Over the course of my lifetime, I have heard many snippets about the plight of amphibians — which have always seemed to taken the brunt of human impacts in this modern world. On Thursday, Februart 18th, Vance Vredenburg gave us some more reasons to be concerned.
One is that until now, amphibians have been in general the survivors. Of the 5 great mass extinctions, they have weathered 4 of them. With humans, the driver of mass extinction of perhaps number six, amphibians are going extinct and being threatened by extinction at rates greater than mammals or birds. And in comparison to the other 5 great mass extinctions this one is unfolding in a very short time span under 10,000 years.
Vredenburg showed us examples of how Rana sierrae, the Sierra Nevada Yellow-legged Frog in particular is being affected. This species, one of 7 native to the Sierra Nevada, has declined by 92.5% since original studies were done by Grinell in the early 20th century. Grinell’s studies and the cataloging done of the Sierra Nevada’s has been great for science.
In what seems to me to be a bit of classic experimental science, Vredenberg and colleagues, went to test whether or not Rana Sierrae was affected by the fish that have been planted in the mountains. Fish panting started in the late 1800’s well before there was any noticeable decline in frog populations, so it had been discounted as a cause. But since the 50s and 60s, fish have been well stocked in the Sierra’s by air where they had never been (the upper Sierra lake’s being mostly inaccessible just by river). So, Vredenberg did a study by removing fish a number of ponds, and then comparing frog populations in control ponds where there were only fish or only frogs.
The results were dramatic, where fish were removed, the frog populations rebounded dramatically. But unfortunately for the frog the story did not end there. Over a number of years a disease has been sweeping through these and other frogs. Starting in 2004, the disease swept through the Sierras, and spread throughout the mountains in under 4 years denuding many lakes of frogs.
The disease is caused by an ancient aquatic fungus whose spores infect the skin of amphibians and create more spores. In that process the skin gets 4-40x thicker which disrupts the amphibian’s breathing and ultimately brings their demise through heart attacks.
Not all species are as affected. Some species are carriers, others are not affected, but the impact on the species it does affect is dramatic. It might be the worst known case of a pathogen affecting a vertebrate population. Diseases tend not to kill off their host species — something seems seriously out of whack.
Why should we care? There are all sorts of practical reasons: frogs have very similar hormones to us (so we ought to be paying attention), the principles of the disease and its spread are applicable to us (and presents a very sobering model), but most of all, we should care for the impractical reason that amphibians are beautiful creatures and we ought to pay attention to how we might be affecting them.
SF Natural History Series 