastornataceae)

Pouteria spp. (Sapotaceae), Ocotea spp. (Lauraceae)

Pouteria spp. (Sapotaceae)

Ferns, herbaceous plants

Citrus sp. (Rutaceae), Hampea spp. (Malvaceae)

Citrus sp. (Rutaceae), Guarea spp. (Meliaceae)

Citrus sp. (Rutaceae), Solanum spp. (Solanaceae)

Eugenia spp. (Myrtaeeae), Ficus spp. (Moraceae)

Psidium guayaba (Meliaceae)

Psidium guayaba (Meliaceae), Eugenia (Myrtaeeae)

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