This passage is excerpted from David B. Wake and Vance T. Vredenburg, “Amphibians in Crisis.” ©2008 by The National Academy of Sciences of the USA.
Amphibians have received much attention during the last
two decades because of a now-general understanding that a
larger proportion of amphibian species are at risk of extinction
than those of any other taxon.* Why this should be has
perplexed amphibian specialists. A large number of factors
have been implicated, including most prominently habitat
destruction and epidemics of infectious diseases; global
warming also has been invoked as a contributing factor. What
makes the amphibian case so compelling is the fact that
amphibians are long-term survivors that have persisted through
the last four mass extinctions.
Paradoxically, although amphibians have proven themselves
to be survivors in the past, there are reasons for thinking that
they might be vulnerable to current environmental challenges
and, hence, serve as multipurpose sentinels of environmental
health. The typical life cycle of a frog involves aquatic
development of eggs and larvae and terrestrial activity as
adults, thus exposing them to a wide range of environments.
Frog larvae are typically herbivores, whereas adults are
carnivores, thus exposing them to a wide diversity of food,
predators, and parasites. Amphibians have moist skin, and
cutaneous respiration is more important than respiration by
lungs. The moist, well vascularized skin places them in
intimate contact with their environment. One might expect
them to be vulnerable to changes in water or air quality
resulting from diverse pollutants. Amphibians are thermal-
conformers, thus making them sensitive to environmental
temperature changes, which may be especially important for
tropical montane (e.g., cloud forest) species that have
experienced little temperature variation. Such species may have
little acclimation ability in rapidly changing thermal regimes.
In general, amphibians have small geographic ranges, but this
is accentuated in most terrestrial species (the majority of
salamanders; a large proportion of frog species also fit this
category) that develop directly from terrestrial eggs that have
no free-living larval stage. These small ranges make them
especially vulnerable to habitat changes that might result from
either direct or indirect human activities.
Living amphibians (Class Amphibia, Subclass
Lissamphibia) include frogs (~5,600 currently recognized
species), salamanders (~570 species), and caecilians (~175
species). Most information concerning declines and extinctions
has come from studies of frogs, which are the most numerous
and by far the most widely distributed of living amphibians.
Salamanders facing extinctions are centered in Middle America.
Caecilians are the least well known; little information
on their status with respect to extinction exists.
The Global Amphibian Assessment completed its first round
of evaluating the status of all then-recognized species in 2004,
finding 32.5% of the known species of amphibians to be
“globally threatened” by using the established top three
categories of threat of extinction (i.e. Vulnerable, Endangered,
or Critically Endangered); 43% of species have declining
populations. In general, greater numbers as well as proportions
of species are at risk in tropical countries. Updates from the
Global Amphibian Assessment are ongoing and show that,
although new species described since 2004 are mostly too
poorly known to be assessed, >20% of analyzed species are in
the top three categories of threat. Species from montane
tropical regions, especially those associated with stream or
streamside habitats, are most likely to be severely threatened.
*A group of one or more populations of an organism or organisms seen by taxonomists to form a unit
Amphibians have received much attention during the last
two decades because of a now-general understanding that a
larger proportion of amphibian species are at risk of extinction
than those of any other taxon.* Why this should be has
perplexed amphibian specialists. A large number of factors
have been implicated, including most prominently habitat
destruction and epidemics of infectious diseases; global
warming also has been invoked as a contributing factor. What
makes the amphibian case so compelling is the fact that
amphibians are long-term survivors that have persisted through
the last four mass extinctions.
Paradoxically, although amphibians have proven themselves
to be survivors in the past, there are reasons for thinking that
they might be vulnerable to current environmental challenges
and, hence, serve as multipurpose sentinels of environmental
health. The typical life cycle of a frog involves aquatic
development of eggs and larvae and terrestrial activity as
adults, thus exposing them to a wide range of environments.
Frog larvae are typically herbivores, whereas adults are
carnivores, thus exposing them to a wide diversity of food,
predators, and parasites. Amphibians have moist skin, and
cutaneous respiration is more important than respiration by
lungs. The moist, well vascularized skin places them in
intimate contact with their environment. One might expect
them to be vulnerable to changes in water or air quality
resulting from diverse pollutants. Amphibians are thermal-
conformers, thus making them sensitive to environmental
temperature changes, which may be especially important for
tropical montane (e.g., cloud forest) species that have
experienced little temperature variation. Such species may have
little acclimation ability in rapidly changing thermal regimes.
In general, amphibians have small geographic ranges, but this
is accentuated in most terrestrial species (the majority of
salamanders; a large proportion of frog species also fit this
category) that develop directly from terrestrial eggs that have
no free-living larval stage. These small ranges make them
especially vulnerable to habitat changes that might result from
either direct or indirect human activities.
Living amphibians (Class Amphibia, Subclass
Lissamphibia) include frogs (~5,600 currently recognized
species), salamanders (~570 species), and caecilians (~175
species). Most information concerning declines and extinctions
has come from studies of frogs, which are the most numerous
and by far the most widely distributed of living amphibians.
Salamanders facing extinctions are centered in Middle America.
Caecilians are the least well known; little information
on their status with respect to extinction exists.
The Global Amphibian Assessment completed its first round
of evaluating the status of all then-recognized species in 2004,
finding 32.5% of the known species of amphibians to be
“globally threatened” by using the established top three
categories of threat of extinction (i.e. Vulnerable, Endangered,
or Critically Endangered); 43% of species have declining
populations. In general, greater numbers as well as proportions
of species are at risk in tropical countries. Updates from the
Global Amphibian Assessment are ongoing and show that,
although new species described since 2004 are mostly too
poorly known to be assessed, >20% of analyzed species are in
the top three categories of threat. Species from montane
tropical regions, especially those associated with stream or
streamside habitats, are most likely to be severely threatened.
*A group of one or more populations of an organism or organisms seen by taxonomists to form a unit