Since the 1980s, scientists have noticed fewer amphibians in many areas around the world. This includes smaller groups of amphibians and large-scale disappearances. This loss of variety in living things is considered one of the biggest threats to life on Earth. Possible reasons include changing or destroying their homes, diseases, overuse by humans, pollution, pesticides, non-native animals, and exposure to ultraviolet-B radiation (UV-B). However, many reasons for these declines are still not fully understood, and researchers continue to study the issue.
Studies show that today’s amphibian extinction rate might be 211 times higher than the normal rate seen in Earth’s history. This number increases to 25,000–45,000 times if species at risk of disappearing are also counted.
Scientists first noticed fewer amphibians in parts of Europe in the 1950s. However, people only began to recognize this as a global problem and label it as a major extinction event in the 1980s.
Observations
Over the past 30 years, the number of amphibians, such as frogs, toads, salamanders, newts, and caecilians, has decreased worldwide. In 2004, scientists completed the first global study of amphibian populations called the Global Amphibian Assessment. This study found that 32% of amphibian species are at risk of extinction, 43% are losing numbers, and between 9 and 122 species have gone extinct since 1980. By 2010, the IUCN Red List, which includes updates from this study, listed 650 amphibian species as "Critically Endangered" and 35 as "Extinct." Although amphibians face serious threats, recent studies show that the public is becoming less concerned about this and other environmental issues, which makes it harder for scientists and conservation workers to protect these animals.
Scientists first noticed widespread declines in amphibian populations in the late 1980s. At that time, many herpetologists (scientists who study amphibians and reptiles) reported fewer amphibians in many areas around the world. The golden toad (Bufo periglenes), which lived only in Monteverde, Costa Rica, was one of the species that disappeared quickly. Its population dropped sharply in 1987 and it was gone by 1989. Another species, the Monteverde harlequin frog (Atelopus varius), also disappeared at the same time. These extinctions happened in the Monteverde Cloud Forest Reserve, a protected area with little human activity, which surprised scientists and raised concerns.
Many scientists believe that amphibians act as "canaries in a coal mine," meaning their declines can signal problems for other animals and plants. When amphibian declines were first reported in the late 1980s, some scientists doubted how serious the issue was. They argued that populations naturally change over time and that there was not enough long-term data to prove that declines were real or needed urgent action. However, over time, scientists agreed that amphibian declines are a real and serious threat to biodiversity. This agreement came after more studies, observations of large die-offs in untouched areas, and recognition that amphibian declines are happening globally.
Causes
Habitat loss, disease, and climate change are believed to be the main reasons for the large drop in amphibian populations in recent years. Population declines have been especially severe in the western United States, Central America, South America, eastern Australia, and Fiji, though amphibian extinctions have been reported worldwide. Human activities are causing the loss of many species, but amphibians seem to be affected more than other groups of living things. Amphibians have a life cycle with two stages: one in water (larvae) and one on land (adults). This makes them sensitive to changes in both water and land environments. Their skin is very thin, which may make them more likely to absorb harmful substances in the environment than birds or mammals.
Many possible reasons for amphibian declines have been studied. Most of these causes are linked to population drops, but each may affect amphibians in different situations. Some causes, like habitat changes, introduced animals, pollution, pesticides, or over-harvesting, are well understood and also affect other species. However, some amphibian declines have happened in areas that seem untouched by these problems. These declines are complex, but may be linked to new diseases, climate change, increased ultraviolet-B radiation, or chemicals carried by wind over long distances.
Artificial lighting may also be a cause. Insects are drawn to lights, which makes them less available in amphibian habitats.
Habitat changes or destruction are a major problem for amphibians worldwide. Because amphibians need both water and land to survive, threats to either habitat can harm their populations. This may make them more vulnerable than species that only need one type of habitat. Large-scale climate changes may also alter water habitats, making it impossible for amphibians to reproduce.
Habitat fragmentation happens when habitats are separated by changes to the environment, such as when a small forest area is surrounded by farmland. Small populations in these fragments are more likely to face inbreeding, genetic drift, or extinction from small environmental changes.
Research from 2007 and 2018 suggested that the return of certain types of chytrid fungi may explain a large part of the overall decline in amphibian populations.
Several diseases have been linked to large die-offs or population drops in amphibians, including "red-leg" disease (Aeromonas hydrophila), Ranavirus (family Iridoviridae), Anuraperkinsus, and chytridiomycosis. It is not clear why these diseases have suddenly started affecting amphibians, but some evidence suggests they may have been spread by humans or may be more harmful when combined with other environmental factors.
There is strong evidence that parasitic flatworms (a type of fluke) have caused developmental problems and population drops in amphibians in some areas. These flatworms, called Ribeiroia, have a life cycle with three hosts. The first host is aquatic snails. The early stages of the flatworms then infect aquatic tadpoles, where they form cysts in developing limbs. These cysts cause limb abnormalities in frogs after they grow into adults. These abnormalities increase the chance of frogs being eaten by birds, the final host of the flatworm.
