manipulated in a series of plots showed that the in-
creased risk in these areas was a hazard associated
with habitat choice rather than aggregation (Pounds
and Crump 1987).
Reptiles. The most obvious predators of snakes and
lizards are birds, although certain snakes and mam-
mals and a host of parasites are also important en-
emies. Little is known about predation on embryos
and juveniles; an unidentified shrew attacked a
hatchling Blunt-headed Tree Snake (C. Rojas, pers.
comm.). Brown Vine Snakes are important diurnal
predators of anoles (Henderson 1974), and Blunt-
headed Tree Snakes capture anoles on their noctur-
nal sleeping perches (Scott 1983d, J. A. Pounds, pers.
obs.). Skink-eaters are remarkably skilled predators
of lizards; they often subdue and swallow small skinks
in less than 10s (Henderson 1984). Snake-eating snakes
include Double-ringed False Corals, Mussuranas, and
Montane Mussuranas. Mussuranas are known for
their ability to subdue large pit vipers, including
Terciopelos (Scott 1983a). Avian predators of snakes
and large lizards include raptors such as Swallow-
tailed Kites, Black-chested Hawks, and White Hawks.
Predators of anoles and other small lizards include
Sunbitterns, primarily insect-eating birds such as Blue-
crowned Motmots and Streak-breasted Treehunters,
and fruit-eaters such as Resplendent Quetzals and Clay-
colored Robins. Many kinds of parasites attack reptiles
(Hoff et al. 1984), but little is known of their impacts.
Malarial protozoans, which are most diverse in the
tropics, are chiefly parasites of lizards (Telford 1977,
Schall and Vogt 1993).
5.4.3. Possible Causes of Declines
Discussion of possible causes of amphibian declines
at Monteverde has focused on ultraviolet (UV) radia-
tion, atmospheric pollution, epidemic disease, and
unusual weather (Pounds and Crump 1994). Here I
examine these factors in light of recent evidence. I
consider both amphibian and reptile declines, which
may be components of a single phenomenon. Both
began in the late 1980s, have taken place in seemingly
undisturbed upland habitats, and have led to disap-
pearances of populations (see Sees. 5.1.1., 5.4.1). Prey
scarcity may have reduced the abundance of frog-
eating snakes, but it is doubtful that the declines of
anoline lizards are a secondary consequence of am-
phibian declines. The combination of anuran and
lizard declines may parallel the case of Australia
(Czechura 1991). Although it seems unlikely that a
single cause can explain the patterns at Monteverde,
there may be a principal underlying factor that has
set the stage for proximate causes of mortality.Ultraviolet radiation, which can damage amphib-
ian embryos and increase their vulnerability to para-
sitic fungi (Blaustein et al. 1994b, Blaustein and Wake
1995), has probably not played a major role. Many
species that have vanished from the area breed under
the forest canopy and conceal their eggs. The Tilaran
Rain Frog, for example, buries its eggs in a subterra-
nean nest (see Sec. 5.5.3). Moreover, the rapidity of
the anuran declines implies high adult mortality,
which has not been linked to UV radiation (Pounds
and Crump 1994). Even if UV light could affect adults
of some species, Golden Toads normally spend most
of their time in underground retreats and are rarely
exposed to direct sunlight. Lizards that have suffered
population extinctions include the Montane Anole,
a basking species whose peritoneum (body-cavity lin-
ing) contains melanin that shields out UV radiation,
and the Cloud Forest Anole, a shade-dwelling species
that avoids direct sunlight (see Sees. 5.4.1, 5.6.2).
Although no unusual acidification has been ob-
served at Monteverde (Crump et al. 1992), unexpect-
edly high concentrations of nitrates and phosphates
in cloud water are evidence of atmospheric pollution
(see Chap. 2, Physical Environment). The possibility
of inputs that are more toxic should be investigated,
especially given concern that some widespread chemi-
cals may disrupt endocrine function in many organ-
isms, including amphibians and reptiles (Stebbins and
Cohen 1995). The threat of airborne pollution is great
in cloud forests, because mist and cloud-water depo-
sition can deliver contaminants at much higher con-
centrations than heavy rainfall (Glotfelty et al. 1987).
Epidemic disease may have played a role in the
declines. The 1987 crash of anuran populations was
so unexpected that it went unnoticed at first—nobody
searched for afflicted animals. In 1982-83, 40 dead
or dying Harlequin Frogs contained larvae of a para-
sitic fly (Notochaeta bufonivora; see Sec. 5.4.2), but
the mortality caused no major decline. Reports of dead
or dying frogs in the mountains of southern Costa Rica
and western Panama (Cordillera de Talamanca) sug-
gest epidemics (Berger et al. 1998, Lips 1998), but
there is no evidence that a single outbreak, spreading
in a wavelike fashion, has caused all the declines in
lower Central America. Sampling has covered too few
areas and mostly narrow time windows.
An alternative hypothesis is that an ecological
change over a large area (e.g., atmospheric pollution
or climate change) may have encouraged epidemics
of the same, or different, microparasites at different
times and places. Because this underlying factor
would only load the dice, not dictate the occurrence
of an outbreak, close synchrony in the declines should
not be expected. Outbreaks similar to those in Central
America have taken place in Australia (Laurance et al.161 Amphibians and Reptiles