The world is losing its frogs, perhaps more rapidly than any other class of animals. Pollutants threaten some species, habitat loss others. However, possibly the biggest threat has been the spread of the chytrid fungus, thought to have driven many frog species to extinction, and pushed others close. So the news some species are rebounding is a huge relief to environmental scientists who have spent decades able to do little more than record the disaster. The announcement comes with evidence it is the frogs, not the fungus, that is changing.
A facet of natural selection is that when a new phenomenon wipes out most of a population, those with an immunity, even a partial one, have an advantage. They get to spread their genes to fill the partially empty niche left open by the deaths of other members of their species. Much as we fear the appearance of antibiotic resistance of lethal bacteria, the process can work in our favor when disease threatens a species we value.
Knowing this, when zoologists heard tropical forests going silent as frogs by their millions died they predicted the storm would eventually pass. Those few frogs with a capacity to resist the cause of their death would found new populations. The question was how long it would take, and how much we would lose in the meantime.
There have been hints in recent times the tide has turned, and certainly, a few species have bounced back. A paper in Science reports on comparisons of frogs at three Panamanian rainforest sites since the Batrachochytrium dendrobatidis caused near total collapse two decades ago. It reports; “Some amphibian host species are recovering, but the pathogen is still present and is as pathogenic today as it was almost a decade ago.”
Meanwhile, some species have evolved better defenses, presumably because a level of immunity was present in the populations beforehand, which has now spread under the fungus' selection pressure.
The distinction is important. Only a bad parasite kills its host – much better to keep them alive to improve the chances of transmission. Consequently, epidemics often start with a very virulent disease that evolves to be less deadly over time, since the less lethal strains are the ones most likely to be passed on.
The paper reveals nine out of 12 river bank-dwelling frog species investigated are recovering, and infection is less common, but this is occurring because the frogs are evolving, rather than the fungus. Comparisons of fungal samples collected in 2004 and almost a decade later found no significant difference in growth rates or toxins released. The frogs, meanwhile, are releasing more inhibitory secretions, sometimes dramatically so, enabling their survival.