Decline in amphibian populations

Since the 1980s, scientists have noticed that numbers of amphibians, such as frogs and salamanders, are getting smaller all around the world. This loss of different kinds of living things is very worrying because it shows how nature is changing in ways that can harm the balance of our planet. There are many reasons why amphibians are disappearing, including places where they live being changed or destroyed, diseases, pollution, use of chemicals to help plants grow, animals from other places that compete with them, and too much harmful radiation from the sun.

Studies show that the speed at which amphibians might vanish is much faster than normal. Some estimates say it could be more than two hundred times quicker than usual! People have seen problems with amphibian numbers since the 1950s in Europe, but it was in the 1980s that the world started to understand this as a big problem for all living things.

Observations

In the past three decades, many kinds of amphibians such as frogs, toads, salamanders, newts, and caecilians have been disappearing around the world. A big study in 2004 found that about 32% of amphibian species were in danger, and at least 43% were losing numbers. Sadly, between 9 and 122 species have vanished since 1980. By 2010, a special list called the IUCN Red List showed that 650 amphibian species were in very serious trouble, and 35 were gone forever.

These declines were first noticed in the late 1980s by scientists who study animals. One famous example is the golden toad, which lived only in Costa Rica. It was seen until 1987, but then it suddenly disappeared by 1989. Other frogs in the same area also vanished around the same time. This was worrying because these animals lived in a protected forest with no obvious human harm.

Many scientists think amphibians are like early warning signs for nature. When their numbers fall, it may mean other animals and plants are also in danger soon.

Causes

Habitat loss, disease and climate change are thought to be responsible for the drastic decline in populations in recent years.

Declines have been particularly intense in the western United States, Central America, South America, eastern Australia and Fiji (although cases of amphibian extinctions have appeared worldwide). While human activities are causing a loss of much of the world's biodiversity, amphibians appear to be suffering much greater effects than other classes of organism. Because amphibians generally have a two-staged life cycle consisting of both aquatic (larvae) and terrestrial (adult) phases, they are sensitive to both terrestrial and aquatic environmental effects. Because their skins are highly permeable, they may be more susceptible to toxins in the environment than other organisms such as birds or mammals.

Numerous potential explanations for amphibian declines have been proposed. Most or all of these causes have been associated with some population declines, so each cause is likely to affect in certain circumstances but not others. Many of the causes of amphibian declines are well understood, and appear to affect other groups of organisms as well as amphibians. These causes include habitat modification and fragmentation, introduced predators or competitors, introduced species, pollution, pesticide use, or over-harvesting. However, many amphibian declines or extinctions have occurred in pristine habitats where the above effects are not likely to occur. The causes of these declines are complex, but many can be attributed to emerging diseases, climate change, increased ultraviolet-B radiation, or long-distance transmission of chemical contaminants by wind.

Artificial lighting has been suggested as another potential cause. Insects are attracted to lights making them scarcer within the amphibian habitats.

Habitat modification

Main article: Habitat destruction

Habitat modification or destruction is one of the most dramatic issues affecting amphibian species worldwide. As amphibians generally need aquatic and terrestrial habitats to survive, threats to either habitat can affect populations. Hence, amphibians may be more vulnerable to habitat modification than organisms that only require one habitat type. Large scale climate changes may further be modifying aquatic habitats, preventing amphibians from spawning altogether.

Habitat fragmentation

Main article: Habitat fragmentation

Habitat fragmentation occurs when habitats are isolated by habitat modification, such as when a small area of forest is completely surrounded by agricultural fields. Small populations that survive within such fragments are often susceptible to inbreeding, genetic drift, or extinction due to small fluctuations in the environment.

Disease

Research from 2007 and 2018 indicated that the reemergence of varieties of chytrid fungi may account for a substantial fraction of the overall decline.

A number of diseases have been related to mass die-offs or declines in populations of amphibians, including "red-leg" disease (Aeromonas hydrophila), Ranavirus (family Iridoviridae), Anuraperkinsus, and chytridiomycosis. It is not entirely clear why these diseases have suddenly begun to affect amphibian populations, but some evidence suggests that these diseases may have been spread by humans, or may be more virulent when combined with other environmental factors.

Trematodes

Further information: Ribeiroia

There is considerable evidence that parasitic trematode platyhelminths (a type of fluke) have contributed to developmental abnormalities and population declines of amphibians in some regions. These trematodes of the genus Ribeiroia have a complex life cycle with three host species. The first host includes a number of species of aquatic snails. The early larval stages of the trematodes then are transmitted into aquatic tadpoles, where the metacercariae (larvae) encyst in developing limb buds. These encysted life stages produce developmental abnormalities in post-metamorphic frogs, including additional or missing limbs. These abnormalities increase frog predation by aquatic birds, the final host of the trematode.

A study showed that high levels of nutrients used in farming and ranching activities fuel parasite infections that have caused frog deformities in ponds and lakes across North America. The study showed increased levels of nitrogen and phosphorus cause sharp hikes in the abundance of trematodes, and that the parasites subsequently form cysts in the developing limbs of tadpoles causing missing limbs, extra limbs and other severe malformations including five or six extra or even no limbs.

