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CULPRITS
Why frogs are vanishing
Before we
look into the reasons why, should we even care that frogs are vanishing?
o If we humans are responsible, we have a moral obligation to prevent
amphibian disappearances.
o Frogs may directly benefit humans. Australia’s gastric-brooding
frog, now extinct, had an enzyme-suppression mechanism that might
have helped people with gastric ulcers.
o Frogs eat disease-carrying insects.
o Frogs are critical links between predators and the bottom of
the food chain (algae, plants, detritus, and such). Frogs feed on many
organisms, and many organisms feed on them. Remove the frogs, and important
links are knocked out of the food chain.
o Frogs are important indicators of environmental health. Because
most frogs spend their early days in water and their adult life on land,
changes in both worlds may affect them.
Frog numbers are dropping in at least 140 countries. Biologists
have identified a number of reasons but suspect that a combination of
factors may be responsible.
James
P. Blair |
LOSS
OF HABITAT
The
number one cause of amphibian decline is habitat loss, and we humans
are to blame. By cutting down forests, damming rivers, and draining
marshes to build houses and shopping centers, we ravage landscapes
around the world.
Evidence: Most amphibians feed and breed in wetlands, so
loss of wetlands equals loss of amphibians. In the past half-century
the lower 48 states have lost more than half of their estimated
original wetlands.
Evidence: Urban development in southern California has gobbled
up 75 percent of the range of the endangered arroyo toad (Bufo
microscaphus californicus). Now the toad must compete with weekend
gold miners, fishermen, campers, and off-road fanatics for use of
its few remaining streams.
Evidence: In the United Kingdom, where many breeding ponds
have been filled in (as many as 80 percent in some areas), all six
native amphibian species have suffered dramatic population declines.
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Erik
Enderson |
DISEASE
Amphibian diseases may be carried from continent
to continent by aquarium fish and exotic animals sold as pets. Disease
may also spread to frog populations on people’s boots or by
birds, insects, and livestock. Before tadpoles are moved from one
area to another to reestablish frog populations, biologists give
the youngsters a clean bill of health to prevent the spread of disease.
Evidence: A deadly virus is the likely culprit in several
recent die-offs of frogs, including spring peepers at Acadia National
Park in Maine, wood frog tadpoles in Massachusetts, mink frogs in
Minnesota, wood frogs and bullfrogs in North Carolina, and numerous
frogs at the Great Smoky Mountains National Park in Tennessee.
Evidence: Several species, such as Costa Rica’s golden
toad, may have disappeared because of chytrid (KIH-trid) fungus
(see photo). This infectious fungus spreads in water and can wipe
out entire populations or species even from pristine habitats. The
frogs most affected live in streams in mid- to high-elevation rain
forests. Climate change may also play a role.
Evidence: Scientists have detected chytrid fungus on almost
a hundred species of amphibians on six continents. More than 40
of those species are in Australia. Chytrid fungi are common in nature,
but this is the first one known to have attacked vertebrates.
Evidence: Most studies of amphibian disease are done in a
lab or on dead specimens. When Don Nichols, a pathologist at the
National Zoological Park in Washington, D.C., put antifungal medication
on captive frogs infected with chytrid fungus, the frogs recovered.
This method is being used to treat infected frogs in other zoos
and private collections.
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Eric
L. Williams
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CLIMATE
CHANGE
Amphibians have vanished even from wilderness areas.
As Earth’s ozone layer thins, increased ultraviolet radiation
may undermine the hatching success of the eggs of certain amphibians.
(Most amphibian eggs have a gelatinous coating but no protective
shell.) UV radiation can also alter the DNA in frogs’ cells
and suppress immune responses. At risk: Frogs that live at cooler,
higher elevations and extreme latitudes, where the ozone layer is
thinner but where amphibians must bask in the sunlight to regulate
body temperature.
Evidence: Increased exposure to ultraviolet radiation may
damage the eggs of the Cascade frog (Rana cascadae), which
lays its eggs in shallow water in high mountain meadows.
Evidence: Global warming may have caused the erratic weather
that ruined the breeding efforts of the golden toad in Costa Rica’s
cloud forest. In 1986-87 warm, dry weather dried up the pools before
the toads’ larvae had matured. Of a potential 30,000 toads,
only 29 survived. The drought may have caused the toads and other
amphibians to concentrate at the remaining pools, which facilitated
the transmission of disease and resulting death from the chytrid
fungus.
Evidence: Eggs and tadpoles need moisture to develop. In
southern California the endangered arroyo toad (Bufo microscaphus
californicus) lays its eggs in shallow pools created by spring
rains. During dry periods the toad can’t reproduce.
Evidence: In times of drought, frogs can become more susceptible
to disease or even dry up and die. When drought threatened a colony
of Chiricahua leopard frogs (Rana chiricahuensis) in southeastern
Arizona, a conservation-minded rancher trucked in a thousand gallons
of water a week for two years.
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James
P. Blair |
POLLUTION
Since frogs breathe at least partly through their
thin, porous skin, they’re very susceptible to toxins. Industrial
pollution may be wiping out local populations. In farm areas, herbicides
and pesticides run off into the ponds, watering tanks, and irrigation
canals where frogs live and breed. Some herbicides interfere with
respiration, and certain pesticides and their by-products produce
severe limb deformities that prevent frogs from fleeing predators.
Evidence: Pesticides drifting up from farmland may be killing
amphibians in California’s Sierra Nevada and other wilderness
areas.
Evidence: Acid rain from air pollution can turn lakes, streams,
and wetlands into death traps. In Canada, Europe, and the northeastern
United States, acid rain has damaged forests and polluted soil and
water, making some areas unlivable for frogs.
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Cecil
Schwalbe
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INTRODUCED
AQUATIC PREDATORS
When sport and bait fish, crawfish, and bullfrogs
(Rana catesbeiana) invade lakes and wetlands where they’ve
never lived, these predators often devour native frogs and their
prey. Humans introduce most aquatic predators for sport and food,
while others escape from captivity or sneak in as stowaways on potted
plants.
Evidence: Introduced trout have nearly wiped out the mountain
yellow-legged frog (Rana muscosa) in California’s Sierra
Nevada.
Evidence: In southeastern Arizona sport fish and the bullfrog,
introduced from the eastern United States a century ago, have nearly
eaten the Chiricahua leopard frog (Rana chiricahuensis) out
of its native waters. The aggressive, fast-growing bullfrog devours
anything it can fit in its mouth: tarantulas, native frogs, rattlesnakes,
bats (see photo), songbirds, and many other creatures. A bullfrog
can lay as many as 20,000 eggs per clutch.
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OVERHARVESTING
People harvest frogs for food as well as for hides,
pets, and research. Some states and countries have banned the commercial
harvesting of frogs for dissection in biology classes. Frog-friendly
alternatives include videotapes, 3-D anatomical models, and interactive
CD-ROMs.
Evidence: In the late 1800s the massive collecting of California
red-legged frogs (Rana aurora draytonii) for food depleted
local populations, which led to the introduction of bullfrogs (Rana
catesbeiana).
Evidence: Between 1981 and 1984 Americans devoured more than
6.5 million pounds of frog legs a year, which led to the death of
some 26 million frogs annually. Ninety percent came from India and
Bangladesh, which banned exports after frog declines led to growing
hordes of mosquitoes, malaria, and increased use of pesticides.
Now Indonesia supplies most of the frogs for restaurants.
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