How will climate change impact amphibians? (focus on fungal disease)

Chytridiomycosis is a disease that is killing amphibians worldwide at alarming rates. To read more about amphibians see this blog post. The disease is caused by Batrachochytrium dendrobatidis or Bd for short. Bd is a fungus that is specific to amphibians. Currently, the disease is responsible for the greatest disease-caused loss of biodiversity in recorded history. Species have had massive drops in populations and seem to have disappeared overnight.Bd occurs inside the cells of the outer skin layer. It causes microscopic changes in the skin and enlarges this layer. This is deadly because amphibians absorb water and salts through the skin. In the family of lungless salamanders, and one lungless species of caecilian Bd causes them to suffocate. Bd travels into the mouthparts of tadpoles or other larval amphibians. When tadpoles change into adults the infection spreads to the skin. Spores, which are the reproductive part of the fungus, cause the skin to become enlarged. This reduces osmotic regulation and electrolyte blood levels drop which leads to cardiac arrest. Chytridiomycosis has almost a 100 percent mortality rate.


Cells infected with Batrachochytrium dendrobatidis 


Currently,  amphibians are facing huge declines in Australia, US, and Central America amphibian populations. Chytridiomycosis was the main reason for the extinction of sharp-snout frog (Taudactylus acutirostris) and the golden toad (Incilius periglenes). 278 species have been documented being affected by chytridiomycosis on every continent except Antartica. Biologist believes that Bd has evolved dramatically since 1938 which is why we are seeking these impacts now. Since the 1970s there have been declines in Australia. Now due to climate change, there is a Northward expansion. Due to increase temperature species are expanding their ranges northward, which puts them into contact with species they wouldn’t have had contact with in the past. This means that more species are being exposed to Bd and therefore developing chytridiomycosis.


Golden toad (Incilius periglenes)


The climate-linked epidemic hypothesis says that there is a relationship between climate and infectious disease. Climate fluctuations, towards warmer temperatures, have been shown to encourage outbreaks of certain pathogens. More climates due to climate change are now at the optimal temperature for Bd, which is 17C to 23C. Bd doesn’t die until 28C. Due to climate change water temperatures have increased which allows Bd to grow more over winter and worsens the infection upon metamorphosis. The hottest months in the northern hemisphere are July and August. Metamorphosis occurs then in salamanders and frogs and the disease grows very quickly due to ideal temperatures. Due to climate change, there is a high abundance of mist and dewfall in May when amphibians breed which allows the disease to spread easier. The spores can swim up to 2 cm in a natural environment. This abundance of precipitation is due to more water evaporating as a result of increased temperatures.



In Costa Rica, the Monteverde harlequin frog (Atelopus varius) and golden toad have vanished. 67% of Atelopus (harlequin frogs) in Central America have gone extinct due to Bd. There were 110 Atelopus species endemic to central America. Atelopus populations might survive in warm weather if Bd is absent from particular sites within their range. 80% of species that have disappeared were seen for the last time right after a relatively warm year. Large-scale temperature shifts are a likely common denominator in loss of amphibians. Factors that might influence the timing of extinction disappearances are altitude, latitude or range size. In Costa Rica’s Monteverde cloud forest, reduced mist in warm years is linked with shifts in populations amphibians. This includes the disappearance of the golden toad and the Monteverde harlequin frog. However not all Atelopus extinctions have occurred in habitats where mist is vital.


The climate-linked epidemic hypothesis predicts amphibian declines in unusually warm years because shifts in temperature or related variables often influence disease dynamics. The Jambato toad (Atelopus ignescens) was last seen in 1988, following a temperature peak in 1987. Before 1988, the Jambato toad was present during 64% of visits to sites which supports an association between disappearances and warm years. Warm or dry conditions may stress amphibians, possibly increasing susceptibility to Bd. Warm years could favor Bd directly. The idea is that lower temperatures benefit Bd might be an oversimplification of the pathogen’s response to climate.


The Jambato toad (Atelopus ignescens)


Cloud cover favors Bd as it promotes Bd survival and growth and reproduction. Amphibians may seek warmth to combat Bd but cloud cover reduces their defenses. Cooling of an area promotes Bd growth while growth peaks at 23C. There are more effects mountainous regions because air rises strongly. cloud cover also reduces heat loss at night. Lowlands are too warm for Bd in the daytime but highlands are too cool for Bd at night. Atelopus extinctions have occurred at elevations where the minimum temperature is moving closer the to the optimal temperature for Bd growth. Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America. It is clear that widespread amphibian extinctions from the disease are driven by global warming.





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