A catastrophic decline of many species of amphibians is underway throughout the world. Around 90 frog species are assumed to have disappeared from the earth since the latter part of the 20th Century, while there has been a devastating population collapse in the majority of the remaining species. Some have fallen by as much as 90%. Frogs serve as a crucial link in the food chain, feeding on smaller creatures while serving as prey for larger predators. If they disappear many more animal species will vanish as well.
Populations of frogs, toads and salamanders are declining in many places (in North, Central and South America, Europe, Asia, Africa and Australia), even from wilderness areas apparently unexposed to levels of disturbance and pollution characteristic elsewhere. Conservationists strive to understand why this animal group is dying out, with possible explanations being increased ultraviolet light from holes in the ozone, chemicals and pesticides, acid rain, viruses, habitat loss and adverse climate events; the answer is likely all the above: the combined effects of degradations to the environment by humans over the past half century; climate change alone cannot explain the sharpness of the decline. Some amphibian experts have warned that the disappearance of frogs and toads is an early warning signal of changes in the Earth's ecosystem.
Amphibians are sensitive environmental indicators with respect to water and atmosphere. Depending on the species, they may be vulnerable to environmental disturbances in both phases because they have life stages both in (egg, tadpole) and out of water, and thus come into contact with a wide variety of substances and influences. Because their skins are porous, heavy metals and other contaminants in soil or water can easily pass into their bodies, as with the chemical residues in the many creatures they eat.
At least one-third of North America's 86 frog and toad species are in serious decline. A 2013 study found that those species fairing "well" are decreasing in population by around 2.7% a year, while the worst cases are being chopped by nearly 12% annually. On average, 3.4 percent of North American amphibian species are disappearing from local amphibian habitats each year. That is the equivalent of losing half the species in any wetland, stream reach or forest site every 20 years. Researchers focused on how colonization and persistence of local populations were related to annual variation in five climate variables thought to affect key components of amphibian life cycles: winter severity, snowfall, breeding water availability, summer soil moisture and maximum temperature.
Similarly, ten percent of Australia's 194 frog species are in serious decline. What was a "mystery disease" was identified as the chytrid fungus in 1998 by scientists at the Australian Animal Health Laboratory. The disease is called chytridiomycosis and assigned to infection by the fungus Batrachochytrium dendrobatidis (Bd). The fungus is selective; in some places where it arrived, every single frog of an entire species has died within a 12-month period; in other places there is no effect at all – lots of frogs, some of them are even infected and not dying. The fungus is responsible for at least five extinctions in Australia. It is probably the major factor in the decline of at least a couple of dozen other species. Many formerly common frogs are now infected with it and are just not as common as they used to be.
The Savannah River Ecology Laboratory (SREL) began research in Rainbow Bay (USA) in 1978. This 20-year study is the longest running project of its kind in the world and is considered to be a model for the type of long-term studies that are required to understand the effects of climatic fluctuations and habitat disturbances on natural amphibian populations. Such long-term intensive studies of a single natural community have revealed that amphibian species can go through major population fluctuations from year to year and that a species dominant in some years may be almost absent in others, and vice versa. Hydroperiod (the number of days in a year that a wetland holds water) is the single most important factor that influences what species are "successful" at Rainbow Bay and how these species interact. The importance of hydroperiod is clearly evident in the amphibian community. If the hydroperiod is too short, no species reproduce successfully; if too long, then only a few species may do well. The number of species successfully reproducing in the pond and the number of young emerging from the pond appear to be highest at intermediate hydroperiods. Statistical analyses show that, due to the extreme levels of natural fluctuations in amphibian numbers, long time periods (greater than 20 years) may be necessary to identify population trends, especially if changes are subtle.
In 1995, large numbers of deformed frogs were found in Minnesota, Wisconsin and Quebec. An emergency research team found frogs with missing legs, extra legs, misshapen legs, paralyzed legs that stuck out from the body at odd places and legs that were fused to the body. They also found frogs with missing eyes. One memorable specimen was a one-eyed frog that turned out to have the second eye growing inside its throat. Virtually all deformed frogs were dying quickly, because they could not feed themselves or escape from predators. The researchers believed that the most probable cause was an environmental degradation caused by some kind of chemical pollutant in water, where the frogs breed and develop, and in which they spend every stage of life.
In 1994, scientists noted that eggs of frog and toad species which do not appear to be declining have high concentrations of an enzyme, photolyase, which protects them against the effects of ultraviolet radiation. The hatching rates of vulnerable frogs' eggs were significantly improved by shielding them from the sun. This suggests that the cause could be damage to the ozone layer.
Frogs play an important role in the control of certain crop pests.
Frogs, as with the traditional role of the canary in mines, may prove to be an indicator of the increase in the level of water pollution beyond a threshold at which the life of many species can be sustained over long periods.
The Vice President of Conservation International stated that "you have to assume that what impacts frogs will eventually impact everything else in the ecosystem. That makes it a biodiversity and a human health issue." To prevent the loss of species conservationists create protected areas. But species like frogs and toads are declining in such areas which calls into question whether protected areas are enough to conserve species and the attitude that "here" is where you keep clean and "there" is where you pollute. This distinction is not valid anymore.
Declining frog numbers may mean increasing human disease from insect borne killers like malaria and dengue fever.