Bird-pollinated Besleria Besleria triflora (Ges-
neriaceae), a shrub pollinated by short-billed hum-
mingbirds, is andromonoecious: individual plants bear
both hermaphroditic and male flowers. Male flowers
in andromonoecious species have been assumed to
permit plants to increase the total amount of pollen
produced, but this is not the case for B. triflora. Male
flowers produce nectar but little pollen. In Monte-
verde, the role of these male flowers is to attract pol-
linators to the plant (Podolsky 1992). Three factors
contribute: (1) the larger the floral display, the more
pollinator visits a flower in that display receives; (2)
hermaphroditic flowers (which can produce fruit)
receive more of the desirable outcrossed pollen if the
flowers near them are nectar-supplying male flowers,
which help attract the birds; and (3) the structure of
these male flowers ensures that they do not remove
pollen carried by the visiting birds. Rather, the pol-
len carried by a bird stays in place while the bird sips
nectar from male flowers but can be removed by the
stigmas of hermaphroditic flowers when they are
visited.
Buzz-pollinated Saurauia A similar pollinator-
attracting role seems to explain the unusual andro-
dioecious sexual system of a small tree of Saurauia
(Actinidiaceae; Cane 1993). In this apparently rare
sexual system, individual plants bear only one of two
flower types: male flowers or flowers that appear to
be hermaphroditic. In this case, the flowers of her-
maphroditic plants function only as females because
their pollen is sterile. Paradoxically, it is their sterile
pollen that ensures these functionally female flowers
receive fertile pollen from male plants: bees bearing
fertile pollen come to collect the sterile pollen, as all
the flowers of this buzz-pollinated species are nectar-
less (Cane 1993).
8.1.3. Biogeographic and
community patterns
Pollinators and forest strata. Monteverde's most com-
mon pollination system is pollination by small di-
verse insects (33% of the flora). It is also the most
frequent pollination system for all plant life-forms.
However, pollinator frequencies differ among the for-
est strata (Koptur et al. 1988; Table 8.1). In the under-
story, the frequencies of pollination by birds, butter-
flies, small bees, and bees are high compared to the
whole flora. Lianas have relatively high incidences of
pollination by beetles and large bees. Overall, Monte-
verde's upper strata (subcanopy and canopy trees)
have a high frequency of pollination by small diverse
insects, with pollination by hawkmoths, moths, and
small bees also relatively high for trees. For large trees
(258 species), pollination by small diverse insectsis especially high (48%), beetle pollination is rare
(0.8%), and pollination by butterflies and birds is
absent. Epiphytes (in the subcanopy and canopy),
however, add hummingbird pollination to the upper
strata (Table 8.1). La Selva's taller forest shows even
more pronounced vertical stratification of pollina-
tion systems (Bawa 1992, Kress and Beach 1994,
Kato 1996).Pollinator effectiveness. The flowers of many plant
species at Monteverde are visited by a wide variety
of animals, and these may vary in their effectiveness
as pollinators. Meticulous study is needed to demon-
strate which among a variety of potential pollen trans-
port agents is the most effective pollinator (Schemske
and Horvitz 1984, Herrera 1987, Murawski 1987,
Bawa 1994). Unspecialized flowers that are accessible
to a variety of small diverse insects merit particular
attention. Monteverde offers opportunities to test the
underlying assumptions that (1) fewer pollinators are
available at middle elevations, (2) a majority of a plant
species' small flower visitors are able to effect polli-
nation, and (3) most small insect flower visitors to
such species are essentially interchangeable (Bawa
1994).Effects of elevation: pollination of trees. Pollination
system frequencies among trees apparently shift with
elevation (Bawa 1990). Pollination of medium and
large trees by small diverse insects and small bees is
nearly twice as common in Monteverde (500 tree spe-
cies examined) than at La Selva (125 tree species ex-
amined); (Kress and Beach 1994). Pollination by wind
is nearly six times more frequent in Monteverde than
in La Selva. Much lower in frequency is pollination
by beetles, bats, and large bees. The two other neo-
tropical montane forests studied to date resemble
Monteverde in the importance of small diverse insects
and small bees as pollinators of trees (Sobrevilla and
Arroyo 1982, Tanner 1982). In Jamaica, fewer polli-
nation systems occur, and pollination by wind is
more pronounced, as expected for island trees (Tan-
ner 1982).
Three factors could contribute to these elevational
differences. First, as physical conditions change with
elevation, so should the most effective pollinator
type. At high elevations (2750-4000 m), pollination
by birds is more frequent and more effective than pol-
lination by bees and hawkmoths, probably because
birds can be active in cloudy, rainy, cool conditions
(Cruden 1972, Cruden et al. 1976). Monteverde's pre-
ponderance of pollination by small bees and small
diverse insects that forage as generalists is explained
in part by the windy, misty, cool, and unpredictable
conditions (Koptur et al. 1988).252 Plant-Animal Interactions