Figure 5.3. A Harlequin Frog (Atelopus varius) in the Monteverde cloud forest. Photograph by Michael
and Patricia Fogden.et al. 1986, 1989; Appendix 8). In compiling the list
of species we would ordinarily expect to find,
we omitted the rare Highland Fringe-limbed Treefrog,
Monteverde Rain Frog, and Black Narrow-mouthed
Toad. We included the highly arboreal Giant Fringe-
limbed Treefrog and Crowned Frog, because their call-
ing sites and distinctive vocalizations were well known
(M. P. L. Fogden, unpubl. data). We assessed the num-
ber of species missing in 1990 and the number miss-
ing throughout 1990-94. Although it is possible to
overlook burrowing forms that appear only briefly at
temporary pools (e.g., the Golden Toad and the Sheep
Frog), this is unlikely during five consecutive years.
To estimate the number of disappearances expected
in the context of natural demographic variability, we
examined long-term studies of other amphibian assem-
blages. We chose studies that illustrated how unstable
amphibian populations can be (Pechmann and Wilbur
1994) and that were conducted on spatial scales appro-
priate for comparison with Monteverde. From these
studies, we estimated the average probability that a
single species would disappear in response to a natural
environmental disturbance such as a drought. Substi-
tuting this probability in the equation for a binomial
distribution, we calculated the likelihood that a particu-
lar number of species would disappear simultaneously.
The resultant probability distributions suggest that
normal population dynamics cannot easily explain
the number of disappearances at Monteverde. Of the
50 species of frogs and toads that we would expect to
find, 25 were absent in 1990. Five of the missing spe-
cies reappeared during 1991-94, so 20 (40%) were
absent throughout the study (Appendix 8). Since com-
pletion of that study, one species has reappeared
(Ferrer's Leopard Frog, in 1997). Thus, 11 years after
populations crashed, the diversity of frogs and toads
is still impoverished. The reappearances were prob-
ably due to recolonization from outlying areas. Most
of these events took place on the periphery of the
study area after several years of absence. Although
even limited recolonization might seem encouraging,
the species that reappeared (except the Bare-hearted
Glass Frog) also inhabited nearby lowlands, where we
have seen little evidence of declines. Prospects are
dim for high-elevation species such as the Golden
Toad and the Green-eyed Frog (see Sec. 5.3).
A comparison of the above patterns with those for
breeding birds in the same area puts the loss of an-
uran diversity in perspective and underscores the im-
portance of further study (Pounds et al. 1997). The
relative frequency of absences was much greater for
frogs and toads than it was for birds. That for anurans,
however, was similar to that for invasive open-country
birds whose habitats in the Pefias Blancas valley had
largely reverted from farmland to forest after being
annexed to the Monteverde Cloud Forest Preserve152 Amphibians and Reptiles