Frogs are like other animals — they need food, a place to live, protection from disease, a safe place to raise babies and all the other things that make life possible on this planet. But the class of animals that survived the Cretaceous-Palaeogene extinction event that killed off 75 percent of all plants and animals on the planet (including the dinosaurs) is now shedding species at an alarming rate — the world has lost around 200 frog species since the year 1970. Some are calling frogs the "canary in the coal mine," claiming that they might have a special sensitivity to pollutants, disease and habitat change. But remember: 66 million years ago, frogs survived what many groups of animals couldn't. So, why are we losing the world's frogs now?
The short answer is, frog species are tanking for the same reason most other animals are disappearing: They don't have anywhere to live.
"But that's not true!" you say. "There are some woods outside my house some frogs can live in!"
Well, that's very nice of you to offer, but here's the thing about frogs and other amphibians: They have what's called "high species turnover," which makes it difficult for them to set up shop just anywhere.
"Species turnover is a way of thinking about diversity," says Dr. John Maerz, professor of vertebrate ecology in the Warnell School of Forestry and Natural Resources at the University of Georgia. "Imagine you are walking from an area with species A. How far would you have to walk before species A would be replaced by species B? The longer the distance, the lower the turnover rate. Amphibians have high turnover — in mountainous regions, particularly in the tropics, you can have species that are unique to a small geographic area like a single mountain range, so any loss of habitat has a relatively higher risk of causing that species to decline or go extinct."
Frogs Need Land and Water
Not only have individual frog species evolved very specific needs in terms of their geographical habitat, all amphibians can be especially sensitive to habitat changes because they spend time migrating between land and water during the year. Water sources like wetlands, ponds and streams are required for the laying of eggs and rearing of young, but many adults spend most of their time on land. Anything we do to a frog's habitat that keeps it from getting from one location to another — building roads and draining wetlands are two prime examples — can do some serious damage to a population, or even an entire species.
Because many frogs are adapted to pretty narrow climatic niches, and because they need access to water, it makes them arguably even more vulnerable to climate change than the rest of us. The increasing frequency or severity of drought isn't doing them any favors.
"We don't know of any examples of amphibian extinctions linked to climate change, but there is evidence of effects on survival rates and abundance," says Maerz. "For species near coastal areas, there is great concern that rising sea level and storm surges threaten freshwater habitats."
A Deadly Fungus
We humans don't just bring bulldozers and destructive weather to frog populations, we introduce and spread disease. A chytrid fungus, Batrachocytrium dendrobatidis (Bd), grows on the skin of a frog — and since amphibians take in water and oxygen through their skin, the fungus throws off their electrolytes, which eventually leads to heart failure. Bd has been known to decimate entire populations of frogs in a single year, and although nobody's entirely sure how it spreads, the best guess scientists have is that humans bring it with us wherever we encounter frogs — either accidentally, or when we take them in as pets, use them as food or in laboratory experiments.
But for all this, frogs persist. One assumption is that amphibians are generally less sensitive to pollutants than other animals, but, says Maerz, the science doesn't back this up:
"Amphibians are more sensitive to some other environmental pollutants compared to say fish, reptiles, or mammals, but there are other pollutants for which they are less sensitive. Some pollutants are responsible for local amphibian declines, but I am unaware of any cases where pollution cause extinction."
"I think frogs can withstand and survive some of the threats we're throwing at them," says Brian Crawford, a postdoctoral research scientist in the Georgia Cooperative Fish & Wildlife Research Unit at the University of Georgia. "It's just when you stack all the different threats — big and small — on top of one another that you realize the chance of a population or a species going extinct is pretty large, and the chance that a species could withstand all of these things at once is pretty small."