A study found that high levels of nutrients from farming and ranching activities increase parasite infections that cause frog deformities in ponds and lakes across North America. The study showed that increased nitrogen and phosphorus levels lead to more trematodes, which then form cysts in tadpoles’ limbs, causing missing or extra limbs and other severe deformities.
There is evidence that chemical pollutants cause frog deformities, such as extra limbs or malformed eyes. Pollutants affect frogs in different ways. Some change the nervous system, while others disrupt hormone production. Experiments have shown that exposure to common herbicides like glyphosate (Roundup) or insecticides like malathion or carbaryl greatly increases tadpole deaths. Studies also show that adult amphibians are affected by non-active ingredients in Roundup, especially POEA, a surfactant. While some frogs naturally change sex in healthy environments, certain estrogen-like pollutants can force these changes. In a study at Uppsala University in Sweden, over 50% of frogs exposed to estrogen-like pollutants found in natural water in Europe and the United States became female. Tadpoles exposed to even weak concentrations of estrogen were twice as likely to become female, while almost all frogs in the heaviest dose group became female.
Most pesticide effects are likely limited to areas near farms, but evidence from the Sierra Nevada mountains in the western United States shows that pesticides can travel long distances into untouched areas, such as Yosemite National Park in California.
Some recent evidence suggests that ozone may be a factor in the worldwide decline of amphibians.
Like many other living things, increased ultraviolet-B (UVB) radiation, caused by the thinning of the ozone layer and other factors, may harm amphibian DNA, especially their eggs. The amount of damage depends on the life stage, species, and other environmental conditions. Salamanders and frogs that produce less photolyase, an enzyme that repairs DNA damage from UVB, are more likely to be harmed by ozone loss. Exposure to UVB may not kill a species or life stage but may cause non-lethal harm.
More than 36 amphibian species have been studied, with serious effects reported in over 40 peer-reviewed journal articles by researchers from North America, Europe, and Australia. Experimental studies on UVB effects on eggs have been criticized because egg masses were placed in water much shallower than natural conditions. While UVB radiation is a significant stressor for amphibians, its impact on eggs may have been exaggerated.
Human-caused climate change has likely played a major role in amphibian declines. For example, in the Monteverde Cloud Forest, unusually warm years led to the disappearance of the Monteverde Harlequin frog and the Golden Toad. Increased cloud cover, caused by geoengineering and global warming, has warmed nights and cooled daytime temperatures, which may have helped the fungus Batrachochytrium dendrobatidis (the cause of chytridiomycosis) grow and spread.
Although the immediate cause of the die-offs was the
Symptoms of stressed populations
Amphibian populations that are starting to decline often show certain signs that can help identify groups in need of protection. One sign is developmental instability, which has been shown to be a result of environmental stress. This stress can increase the chance of amphibians getting sick from diseases like chytridiomycosis, leading to population declines. For example, a study in Queensland, Australia, found that two amphibian species, Litoria nannotis and Litoria genimaculata, had much higher levels of uneven limb development in the years before their populations declined compared to years without die-offs, which occurred about 16 years earlier. Recognizing these signs early could improve efforts to protect amphibians.
Conservation measures
In 1990, a group called the Declining Amphibian Population Task Force (DAPTF) was created to address reports that amphibian numbers were decreasing. DAPTF worked to monitor amphibian populations more closely to understand the problem better. They also formed smaller groups to study different issues. Their findings were shared in a newsletter called Froglog.
Much of this research helped create the first Global Amphibian Assessment (GAA), published in 2004. This study evaluated every known amphibian species using the IUCN Red List criteria. It found that about one-third of amphibian species were at risk of extinction. Because of these findings, an Amphibian Conservation Summit was held in 2005. Leaders at the summit said it was "morally irresponsible" to document amphibian declines without also taking action to stop them.
The summit led to the creation of the Amphibian Conservation Action Plan (ACAP) and the merging of DAPTF and the Global Amphibian Specialist Group into the IUCN SSC Amphibian Specialist Group (ASG). ACAP outlined steps to address the crisis, including actions in different areas of conservation. ASG is a worldwide network of scientists who help guide efforts to protect amphibians globally.
ACAP (Gascon et al. 2007) warned that time was limited and recommended starting captive breeding programs for all amphibian species in need. On February 16, 2007, scientists from around the world met in Atlanta, U.S., to form a group called Amphibian Ark. This group aimed to save more than 6,000 amphibian species by starting breeding programs in captivity. Between 2007 and 2019, the number of breeding programs increased by 57%, adding 77 more species to the effort.
In places like Australia, where frog extinctions have been reported, few policies exist to prevent these losses. However, local efforts to reduce global warming have also helped protect frogs. In South America, where amphibian populations are also declining, no clear policies exist to save frogs. Some experts suggest that national governments should create rules and institutions to guide local efforts.
A major challenge is designing protected areas that provide safe conditions for amphibians. Using protected areas has often been a short-term solution because amphibians in these areas may become inbred. Most amphibians need large, diverse environments to maintain genetic diversity, which is essential for their survival.
Teaching local people how to protect amphibians is important. Laws should also be created to protect amphibians and limit the use of harmful chemicals, such as certain fertilizers and pesticides, in areas where amphibians live.