GLOBAL DECLINES OF
AMPHIBIANS
Vance T. Vredenburg and David B. Wake
University of California, Berkeley
I.
II.
III.
IV.
V.
Amphibian Biodiversity
Dimensions of the Problem
Factors Responsible for the Declines
Challenges and Opportunities for the Future
Implications for the Biodiversity Crisis in General
GLOSSARY
emerging disease An infectious disease that has
newly appeared in a population or that has been
known for some time but is rapidly increasing in
incidence or geographic range.
exotic species Organisms living in habitats where
they do not occur naturally.
metapopulation A collection of local populations
linked through emigration and dispersal whose
long-term survival depends on the shifting balance
between local extinction and recolonization.
phenological shift The relationship between a periodic biological phenomenon, such as breeding and
climatic conditions.
synergistic decline factor A factor (i.e., predation,
pollution, and disease) whose effect is enhanced
when it is in the presence of another factor.
UV-B hypothesis The proposal that human-induced
climate modification, resulting in increased levels
of harmful ultraviolet radiation (UV-B), is negatively affecting amphibians.
Encyclopedia of Biodiversity
Copyright & 2007 Elsevier Inc. All rights of reproduction in any form reserved.
GLOBAL DECLINES OF AMPHIBIANS refers to the
phenomenon where amphibian species are experiencing severe population declines around the
world. A recent assessment of the world’s amphibians
(Stuart et al., 2004) found that 32% of 5743 species
were globally threatened and that at least 43% of all
species were experiencing population declines in some
part of their range. These declines, more dramatic than
those described for birds, mammals, or reptiles, forecast the impending extinction of many species during
the coming decades. Many of these declines have occurred in protected areas and involve idiosyncratic or
enigmatic causal agents. However, other declines are
due to obvious reasons, particularly habitat destruction; but the number of species experiencing enigmatic
declines is increasing and these have caused the greatest alarm (Semlitsch, 2003; Lannoo, 2005).
I. AMPHIBIAN BIODIVERSITY
The world’s living amphibians include more than 6000
species placed in three distinct clades, the frogs and
toads (Salientia), salamanders (Caudata), and caecilians (Gymnophiona). Of the three groups, frogs
and toads exhibit the most varied reproductive modes
and habitat associations and constitute the majority
(45300 species). Salamanders and caecilians, also
diverse, have fewer species and are more restricted,
but still have a widespread distribution (555 and 171
species, respectively; AmphibiaWeb, 2006). Most of
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the world’s amphibian diversity occurs in the tropics,
especially in Central and South America, but other
amphibian biodiversity hotspots include sub-Saharan
Africa, Madagascar, Sri Lanka, Southeast Asia, and
Australia (Fig. 1). Salamanders are generally thought
to be restricted to North Temperate regions, where all
10 families occur, but the largest family is well represented in tropical America, where more than 40% of
all salamanders occur. Salamanders are especially
abundant in North America, whereas caecilians are
restricted to tropical regions.
Amphibians are often characterized as tetrapods
with aquatic larvae and terrestrial adults, but alternative life histories are common. Some species of the
three main clades are permanently aquatic and some of
these give birth to metamorphosed offspring. In contrast, some members of all three clades (including a
majority of the species of salamanders and caecilians)
are strictly terrestrial and lack aquatic larvae; eggs well
provisioned with yolk are laid on land and develop
directly into miniatures of adults. Both frogs and salamanders may deposit eggs in arboreal microhabitats.
Egg-laying sites vary greatly, and eggs, and sometimes
tadpoles, of some species are transported on the legs or
backs of either parent. Some members of all three clades give birth to metamorphosed young that have been
nourished during development in the reproductive
tract of the female, but the mode of nourishment varies, from cannibalism (in Salamandra), to ingested
trophic oviductal secretions (caecilians), or absorption
of oviductal secretions (a few frogs). Free-living larvae
typically metamorphose after one season, but multiyear tadpoles are known for several species of frogs.
Several salamanders never metamorphose, but remain
in gilled or semigilled, pedomorphic states throughout
their lives. Frogs have evolved many unusual life histories, frequently involving elimination of eggs or
larvae from aquatic habitats. Eggs of different species
of frogs are transferred to male vocal sacs, to compartments in the skin of the back of females, to the
FIGURE 1 Global amphibian species diversity by country visualized using density equalizing cartograms. Country size is purposefully distorted in proportion to the number of amphibian species that occur in each country. Prepared by: M. Koo, Museum of
Vertebrate Zoology; technique after Gastner and Newman (2004); data source: Stuart et al. (2004).
