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Chytrid Fungus and Its Role in Declining Amphibian Populations
By: Mike Tom, Rincon Consultants
Currently, amphibian populations all over the world are in decline. Declines are to a point that some
consider the current situation as a sixth mass extinction. One of the many correlates of this decline is
the presence of the disease Chytridiomycosis in amphibian populations. The Chytrid fungus,
Batrachochytrium dendrobatidis, was identified as the species causing Chytridiomycosis in amphibians.
This fungal species infects the keratin of the skin, which is the major respiratory organ for amphibians.
Infection by Chytrid fungus can cause reddening of the ventral skin as well as sloughing of skin over the
entire body. On occasion, ulceration and hemorrhaging is also observed. These symptoms can
ultimately lead to death. In addition to physiological effects, behavioral changes in animals infected
with the fungus can occur, including lethargy, a failure to seek shelter or escape danger, abnormal
posturing and the inability to turn themselves upright. These altered behaviors ultimately reduce the
fitness of individuals and can increase exposure to predation.
Theories of how this disease has become so widespread are currently speculative, but it was first
observed in amphibians in 1998. Susceptibility to the disease can vary among species, populations and
individuals. Attention has increased around the study of the relationships among climate, infectious
disease and amphibian declines, resulting in the development of the “climate-linked epidemic
hypothesis.” This hypothesis posits that changes in climate alter disease dynamics, which can increase
vulnerability to certain pathogens. Two main factors typically affect the spread of fungal diseases:
temperature and moisture levels. Chytrid’s optimal growth occurs between 17°C and 25°C (62°-77°F).
Chytrid fungus also has the capacity to overwinter in hosts and can persist at temperatures as low as 4°C
(39°F). Our current global warming trend may be creating conditions that are optimal for the spread of
Chytrid. For instance, studies in tropical areas, such as the cloud forests of South America, have
correlated daytime cooling and nighttime warming trends and increasing amounts of cloud cover with
high incidences of Chytrid. These regions are also experiencing large declines in amphibian populations.
Elsewhere, in more moderate climates such as the Central Coast of California, increasing temperatures
and shorter and milder winters are also being correlated with higher incidences of Chytrid.
Chytrid is also found in animal taxa other than amphibians. Researchers observed in ponds that even
after entire populations of amphibians were killed by Chytrid, any amphibians that later tried to
recolonize that same pond also died from the disease. One possible reservoir is crayfish, which have
been found infected with Chytrid but asymptomatic. Crayfish, not native to the western United States,
occur in freshwater locations including ponds, streams and irrigation canals throughout California, but
they do not occur in high-elevation lakes and streams. So, how does Chytrid persist in isolated, highelevation amphibian habitat?
Chytrid has been detected in high-elevation amphibian species such as the Sierra Nevada mountain
yellow-legged frog (Rana sierrae) and the southern mountain yellow-legged frog (Rana muscosa). Both
are California Species of Special Concern, and certain populations of the southern mountain yellowlegged frog are federally endangered. Chytrid is now common in populations of the two species and
outbreaks are resulting in mass die-offs. Recent research has shown that some frog species with very
low susceptibility to Chytrid infections may act as reservoirs for the fungus. In California, the three
treefrog species, formerly recognized as the single species – Pacific treefrog (Pseudacris regilla) – have
shown very low sensitivity to Chytrid. Treefrogs are widely distributed throughout the state and occur
from sea level to high-elevation areas (up to around 11,600 feet). They also inhabit a wide variety of
vegetation communities such as forests, woodlands, chaparral, grasslands, pastures, desert streams and
oases, and urban areas. Treefrogs commonly co-occur with both species of mountain yellow-legged
frog, and this may explain why Chytrid persists in isolated high-elevation areas.
Another possible example of an amphibian reservoir for Chytrid is the bullfrog (Lithobates catesbeianus).
Bullfrogs have shown resiliency to Chytrid. Bullfrogs, also not native to California, can co-occur with
native species such as the California red-legged frog (Rana draytonii). The implications of bullfrogs as a
reservoir for Chytrid are not well studied, but one can see the potential for bullfrogs to spread the
disease resulting in a greater competitive advantage over the native frog species.
So how can we, as biologists or people frequenting aquatic habitats, help prevent the spread of Chytrid?
The Declining Amphibian Populations Task Force has developed a fieldwork code of practice to reduce
the risk of spreading diseases from one aquatic system to another. The Declining Amphibian
Populations Task Force Fieldwork Code of Practice can be found here.