iStockphoto
Amphibian species have been facing a steep decline for decades, in large part because of a fungus, climate change, and environment disruption. As many as one-third of the world’s 6,300 amphibian species are threatened with extinction, and researchers fear their loss could wreak havoc on our ecosystem and food webs. Here are landmark studies that have defined the problems and—we hope—will help humans to figure out how to save their froggy friends.
1. The long-term perspective
In August
2008 two researchers at University of California at Berkeley published a
meta-analysis titled “Are we in the midst of the sixth mass extinction?” The
global assessment highlighted the threat of chytridiomycosis, an infectious,
rapidly spreading disease caused by a waterborne fungus.
2. The seminal study
The study
“Status
and Trends of Amphibian Declines and Extinctions Worldwide” [subscription
required] showed one-third of the world’s 6,300 amphibian species are
threatened with extinction (compared with just 12 percent of all bird species
and 23 percent of mammal species). The authors concluded that scientists must
begin captive breeding.
3.
Problems with captive breeding
A
September 2008 study published in Current
Biology said captive breeding program accidentally introduced the chytrid
fungus that causes chytridiomycosis into Mallorca in 1991; an endangered frog
species was housed in the same room as a group of toads, and the frogs spread
the chytrid fungus to the toads. The fungus was not known at this time, so
health screening of the toads did not reveal the problem.
4.
Extinction rates
According
to a 2007 study in the Journal of
Herpetology, amphibian species are becoming extinct 211 times faster than
the “normal extinction rate,” the standard rate of extinction in history before
humans became a primary contributing factor. And if you count those species “in
imminent danger of extinction,” that rate climbs to a whopping 45,474 times
faster than normal.
5.The global-warming theory
It is
unclear how the chytrid fungus spreads. A 2006 study in Nature [subscription required] blamed global warming, for creating ideal conditions shifts
temperatures to those more agreeable to the fungus’ growth and
reproduction—between 63° and 77°F. But there were many outspoken skeptics of
this claim (see study #6),
6.
Skeptics of the global-warming theory
A March
2008 PLoS Biology paper said there is
“no evidence to support” the global-warming hypothesis.Instead, researchers said, the pattern of the
fungus’ spread was typical of an emerging infectious disease; they call their
theory the “spreading pathogen hypothesis.” The authors suggested governments
and environmental agencies can help prevent the fungus’ spread by
regulating potential infection routes, such as the ornamental plant and
aquarium wildlife trade.
7. Problems with insecticides
October
2008’s Ecological Applications [subscription required] study
suggests malathion, the most common insecticide in the U.S., can
devastate tadpole populations even at doses too small to kill individual
tadpoles. Researchers created simulated ponds in 300-gallon outdoor tanks,
placed tadpoles inside, and exposed the ponds to no malathion, a single
moderately concentrated dose, or low concentrations in weekly doses similar to the
exposure tadpoles experience in (human-altered) nature. Even the small amounts
of malathion set off a chain of events that caused a decline of tadpoles’
primary food source: bottom-dwelling algae. Consequently, half the tadpoles in
the experiment did not reach maturity and would have died in nature.
8. A
fungus-free country
Oddly, none
of the many known amphibian species in Madagascar have been driven to
extinction; the island also shows no signs of the chytrid fungus, according to
a May 2008 PLoS Biology paper.
Because the amphibians in Madagascar are doing so well, the authors argue the
region is one of the best places to focus future research efforts. They hope to
find out what helps amphibians in Madagascar thrive, as they suffer steep
declines elsewhere.
9.
Beating the fungus
Introducing
probiotic bacteria into the ecosystem could help lessen the effects of
chytridiomycosis, according to research presented in June 2008 at the 108th
General Meeting of the American Society for Microbiology. The tests indicated
that adding pedobacter, a bacterial species that occurs naturally on the skin
of red-backed salamanders, to the skin of mountain yellow-legged frogs
decreased chytrid’s deadly effects.
10. Evolving more slowly than the environment
A May
2007 study in BioScience attributed
amphibians’ decline to their inability to adapt to the current rapid rate of
global change. The authors noted the aforementioned pesticide pollution and
chytrid infections, as well as habitat loss and UV-B light exposure that causes
mutations in amphibian eggs. Amphibians are particularly vulnerable because
they have permeable skin, ability to live on both land and water, and eggs
without shells. Perhaps most detrimental is their complex life cycle, which
makes evolution an even slower process.
Read the companion story Are Frogs Hopping Straight Into Extinction?