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stomach, or to pouches in skin of the back of females.
Larvae of salamanders, caecilians, and a few frogs are
carnivorous, but most tadpoles feed on suspended or
attached vegetation and detritus. Tadpoles display
great diversity in behavior and microhabitat. Some are
semiterrestrial burrowers and some are even semiarboreal (McDiarmid and Altig, 1999).
Amphibians are represented in diverse aquatic and
terrestrial ecosystems and are frequently important
components of communities and food webs. In some
parts of the world they are the dominant predator, both
in terms of numbers and total mass. They are diverse in
behavior. Most salamanders have the structure of a
generalized tetrapod with four legs, a relatively short
trunk, and a tail, but some are extremely elongated with
very small limbs or only forelimbs, and some reach very
large size—in excess of 1.5 m. Caecilians are limbless
and their eyes are covered by skin. They have larger
numbers of trunk vertebrae and are very elongated, but
they either lack or have an exceedingly short tail. Frogs
have a characteristic form consisting of a large head, a
very short trunk, and four legs. The hind limbs contain
four major segments and are elongated, suspended from
elongated and specialized pelvic girdles, enabling the
frogs to jump and swim. However, despite the constraints of body form, frogs are diverse in morphology,
coloration, and behavior. Adult amphibians are effective
predators and both salamanders and frogs have tongues
specialized for rapid, long-distance prey capture. Caecilians generally feed on subterranean prey such as
earthworms.
II. DIMENSIONS OF THE PROBLEM
A. Geography
The geographic extent of the declines is worldwide.
The areas most affected are located in Central and
South America, the Caribbean, the wet tropics of eastern Australia (Fig. 2), and western North America
(Stuart et al., 2004). Little is known about the status of
FIGURE 2 Percent of threatened amphibian species by country visualized using density equalizing cartograms. Country size is
purposefully distorted in proportion to the percent of threatened amphibian species (see Fig. 1).
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species in Africa and Asia due to the lack of long-term
studies. The first reports of massive collapse of amphibian faunas came from montane areas in Central America
and Australia. The loss of more than 50% of the species
in a large tropical montane fauna (Monteverde Cloud
Forest Reserve) in Costa Rica in the course of a single
year (1987) was a profound shock (Semlitsch, 2003),
and included the first prominent extinction (the Golden
Toad, Bufo periglenes). Collapse of amphibian fauna in
montane and lower montane Central America and
South America is continuing (Lips et al., 2006; Pounds
et al., 2006). Several species of frogs declined dramatically, some to the point of apparent extinction, in eastern Queensland, Australia, starting at about the same
time (1980s). Concern has been expressed about the
declines of frogs in California over many years, a phenomenon which accelerated in the 1980s and early
1990s. Now there have been reports of mainly geographically limited declines from many parts of the world.
both terrestrial and aquatic food webs. Adults are important predators, as are larval salamanders and some
larval frogs; whereas more generally, tadpoles play important roles in controlling vegetation levels in both
lentic and lotic ecosystems. For example, streams that
have lost tadpoles may become choked with aquatic
vegetation. The largely aquatic larval forms of amphibians gather productivity from aquatic habitats;
and when they metamorphose into terrestrial adults,
they make this productivity available to terrestrial
predators. In some areas, aquatic systems are more
productive than the surrounding terrestrial systems
and amphibians help connect these habitats. In some
cases, the loss of amphibians is cascading into declines
and disappearances of their terrestrial predators (i.e.,
snakes). Amphibians are also important predators, for
example, on aquatic insects, and their loss may greatly
affect insect population dynamics.
C. Systematics
B. Ecology
It is still too early to accurately characterize the
ecology of the species that have declined, but there are
some characteristics that are widely shared. Most attention has been drawn to the lower montane to montane species that are associated with streams, but there
are many exceptions. Species that have aquatic breeding habits and stream-dwelling tadpoles are more
likely to be in decline than species that lay eggs on
land and that develop without a larval stage. In Australia, an analysis of the ecological characteristics of 40
species of frogs with an aquatic stage showed that
species with stream-dwelling tadpoles are more likely
to be in decline than other species (Hero et al., 2005).
