Welcome! This blog is being created by students in the courses Population Ecology and Biological Diversity at the University of Oregon. It is one component of their work, and for each course will unfold throughout the term. +Jessica Green & Ann Womack
Wednesday, June 12, 2013
Amphibian decline: Chytrid fungus
(Atelopus varius) Costa Rican Variable Harlequin Toad
When you think about why a species goes extinct, what comes
to mind? Habitat destruction, pollution, overharvesting, climate change,
invasive species, and infectious disease are all factors that contribute to a
species decline. What we are seeing with the amphibians is not the result of
just one cause but rather all of them acting together and the result is
catastrophic. Amphibians, which includes, frogs, toads, salamanders, and
caecilians are facing a mass extinction in our lifetime if we do not act soon.
In this post I will touch on the various factors leading to the extinction of
amphibians but then focus on one of the main causes for amphibian decline;
infectious disease in the form of the chytrid fungus. Many species of amphibians are endemic
(meaning that they are only in one particular area) which makes amphibian
species very vulnerable to changes in their environment. Habitat destruction in
areas such as the rain forests is a major player in causing many of these
endemic species to go extinct. As
amphibians inhabit both water and terrestrial habitats, many types of pollution
can have a huge effect on amphibian populations. One aspect of pollution that dominates in amphibian
loss is the use of pesticides like atrazine. In recent studies, atrazine has
shown to affect the reproductive organs and health of the animals. These
amphibians are changing sex or even growing multiple reproductive organs when
exposed to even low levels of atrazine (Raloff). Climate change is changing the
seasons, temperature, moisture, and many other factors to which amphibians are
very vulnerable. Overharvesting is also
an issue for amphibians in the form of a food source (frog legs), medical, or
educational purposes and until recently, impacts from overharvesting have not
Dead frogs killed by the amphibian chytrid fungus.
Photograph by Joel Sartore, National Geographic
Amphibians have this super permeable skin where water,
oxygen, and other nutrients can travel from the environment into the animal.
This is how these animals can live deep in soils for long periods of time. For example the Spade Foot Toad that lives in
the high desert in Southeastern Oregon will bury itself deep into the ground
and only come out when it rains. They are able to survive for years without
coming back up to the surface. The
Chytrid fungus (Batrachochytrium
dendrobatidis) causes a thickening of the skin which leads to oxygen and
electrolyte deficiencies. There are currently two current ideas regarding the
toxicity of the fungus. The first being that it secretes toxic enzymes and the
second being that the loss of oxygen and electrolytes lead to poor
osmoregulation. Without an appropriate osmoregulation,
the heart will go into cardiac arrest (Lee). The chytrid fungus does not only
infect amphibians, recent research has shown that animals such as the crayfish
can be carriers of this disease and the chytrid fungus can remain in the
environment for long periods of time. This means that even if we transplant
amphibians back into an environment that has lost them; we may still see amphibian
death due to the fungus (McMahon).
Now that we know what the chytrid fungus is and what it has
done to the amphibian populations the next questions are: Where did it come
from? How does it spread across the globe? Several avenues of the spread of
this disease have been postulated but none have been confirmed as the one way
this disease has spread so quickly. The original thought was that African
Clawed Frogs were the original carrier of this disease and it was spread to
other populations due to human uses. These frogs were used until the 1970s as
pregnancy tests. Doctors would inject a female human’s urine into the frog and
if the frog laid eggs then the female human was pregnant (Lee). These frogs
have also been widely sold in pet stores across the globe. The thought is that
these frogs have gotten loose into the wild and spread the chytrid fungus to
other species of amphibians in various locations around the world. African Clawed frogs in the wild have tested
positive for the chytrid fungus which verifies that there is a possibility that
this was one path taken by the chytrid fungus. Another path of the spread of
this disease is believed to be the American Bullfrog. Although the American
Bullfrog is a native species to North America, it has become an invasive
species in areas such as California and is spreading the chytrid fungus to
other amphibians. The American Bullfrog is widely used as frog legs and since
the fungus is not harmful to humans and is not visible to the naked eye, human
consumption of this delicacy has not slowed down (Upton). The American
Bullfrog’s movements can be correlated with the path of the chytrid fungus
(Lee). The chytrid fungus has also been
found hitching a ride on the bottom of bird’s feet which could attribute to the
widespread reaches of the disease. Whatever the path is that delivers the
chytrid fungus, we know that it is deadly and it has been around for about
40,000 years (Lee). Some amphibians seem to simply be carriers and are immune
to the actual disease which may contribute to why this disease has been around
for so many years but is just now making a global appearance. Several
groups of scientists are working on tracking the fungus so that we can be one
step ahead of the infection. One group
has found that 42% of the world’s amphibian population is currently affected by
the fungus and this is spread over 52 countries around the globe as the map
below shows (Olson).
Olson et al.
So why should we care about saving amphibians from this
deathly fungus? Amphibians provide quite a few ecological services. Without
frogs we would see an enormous increase in the amount of insects and possibly
an increase in infectious diseases carried by insects. Like any other group of
animals in the wild, frogs and other amphibians play a role in food webs and
the loss of amphibians could be a possible destruction of the animals that
consume amphibians as their food source.
Many antimicrobial agents and various types of medications have been
developed using amphibians and more are still left to be discovered. Amphibians
have also played an important role in education and research. Amphibian embryos
are transparent and have been widely used in the field of developmental
What are some things we should do to help? Generally we
think that just one person cannot make that big of a difference but we hear all
the time that one person can make all the difference in the world. Here are some
easy guidelines to live by that will impact amphibians in a positive way. Just
doing your part such as conserving resources, reducing your carbon footprint,
and reducing your effect on global warming will greatly help our frogs and
other amphibians. If we stop purchasing wild-caught frogs we do not run the
risk of setting them free and possibly infecting others in the wild. Refraining
from using pesticides or eating frog legs will also help our frogs. There are many ways to help save these
important animals and if we work together we may just be able to.
Save the frogs!
And finally here are a couple of links to learn more about
Chatfield M, Moler P, Richards-Zawacki C, Sturtevant J. “The
Amphibian Chytrid Fungus, Batrachochytrium dendrobatidis, in Fully Aquatic
Salamanders from Southeastern North America”. Plos ONE [serial
online]. September 2012;7(9):1-5.
Lee , Jane. "African Clawed Frog Spreads Deadly
Amphibian Fungus." National Geographic . 15 05 2013: n. page.
Web. 29 May. 2013. <http://news.nationalgeographic.com/news/2013/13/130515-chytrid-fungus-origin-african-clawed-frog-science/>.
McMahon, T., Brannelly, Laura A., Chatfield, Matthew
W. H., et al. Chytrid fungus Batrachochytrium dendrobatidis has
nonamphibian hosts and releases chemicals that cause pathology in the absence
of infection.PNAS 2013 110 (1) 210-215
Olson D, Aanensen D, Fisher M, et al. “Mapping the Global
Emergence of Batrachochytrium dendrobatidis, the Amphibian Chytrid Fungus”. Plos
ONE [serial online]. February 2013;8(2):1-13.
Raloff J. Herbicide makes frogs Mr. Moms. Science News [serial
online]. March 27, 2010;177(7):9.
Upton, John. "Despite Deadly Fungus, Frog Imports
Continue." New York Times 07 04 2012, n. pag. Web. 29 May. 2013.