Decline in amphibian populations

Amphibians, such as frogs and salamanders, are found all over the world. Since the 1980s, scientists have noticed that there are fewer of them than before. This is a concern because it shows that nature is changing in ways that can affect our planet.

There are several reasons why amphibians are disappearing. Their homes can be changed or destroyed. Diseases, pollution, chemicals used to help plants grow, animals from other places that compete with them, and too much harmful radiation from the sun can also be problems.

Studies show that amphibians might be disappearing much faster than normal. Some estimates say it could be more than two hundred times quicker than usual! People have noticed problems with amphibian numbers since the 1950s in Europe. In the 1980s, the world began to understand this as a big problem for all living things.

Observations

In the past few decades, many kinds of amphibians such as frogs, toads, salamanders, newts, and caecilians have been disappearing around the world. A big study showed that many amphibian species were losing numbers, and some have vanished.

These declines were first noticed in the late 1980s by scientists. 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. 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 may be reasons for the big drop in the number of amphibians in recent years.

The drops have been very big in the western United States, Central America, South America, eastern Australia and Fiji (but cases have appeared all over the world). While humans are causing loss of many plants and animals, amphibians seem to be hurt more than others. This is because they live both in water as larvae and on land as adult animals. They are also sensitive to problems in water and on land. Because their skin easily lets things through, they may be hurt more by toxins than animals like birds or mammals.

Many reasons have been suggested for why amphibians are declining. Most of these reasons have been linked to drops in numbers, so each one may play a part in some places but not others. Many reasons are understood and also affect other animals. These reasons include changing habitats, introducing new animals that eat or compete with them, pollution, using pesticides, or taking too many of them. However, some drops have happened in places where these things are not likely to happen. The reasons for these drops are complicated, but may include new diseases, climate change, more ultraviolet-B radiation, or chemicals carried far by wind.

Lights may also be a problem. Insects are drawn to lights, making them harder for amphibians to find.

Habitat modification

Main article: Habitat destruction

Changing or destroying habitats is a big problem for amphibians all over the world. Because amphibians need both water and land to live, threats to either can hurt their numbers. This may make them more vulnerable than animals that only need one type of home. Big changes in climate may also change water homes, stopping amphibians from having babies.

Habitat fragmentation

Main article: Habitat fragmentation

Habitat fragmentation happens when habitats get separated by changes like farms growing around a forest. Small groups of animals left in these pieces can suffer from inbreeding, genetic drift, or even going extinct because of small changes in their home.

Disease

Research has shown that certain types of chytrid fungi may be a big reason for the drop in amphibians.

Several diseases have been linked to big drops in amphibian numbers, including "red-leg" disease (Aeromonas hydrophila), Ranavirus (family Iridoviridae), Anuraperkinsus, and chytridiomycosis. It is not clear why these diseases are now affecting amphibians, but some signs point to humans spreading them, or them becoming stronger when combined with other problems.

Trematodes

Further information: Ribeiroia

There is strong proof that tiny parasites called trematode platyhelminths (a kind of fluke) have caused problems and drops in amphibian numbers in some places. These trematodes of the kind Ribeiroia have a life cycle with three hosts. The first host is water snails. The early stages of the trematodes move to tadpoles, where they stay in the developing legs. These can cause problems in the legs of frogs after they change shape, such as extra or missing legs. These problems make frogs more likely to be eaten by birds, the final host of the trematode.

A study showed that lots of nutrients from farms can make parasite infections worse, leading to frog problems in ponds and lakes in North America. The study showed that more nitrogen and phosphorus make lots more trematodes, and these parasites can cause cysts in tadpole legs, leading to missing legs, extra legs, and other serious problems.

Pollution and chemical contaminants

There is proof that chemical pollutants can cause problems in frog development (like extra limbs or bad eyes). Pollutants affect frogs in different ways. Some change the central nervous system; others mess with hormones. Tests have shown that common herbicides like glyphosate (Tradename Roundup) or insecticides like malathion or carbaryl can kill many tadpoles. Tests also show that adult amphibians can be hurt by some parts of Roundup, especially POEA, which is a surfactant. While some frogs naturally change their sex in clean places, some pollutants that act like estrogen can force this change. In a lab test in Sweden, more than half of the frogs exposed to these pollutants became females. Even the smallest amount of these pollutants made tadpoles twice as likely to become females.

While most pesticide problems are likely close to farms, proof from the Sierra Nevada shows that pesticides can travel far into clean areas, including Yosemite National Park in California.

Some new signs point to ozone as maybe helping cause the world-wide drop in amphibians.

Ozone depletion, ultraviolet radiation and cloud cover

See also: Climate change

Like other animals, more ultraviolet-B (UVB) radiation from less ozone depletion can hurt the DNA of amphibians, especially their eggs. The amount of harm depends on the age, the type of animal, and other things around them. Salamanders and frogs that make less photolyase, an enzyme that fixes DNA damage from UVB, are more likely to feel the effects of losing the ozone layer. UVB radiation may not kill a certain type or age but can still cause harm.

Over thirty types of amphibians have been studied, with big effects reported in more than forty science papers from North America, Europe and Australia. Tests to see UVB effects on eggs have been criticized; for example, the eggs were put in very shallow water, which is not how they usually are laid. While UVB radiation is a problem for amphibians, its effect on eggs may have been made to seem bigger than it is.

Climate change has likely played a big role in amphibian drops. For example, in the Monteverde Cloud Forest, a few very warm years led to the disappearances of the Monteverde Harlequin frog and the Golden Toad. More cloud cover, from geoengineering and global warming, has been blamed for helping the growth of a fungus that causes chytridiomycosis.

Even though the fungus was the direct cause, climate change helped it happen. Scientists included this idea in their theory linking climate to diseases, saying climate change is important in amphibian drops in Costa Rica and elsewhere.

New signs show that global warming can also directly hurt toads' health and chances of living. It also works with changes in the land, pollution, and new animals to cause amphibian drops.

Introduced predators

Main article: Introduced species

Non-native animals that eat or compete with frogs have been found to hurt frogs where they live. The mountain yellow-legged frog in the Sierra Nevada lakes has dropped in numbers because people put in non-native fish (trout) for fun fishing. The young frogs are eaten by these fish in large numbers. This problem in the frogs' three-year metamorphosis is causing drops all through their home.

Increased noise levels

Frogs and toads make lots of noise, and they often use calls to find mates. Some signs show that more noise from humans may be hurting them. In a study in Thailand, more ambient noise levels made some frogs call less and others call more. But this has not been shown to cause the big drops in numbers.

Symptoms of stressed populations

When amphibian populations start to decrease, they sometimes show signs that help scientists notice problems early. One important sign is called developmental instability. This means the animals develop in uneven ways.

This can happen because of stress from their environment. It can make the animals more likely to get sick. Over time, this can lead to fewer amphibians.

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 saw that amphibian numbers were getting smaller, they started a group in 1990 called the Declining Amphibian Population Task Force. This group watched amphibian numbers and studied why they were shrinking. They shared what they learned in a newsletter named Froglog.

In 2004, they finished the first Global Amphibian Assessment. They checked every known amphibian species and found that about one third were in danger of disappearing. Because of this, leaders met in 2005 at an Amphibian Conservation Summit. They decided it was important to help these animals quickly.

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

In places like Australia and South America, where amphibian numbers are also getting smaller, there are not many rules to protect them. Local efforts to reduce global warming may help these animals. Protecting areas for amphibians is important, but sometimes these areas don’t work for long because the animals can lose genetic diversity. Teaching local communities and making laws to limit harmful chemicals are also important steps to save amphibians.