Furthermore, independent of phylogenetic relationships, low ovarian clutch size is the ecological trait
most likely to be associated with population declines.
Upland species with stream-adapted tadpoles are also
found to be associated with declines in lower Central
and South America (Lips et al., 2006; Pounds et al.,
2006), but in these regions entire amphibian faunas
have experienced precipitous declines.
Amphibians are important members of ecosystems,
and their declines are likely to have diverse, and as yet
not fully understood, impacts on communities and
ecosystems. Amphibians are unique among vertebrates
in that many have biphasic life cycles that include fully
aquatic forms (larvae) and terrestrial or semiterrestrial
(adult) forms. This and the fact that they exist in so
many habitats makes amphibians key components in
Amphibian systematics is in a state of flux and new
analyses of the phylogenetic pattern of declines are in
progress. Stuart et al. (2004) found that some taxa were
more likely to be affected than others; but much more
analysis is needed. What is needed in particular is to sort
out the reasons for decline and then determine if there is
a phylogenetic bias. For example, frogs of the genus
Atelopus from the highlands of lower Central America
and the mountains of northwestern South America have
been hard-hit by declines and several species are
thought to be extinct. These species also have streamadapted larvae. Elsewhere, stream adaptation is also associated with declines, so there may be a phylogenetic
bias toward ecology that is imperiling this clade. Salamanders are generally thought to be less affected than
frogs, but neotropical salamanders, all members of a
single clade (the supergenus Bolitoglossa, family Plethodontidae), are also in severe decline. Whether this is the
result of their presence in a general center for declines
(Central America) or of their membership in a particular
clade with a life history bias (all lack aquatic larvae, a
trait associated with relative immunity from declines
elsewhere) is yet to be determined.
III. FACTORS RESPONSIBLE FOR
THE DECLINES
Many potential causes for the widespread declines of
amphibians have been proposed. In general, these can
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be grouped into two major categories: (i) factors general to the overall biodiversity crisis, including habitat
destruction, alteration and fragmentation, introduced
species and overexploitation and (ii) factors associated
with amphibians that might account for declines in
relatively undisturbed habitats. The first category
includes relatively well-understood ecological phenomena, whereas the second includes complex and
elusive mechanisms, such as climate change, increased
ultraviolet radiation (UV-B), chemical contamination,
spread of infectious diseases, and the causes of deformities (or malformations). The underlying mechanisms behind these factors are complex and may be
working synergistically with more evident factors,
such as habitat destruction and introduced species, to
exacerbate declines. Many biologists believe that there
are some dominant causes, such as new infectious
diseases, whereas others are not convinced that there
is a single overarching cause for global declines, but
that many factors are threatening amphibian populations to a greater or lesser extent.
A. Habitat Degradation and Conversion
Perhaps the most obvious factor in the loss of amphibians is habitat degradation and conversion. Loss of
wetlands is a major factor in temperate zones, while
removal of forests is the most serious threat in the
tropics. However, even in areas of high human population density, intense agricultural activity, heavy industrialization, and urbanization, such as Japan and the
southern Korean peninsula, amphibians can persist and
some species continue to thrive. Habitat fragmentation
mostly affects widespread species and those that have
natural metapopulation structures. Many amphibians,
especially terrestrial species with direct development
and no larval stages, are typically highly structured
genetically, with much cryptic diversity, and fragmentation leading to extensive loss of biodiversity even if
species fragments persist. Some habitat changes are
subtle and have unexpected effects. Fire suppression in
parts of western North America has led to encroachment of forests into breeding sites, resulting in shading
that has negative effects on tadpole development.
A major factor in habitat degradation is pollution
from agricultural pesticides and fertilizers (Hayes
et al., 2002). There is now extensive documentation
both of direct effects and of synergistic interactions
with other factors (see below). In northern zones, acid
precipitation has been shown to affect species at the
limits of their ranges.
