Earlier hunting pressure and habitat modification
virtually eliminated large raptorial birds such as hawk-
eagles and large carnivorous mammals in Monteverde.
Their populations are unlikely to rebound in the fu-
ture because pressures on them are even more severe
elsewhere in their ranges. After the loss of carnivorous
mammals on Barro Colorado Island (BCI), Panama, for
example, there was a rapid increase in the abundance
of their prey (rodents such as Agoutis). Because Agou-
tis are major seed predators, the decline in top carni-
vores resulted in a change in the structure of the plant
community on BCI (Leigh et al. 1982). In similar fash-
ion, the disappearance of top carnivores may have
affected animals and plants in Monteverde. Redford
(1992) evoked the image of an "empty forest," one that
still has towering trees but has lost much of its fauna,
including many of the animals on which plants de-
pend for seed dispersal. Fruit-eating bats, which are
less affected by forest fragmentation than less mobile
animals, may continue to serve as seed dispersers
even in isolated forest remnants, although the com-
position of plant communities is likely to change be-
cause of bats' preference for particular fruits (see
Dinerstein, "Influence of Fruit-Eating Bats," p. 434).
Straddling the Continental Divide, Monteverde's
forest must have always been home to species that are
chronically rare because of the restriction of their
habitats to mountain peaks. The Golden Toad and the
Ruddy-capped Nightingale-Thrush exemplify obli-
gate highland species that probably never had large
population sizes. In theory, populations that have
been rare and isolated in the past are less prone to
inbreeding depression because most deleterious al-
leles have been exposed by inbreeding and eliminated
by natural selection. The risk to these historically
small populations is not genetic but demographic;
minor population fluctuations can cause them to
crash (Soule 1986).
At the scale of landscapes, the distribution of Costa
Rica's remaining forests illustrates some of the threats
to biodiversity that are peculiar to Monteverde and
other highland habitats. Not only is the area of suit-
able habitat on mountaintops restricted, but also habi-
tats are isolated on two fronts. Deforested or climati-
cally inhospitable lowlands act as dispersal filters
or barriers between peaks (Fig. 12.2). At the other
altitudinal extreme, there is nowhere to go, so high-
elevation species become stranded on mountain peaks.
If global warming continues to the point where local
climates change appreciably (Kerr 1997) or plant com-
munities are altered, conditions may become unsuit-
able for lower montane forest species (Abrahamson
1989, Peters and Lovejoy 1992, Tosi et al. 1992). In
fact, models predict that even a slightly warmer cli-
mate will increase the altitude at which clouds form
in tropical mountains, drastically diminishing the
cloud-born precipitation vital to cloud forest eco-
systems (Still et al. 1999). Long-term precipitation,Figure 12.2. Soil erosion on the deforested and heavily grazed Pacific slopes below Monteverde.
Photograph by Robert Timm.425 Conservation Biology