In tropical forests used as extractive reserves, the
experiment is underway. Many of these sites have lost
an important component of the larger avian and mam-
malian disperser pool. Unfortunately, in highly threat-
ened tropical forests, it is likely that the time lag
between the loss of large frugivores and the loss of ma-
ture individuals of large-fruited trees is so great that
habitat loss will occur before we are able to document
the erosion of dispersal processes.THE IMPORTANCE OF PACIFIC SLOPE FOREST FOR
MAINTAINING REGIONAL BIODIVERSITY
Carlos F. Guindons recently as 1935, most of the upper slopes
of the Cardillera de Tilaran range were for-
ested. In the 1990s, forest occurs as remnants
varying in size and isolation, especially below 1500
m (see Fig. 12.4). How important are Pacific slope
forest remnants for maintaining the biological diver-
sity of the region? In my study of conservation biol-
ogy of forest fragments, I asked (1) how great is tree
species richness in forest fragments, (2) how is this
diversity distributed, (3) what proportion of tree spe-
cies depend on vertebrates for seed-dispersal, and (4)
what aspects of forest fragmentation influence the use
of fragments by large frugivorous birds?
I selected 30 forest fragments (1000-1500 m in ele-
vation) that varied in size and isolation (see Fig. 12.4).
Tree species richness was determined from a system-
atic 10% sample of each fragment. Transects were
established along parallel lines running the length of
each fragment. All trees located within 2.5 m of the
transect line were measured and identified to spe-
cies. The same transects were used to census bird and
Lauraceae fruit abundance. Bird censuses were con-
ducted by walking the transects at a pace of 10 m/min
and recording all individuals seen or heard within
25 m on each side of the transect.
Tree species richness was high in the fragments.
The sample included 5800 stems representing 59
plant families, 130 genera, and 225 species. Many of
these tree species occur only within narrow climatic
bands along the sharp altitudinal moisture gradient
of the Pacific slope and are not represented within the
protected forest. This shift in species along the alti-
tudinal gradient is evident by comparing the 10 most
common species within three elevational ranges
(Table 12.1). The long-term survival of many of these
tree species, now restricted to forest remnants, wind-
breaks, or pastures, will depend on the ability of pol-
linators and seed dispersers to locate them. Of the 225
tree species identified in the fragments, 88% depend
on vertebrates for seed dispersal, with most (63%) pri-
marily dispersed by birds. The long-term survival of
many of the seed dispersers and pollinators may de-
pend on the continued existence of the trees distrib-
uted along the Pacific slope. The Lauraceae, with 26
species, was the most species-rich tree family within
the 30 forest remnants. All of these are dispersed by
vertebrates, and all but two species by birds. The
Lauraceae also contributed the most to overall stem
density and basal area within many of the remnants.
Sixteen of the 24 bird-dispersed species in the for-
est remnants produce large fruit (>1.7 cm diameter).
Their seeds are dispersed almost exclusively by Black
Guans, Keel-billed Toucans, Emerald Toucanets, Re-
splendent Quetzals, and Three-wattled Bellbirds. All
of these species spend at least part of the year within
forest remnants on the Pacific slope (Table 12.2), where
they feed on the fruits of Lauraceae and other plant
species. Only toucanets were found in all remnants
and during every month of the year. Bellbirds and
toucans were most abundant in the forest remnants
with the most Lauraceae fruit, even when these rem-
nants were isolated and small. Guans and quetzals
were not observed in the more isolated remnants. They
were most abundant in the larger remnants closer to
the extensive forest and at higher elevations. They
were also observed (but not censused) in large rem-
nants located along the cliff-edge that drops down into
the Guacimal and San Luis river valleys at 1200 m.
This suggests that isolation, rather than elevation it-
self, limits the use of lower forest remnants by guans
and quetzals (Table 12.3).
Data from captive birds suggest that they may dis-
perse considerable numbers of seeds while using for-
est remnants along the Pacific slope. A bellbird may
consume more than 30 large Lauraceae fruits (e.g.,
Beilschmiedia brenesii or Ocotea monteverdensis) in
one day. Assuming that an individual consumes only
large fruits and is present in the remnants for four
months of the year, it may disperse more than 3600
seeds. Many of these seeds are dispersed within the
same remnant as the parent tree, but some are dis-
persed to other remnants. In the case of the bellbirds435 Conservation BiologyA