B. Impact of Exotic Species
The establishment and spread of exotic species are a
major threat to worldwide biodiversity, and there are
many examples of amphibians being affected (Kats and
Ferrer, 2003). Exotic species affect amphibians as
competitors, predators, and as vectors for parasites
and disease. Nonnative or exotic amphibians are responsible for many of the declines of native amphibians. The North American bullfrog (Rana catesbeiana)
has been transported around the world by humans
mostly for food. In California, bullfrogs were introduced after native red-legged frogs (R. draytonii) were
hunted to low numbers in the nineteenth century.
Wild populations of nonnative bullfrogs now eat and
outcompete native red-legged frogs and foothill yellow-legged frogs (R. boylii), both of which are in decline. In the last several decades, nonnative bullfrogs
escaped from farms in Venezuela and established wild
populations in areas where native frog populations
collapsed (several species of the genus Atelopus). Some
native amphibians are infected with chytridiomycosis,
a disease caused by a fungal pathogen. Bullfrogs appear to be resistant to chytridiomycosis, and may act
as carriers for the disease. In Australia, sugarcane
farmers introduced cane toads (B. marinus) in an effort
to control insect pests. Unfortunately, the nocturnal
cane toads do not control the largely diurnal pests.
Since the first introductions in the late 1800s, cane
toads have spread throughout the eastern portion of
Australia where they act as both competitors and
predators of native amphibians. Additionally, the toads
produce toxic defensive compounds in their skin, and
they are often deadly to predators that eat them, such
as native amphibians, mammals, birds, and snakes.
Nonnative populations of African clawed frogs (Xenopus leavis) have also been widely established around
the world. They are prolific, highly competitive and,
like the bullfrog, are implicated in the spread of
chytridiomycosis to native species of amphibians.
Fish are generally dominant species in aquatic systems, but unlike most amphibians, they are bounded
by natural aquatic barriers such as waterfalls, dry land,
etc. Humans, however, have introduced fish varieties
to new aquatic habitats and this has had devastating
consequences for some amphibians, especially those
that evolved without fish predators. A well-studied
example is the introduction of trout into historically
fishless freshwater systems. Trout (Salmonidae: Onchorhychus sp., Salmo sp.) are restricted to cold mountain streams in North America and Europe but have
been widely introduced to every continent on Earth
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except Antarctica. Aquatic systems in entire mountain
ranges have been altered. For example, in California,
prior to the mid-1800s, more than 99% of the lakes
and ponds in the Sierra Nevada above 2100 m were
fishless. Historical accounts from the Museum of Vertebrate Zoology (Grinnell and Storer, 1924) state that
the mountain yellow-legged frog (R. muscosa) was
once the most common vertebrate in these highelevation ponds and lakes. Since the mid-1800s, trout
have been introduced throughout the Sierra Nevada
for sport fishing. Now, more than 90% of these naturally fishless lakes contain nonnative trout. The
mountain yellow-legged frog, which is adapted to
living in environments without any fish, has declined
dramatically, and while there are many potential
causes for the decline of this species, field experiments have shown that removal of introduced trout
from entire lakes can lead to recovery of local populations (Vredenburg, 2004).
America (Berger et al., 1998; Lips et al., 2006), and is
implicated in declines in Spain, Australia (Berger et al.,
1998) and California (Rachowicz et al., 2006). It is
unknown whether chytridiomycosis is an emerging
disease that has recently been spread to new habitats,
or if it previously coexisted with amphibians but either
the pathogenicity has recently increased or the amphibian immune function has recently decreased.
D. Factors Associated with Global Climate
Change
There is a growing suspicion among ecologists that
amphibians may be more sensitive to climate change
than other vertebrates, notably such as birds, because
they are more likely to show habitat and microhabitat
specialization and because they are significantly less
vagile.
