The End of Frogs?
We are getting somewhat used to that pesky little virus some call the ‘rona, and pandemic is a word we are all now familiar with, however our little virus is nothing on what is being described as “The most devastating panzootic to date” (panzootic=animal pandemic).
We have shut down countries, cancelled weddings and covered the earth in a shockingly wide variety of facemasks with different patterns. Unfortunately, when it comes to infections in wildlife populations there isn’t always a lot we can do (or can be bothered to do*). Of course with rinderpest (aka cattle plague) which we kindly transferred from our provider of steaks and milk to a wide variety of even toed ungulates, there was a very real threat to our cows, so we put in the effort and seem to have added it to the very short list of diseases humanity has successfully eradicated from planet Earth (smallpox is the only other listee). Of course, with diseases where there is no obvious detriment to humanity far less care is given.
This isn’t ideal for our (often) hopping friends, the amphibians. There are over 8,000 species worldwide, with many more yet to be found, however nearly a third of these are threatened globally, and over 40% are facing a population decrease. Nearly 500 of these species are in fact now critically endangered and in the last few decades a number of species have gone extinct entirely. This is primarily due to a nasty little fungus called chytrid, or Batrachochytrium dendrobatidae if you’re feeling nerdy (a similar species is also affecting salamanders). This fungus gives rise to the disease chytridiomycosis, which involves flagellated zoospores (imagine a sperm) infecting an amphibians skin, creating sporangia (a sort of enclosure where more zoospores are produced), then reinfection with these new zoospores. In response to this, amphibians attempt to shed their skins, relatively unsuccessfully, instead the skin thickens. In humanity having a ‘thick skin’ isn’t always a bad thing, but for amphibians which rely on the permeability of their skin for nutrient uptake, toxin release and in some cases breathing, this skin thickening can be fatal.
The origins of chytrid are unclear, but it seems due to trade in frogs and possibly in part for our peculiar usage* of African clawed frogs in the 1930’s, chytrid was taken from Asia where Amphibian species had evolved to coexist with the fungus, all over the globe, where it is alien and often has a 100% mortality rate.
*people used the fact that urine from pregnant women would induce egg production in these frogs as a basic pregnancy diagnosis (yes, we put human pee on the poor frogs).
Unfortunately, treating frogs one by one is out of the question except in such critically endangered species that the entire population can be collected and kept in captivity. So we have an inevitable wave of amphibian extinctions on the horizon and there is next to nothing we can do.
Part of the problem is that secondary to chytrid, amphibians are seriously struggling:
1. Their rainforests are being cut down and their bogs are being drained. Amphibians rely on both aquatic and terrestrial habitats so they are particularly vulnerable to such habitat modification
2. Their habitats are becoming ever more fragmented and within such fragments they are forced to inbreed, losing the genetic diversity required to evolve and adapt to new pathogens.
3. We are using herbicides, pesticides and fertilisers that can all prove problematic to animals that directly absorb through their skin, such as amphibians. Some herbicides such as atrazine have even been shown to cause frogs to become hermaphroditic (i.e. have both male and female reproductive organs).
4. Climate change is progressing rapidly, the ozone layer is depleted, cloud cover and UV radiation is increasing and their eggs are likely to be suffering from this UVB radiation effecting their DNA.
All of these horrors come together to increase environmental stress and as a consequence make them more susceptible to the nasty chytrid infection sweeping the world.
Some studies are looking at various ecosystem treatment strategies and at least one has got positive results by combining anti-fungal treatment of tadpoles with chemical disinfection of the whole environment. But whether this is possible on the scale required for the long-term survival of the many amphibian species at risk remains to be seen. Another strategy is ‘bioaugmentation’ with bacteria, meaning the use of bacteria to degrade the fungus, again unlikely to be practical on the scales required, and it is likely to increase the complexity of interactions between species in already complex ecosystems.
It is a fairly hopeless situation. There isn’t much we can do for them. Interacting with amphibians in attempts to ameliorate the situation can just aid in transmitting the disease to new populations. The only real hope is to minimise climate change, keep their habitats as pristine as possible to reduce environmental stress, and where possible build up degraded habitats by restoring them with native species, increasing biodiversity and ecosystem resilience. There are also some pretty cool groups undertaking captive breeding programmes such as HARCC and with a little bit of ingenuity there might still be hope for these slimey beauties.
Further reading:
Successful elimination of a lethal wildlife infectious disease in nature
Recent Asian origin of chytrid fungi causing global amphibian declines