Pollution and chemical contaminants

There is evidence of chemical pollutants causing frog developmental deformities (extra limbs, or malformed eyes). Pollutants have varying effects on frogs. Some alter the central nervous system; others cause a disruption in the production and secretion of hormones. Experimental studies have also shown that exposure to commonly used herbicides such as glyphosate (Tradename Roundup) or insecticides such as malathion or carbaryl greatly increase mortality of tadpoles. Additional studies have indicated that terrestrial adult stages of amphibians are also susceptible to non-active ingredients in Roundup, particularly POEA, which is a surfactant. Although sex reversal in some species of frogs occur naturally in pristine environments, certain estrogen-like pollutants can forcibly induce these changes. In a study conducted in a laboratory at Uppsala University in Sweden, more than 50% of frogs exposed to levels of estrogen-like pollutants existing in natural bodies of water in Europe and the United States became females. Tadpoles exposed even to the weakest concentration of estrogen were twice as likely to become females while almost all of the control group given the heaviest dose became female.

While most pesticide effects are likely to be local and restricted to areas near agriculture, there is evidence from the Sierra Nevada mountains of the western United States that pesticides are traveling long distances into pristine areas, including Yosemite National Park in California.

Some recent evidence points to ozone as a possible contributing factor to the worldwide decline of amphibians.

Ozone depletion, ultraviolet radiation and cloud cover

See also: Climate change

Like many other organisms, increasing ultraviolet-B (UVB) radiation due to stratospheric ozone depletion and other factors may harm the DNA of amphibians, particularly their eggs. The amount of damage depends upon the life stage, the species type and other environmental parameters. Salamanders and frogs that produce less photolyase, an enzyme that counteracts DNA damage from UVB, are more susceptible to the effects of loss of the ozone layer. Exposure to ultraviolet radiation may not kill a particular species or life stage but may cause sublethal damage.

More than three dozen species of amphibians have been studied, with severe effects reported in more than 40 publications in peer-reviewed journals representing authors from North America, Europe and Australia. Experimental enclosure approaches to determine UVB effects on egg stages have been criticized; for example, egg masses were placed at water depths much shallower than is typical for natural oviposition sites. While UVB radiation is an important stressor for amphibians, its effect on the egg stage may have been overstated.

Anthropogenic climate change has likely exerted a major effect on amphibian declines. For example, in the Monteverde Cloud Forest, a series of unusually warm years led to the mass disappearances of the Monteverde Harlequin frog and the Golden Toad. An increased level of cloud cover, a result of geoengineering^{[citation needed]} and global warming, which has warmed the nights and cooled daytime temperatures, has been blamed for facilitating the growth and proliferation of the fungus Batrachochytrium dendrobatidis (the causative agent of the fungal infection chytridiomycosis).

Although the immediate cause of the die offs was the chytrid, climate change played a pivotal role in the extinctions. Researchers included this subtle connection in their inclusive climate-linked epidemic hypothesis, which acknowledged climatic change as a key factor in amphibian extinctions both in Costa Rica and elsewhere.

New evidence has shown global warming to also be capable of directly degrading toads' body condition and survivorship. Additionally, the phenomenon often colludes with landscape alteration, pollution, and species invasions to effect amphibian extinctions.

Introduced predators

Main article: Introduced species

Non-native predators and competitors have also been found to affect the viability of frogs in their habitats. The mountain yellow-legged frog which typically inhabits the Sierra Nevada lakes have seen a decline in numbers due to stocking of non-native fish (trout) for recreational fishing. The developing tadpoles and froglets fall prey to the fish in large numbers. This interference in the frog's three-year metamorphosis is causing a decline that is manifest throughout their ecosystem.

Increased noise levels

Frogs and toads are highly vocal, and their reproductive behaviour often involves the use of vocalizations. There have been suggestions that increased noise levels caused by human activities may be contributing to their declines. In a study in Thailand, increased ambient noise levels were shown to decrease calling in some species and to cause an increase in others. This has, however, not been shown to be a cause for the widespread decline.

Symptoms of stressed populations

When amphibian populations start to decrease, they often show certain signs that can help scientists notice problems early. One important sign is called developmental instability, which means the animals develop in uneven ways. This can happen because of stress from their environment, making them more likely to get sick and leading to fewer amphibians over time.

For example, in Queensland, Australia, scientists studied two kinds of frogs, Litoria nannotis and Litoria genimaculata. They noticed that before the frog numbers dropped, the frogs had more uneven limbs than usual. spotting these signs early can help protect amphibian populations better.

Conservation measures

See also: Amphibian Ark

When scientists noticed that amphibian populations were shrinking, they formed a group called the Declining Amphibian Population Task Force in 1990. This group worked to monitor amphibian numbers and study the problem. They shared their findings through a newsletter called Froglog.

In 2004, they completed the first Global Amphibian Assessment, which checked every known amphibian species. They found that about one third were at risk of disappearing. This led to an Amphibian Conservation Summit in 2005, where leaders decided it was important to act quickly to help these animals.

From this summit came the Amphibian Conservation Action Plan and a merged group called the IUCN SSC Amphibian Specialist Group. These groups work worldwide to support amphibian conservation. They also started special breeding programs to protect many amphibian species. By 2019, these efforts had grown significantly.

In places like Australia and South America, where amphibian numbers are also falling, there are still few rules to protect them. Local efforts to reduce global warming may also help these animals. Protecting areas for amphibians is important, but these areas sometimes don’t work long-term because the animals can lose genetic diversity. Educating local communities and making laws to limit harmful chemicals are also key steps in saving amphibians.