1. Elevated UV-B Increased Ultraviolet Radiation
C. Infectious Diseases
Infectious diseases have been associated with collapsing amphibian populations on several continents including Central America and Australia (Berger et al.,
1998). Viruses, bacteria, water molds, fungi, and trematode parasites are among the diverse agents associated with varying levels of mortality and population
decline. Viruses belonging to the family Iridoviridae
have been associated with mass mortality in the common frog (R. temporaria), the Sonora tiger salamander
(Ambystoma tigrinum), and other species in both
captive and wild populations. The bacterial pathogen
characteristic of red-legged disease, Aeromonas hydrophila, has been reported in amphibians in wild populations for several decades, and a pathogenic water
mold, Saprolegnia ferax, appears to be largely responsible for egg mortality in several western North American amphibians. Trematode infestation has been
implicated in limb deformities in the pacific tree frog
(Pseudacris regilla) and several other species of amphibians, but so far no population declines have been
tied to trematode infestations. Extinctions of several
Wyoming Toad (B. baxteri) populations are thought to
be primarily due to the parasitic fungus Basidiobolus
ranarum. The disease that has generated the most
alarm amongst herpetologists and conservation biologists is chytridiomycosis caused by the pathogenic
fungus Batrachochytrium dendrobatidis (Berger et al.,
1998). The disease played a major role in the
amphibian population collapse in Central and South
Global atmospheric changes caused by anthropogenic
activities are well documented and one result is a reduction of stratospheric ozone resulting in an increase
in the amount of biologically damaging UV-B reaching
the Earth’s surface. The increase in UV-B may be causing increased mortality rates in amphibians and this
could help explain enigmatic declines in protected areas. Most of the work testing this hypothesis has focused on comparing egg-hatching rates in species that
lay their eggs in shallow, exposed breeding sites subject to high levels of UV-B. Field experiments have
concluded that many species are sensitive (Blaustein
et al., 1998); however, not all species that are sensitive
are in decline and recent studies suggest that interactions between UV-B and factors such as water chemistry, seasonal variations in breeding, and amount of
precipitation are important.
2. Establishment of Conditions Favorable for
Fungal Growth
Several studies have suggested that a change in climate
may make conditions favorable for the spread of
amphibian diseases and parasites. These may then
overwhelm frog immune systems causing death. For
example, in the America tropics 67% of the 110 species
of Harlequin frogs in the genus Atelopus declined soon
after unprecedentedly high mean air and sea surface
temperatures (Pounds et al., 2006). While warmer conditions directly affect frogs, one hypothesis is that a
warming climate leads to more cloud formation, which
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could favor the growth of the fungal pathogen. The hypothesis states that by increasing the average lowest
nighttime temperature and lowering the average daytime maximum temperatures, the pathogen experiences
optimal growth temperatures.
3. Threats from Shifts in Weather Patterns
Many species react to changes in climate by shifting
their distributions either latitudinally or altitudinally.
Good evidence exists that some lowland butterflies in
western North America are moving both to higher
elevations and higher latitudes, presumably tracking
shifting resources that change as climate changes. In
contrast, in the American tropics there is evidence that
climate change has initiated precipitous declines in
amphibians, even within a single season (Pounds et al.,
1999). Amphibians, especially those in tropical
regions, are narrowly distributed, for example they
are restricted to specific and often narrow elevational
belts on mountains that are themselves isolated. There
may be no place to go when climate becomes warm or
dry, except higher up the mountain. Species already at
the top of such habitats are literally pushed off the
mountain, into extinction. Other species may be limited in their movements to the north or south, by
ranges fragmented by habitat conversion, or by barriers such as rivers that effectively stop range expansion.
Global climate change has diverse direct effects on
amphibians. In Central America, the cloud line has risen
several hundred meters; and the immediate effect was a
drought of unusual severity in a high-elevation cloud
forest in Costa Rica, which has been implicated in the
disappearance of 20 out of the 50 species of amphibians
known from the site, including a local endemic, the
Golden Toad, B. periglenes (Pounds et al., 1999).
In temperate zones, one documented effect of climate change has been the earlier breeding of some
species in the northern parts of their ranges. The longterm implications of this phenological shift are still
unclear.
E. Synergistic Effects
Many factors by themselves pose severe threats to amphibian survival, but more insidious and much harder
to comprehend are synergistic interactions between
factors. Many synergisms have been proposed (Pounds
et al., 1999; Blaustein and Kiesecker, 2002). The effects of infectious diseases may be greater in the presence of elevated UV-B or of chemical pollutants
(e.g., pesticides or fertilizers), which may compromise
immune systems. Unusual weather conditions might
enhance the impact of different stressors. The link between climate change and pathogen growth conditions
in Central America, mentioned above, may be enhanced by the presence of additional stressors like
high environmental loads of pesticides in the area. In
California, pesticide drift from the heavily agricultural
Central Valley has been linked to the disappearance of
high-elevation frog populations, possibly by enhancing
the effects of chytrid infection on populations already
weakened by interactions with exotic predators (introduced trout). Many other kinds of synergistic effects have been suggested. Synergisms are likely to
pose a major threat to the continued existence of many
amphibians.
