Many of us enjoyed watching tadpoles develop into froglets when we were children. Sometimes you got a surprise when that tadpole become a newt, not a frog. Today though there are a lot less frogs. Unfortunately they and their relatives worldwide are in serious trouble.
Frogs, toads and other amphibians have been disappearing on an enormous scale for years. In fact since the late 1970s many, if not most, populations globally have crashed. Sadly some species appear to have gone extinct. The reasons for this are still being debated. There are several theories, ranging from mutated pathogens to global warming but research is still being done on exactly why.
Habitat destruction is an obvious cause. Rainforests have been reduced considerably, as have other wildernesses. Even changes in farming methods can lead to amphibians having nowhere to live. Other human developments can affect them. For example a road can easily prevent adult frogs reaching a pond to breed. As a result they disappear.
However amphibians are dying in record numbers even in virtual intact environments. Once common rainforest frog species are now never seen. Their ecosystem seems to be fine as far as we can tell but there are still no frogs. It appears that there must be other reasons for the decline.
One explanation is that they have semi-permeable skin. This makes them vulnerable. Toxins that would wash off a lizard, bird or mammal are absorbed by an amphibian. With increasing pollution throughout the world this is likely to be a contributing factor.
Then there is disease. Many become susceptible to a variety of pathogens, notable the chytrid fungus. Whole populations have been discovered dying of various infections on a scale never seen before.
Climate change could have implications for animals that are dependent on two different environments. A change in the water could be disastrous, as could a change on the land. Increased UV radiation is another possible cause.
There is a strong possibility that toxins and disease are related. An amphibian’s actual immune system is quite vulnerable itself. When the animal is stressed, by temperature changes, acidification, or other pollutants these might not kill it. However it is going to struggle with any disease a healthy amphibian could deal with.
An experiment on captive leopard frogs (Rana pipiens) showed that a long exposure to cold, or water acidification increased their susceptibility to infections. Any change that stresses amphibians might not kill them, but it make makes them vulnerable.
This would explain why so many fall to the chytrid fungus. This is common worldwide and you would expect most amphibians to throw off its effects. With compromised immune systems they cannot. So you have these mass deaths, apparently due to disease.
There does not seem to be just one answer to why amphibians are declining so. The combination of habitat destruction, climate change, acid rain, appear to work together. These sadly, are all caused by us.