IV. CHALLENGES AND OPPORTUNITIES
FOR THE FUTURE
Perhaps the main surprise associated with amphibian
declines is that so many of the most dramatic instances
have taken place in protected areas, such as the great
national parks of the Sierra Nevada of California, the
Monteverde Cloud Forest Preserve in Costa Rica, and
protected areas in Australia, to give three prominent
examples. Thus, the standard conservation approach
of purchasing and protecting land and habitats is unlikely to assure survival. Instead, conservation strategies must involve researchers with diverse talents in
the fields of infectious disease ecology, reproductive
biology, endocrinology, immunology, and pollution
ecology. Natural historians are critical components of
any conservation strategy, for it is their expertise that
may provide insight into patterns of survival and
recovery of once infected organisms, for example. Ex
situ strategies increasingly seem to be essential, but
captive breeding is difficult and expensive and careful
thought must be given to the selection of the candidate
species (e.g., phylogenetic, ecological, and behavioral
diversity should be represented). A necessary element
is information exchange and discussion among specialists and generalists to attempt to formulate new
pathways for understanding, and community action to
counteract the very real threats facing this ancient
group of organisms.
A. Mitigation of Habitat Changes
Wetland restoration holds great promise for recovery
of many amphibian species. Alteration of wetlands by
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humans has had one of the most substantial effects on
amphibian populations. Wetlands are important components of the Earth ecosystems as they provide vital
ecosystem services (i.e., water purification). Mitigation
in wetlands not only benefits humans directly by helping restore clean water systems, but also provides an
important habitat for many species of amphibians.
B. Removal of Exotic Species
As more amphibian populations collapse, reversing
these declines is becoming increasingly urgent. The
removal of exotic species is something that has been
proved to work in several systems, but is not yet
widely used. For example, removal of exotic fishes has
had positive benefits for amphibians in Spain, Chile,
and in several areas within the United States. In California, removal of nonnative trout from entire lakes in
the Sierra Nevada led to the rapid recovery of threatened mountain yellow-legged frog populations (Vredenburg, 2004).
V. IMPLICATIONS FOR THE BIODIVERSITY
CRISIS IN GENERAL
Amphibian declines may be the window into the
future of what we can expect as humans continue to
alter the environment on a global scale. Only now
are government officials finally willing to acknowledge
that humans have caused so much damage to the
environment that they are even affecting global climate
change. We do not think that amphibians are special
or unusual. In our case, they are simply the organisms
we chose to study, and there is no a priori reason to
think that they are exceptions. We can no longer lock
up nature and expect it to care for itself. Ironically, we
may not only be a primary source of the problems
facing amphibians, but also, their main hope for the
future. It is too early to tell if amphibians have lessons
for conservation strategies in general, but it is very
clear that we are now facing extinctions on a massive
scale, well beyond anything we have ever experienced.
See Also the Following Articles
C. Attenuation of Infectious Agents
Study of disease ecology in wild amphibians has only
recently gained a lot of attention. Disease ecology
seeks to understand the mechanisms that lead to disease outbreaks in natural systems. In some cases, museum collections can be used to look back in history
for pathogens in order to see whether they were
present then or were more common compared to today. The vast increase in connectivity through global
trade between continents may be linked to many of
these outbreaks. Precautions like disinfection and
quarantine programs and the prevention of human
movement of disease vectors (i.e., live bullfrogs) must
be taken to help lower the probabilities of future outbreaks.
D. Captive Breeding
While some biologists view captive breeding as a lastresort conservation action, the International Union for
the Conservation of Nature and Natural Resources
(IUCN) endorses captive breeding as a proactive conservation measure, one that should be initiated while a
species is still available to allow for a husbandry
learning curve.
AMPHIBIANS, BIODIVERSITY OF CAPTIVE BREEDING AND
REPRODUCTION CLIMATE CHANGE AND EXTINCTIONS
DISEASES, CONSERVATION AND ENDANGERED AMPHIBIANS
ENDANGERED REPTILES AND AMPHIBIANS INTRODUCED
SPECIES, EFFECT AND DISTRIBUTION OF ULTRAVIOLET
RADIATION
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