Threats to Frogs
Frogs and other amphibians are facing an abundance of dangers which contribute to their decline, all of which are brought about or exacerbated by human actions. Here's a list of their biggest threats.
Habitat Destruction
The largest threat to frogs and toads is habitat destruction. This can vary from complete destruction of habitats, like deforestation and the filling of ponds and wetlands, leaving amphibians without a place to live, to fragmentation, to alteration. According to EDGE of Existence, a program dedicated to conserving threatened, evolutionary distinct species, “It is now estimated that humans have altered between one third and one half of the Earth’s land surface.” Because of the fact that amphibians live in both land and water, they need a fairly large area of habitat to be preserved. Otherwise, the construction of buildings and roads in their environments can lead to numerous roadkills.
Pollution
Another major threat is pollution. Frogs and toads have very porous skin, allowing them to drink and breathe through their skin. Unfortunately, because of this, harmful chemicals can easily enter the bloodstream of frogs and toads, making them extremely susceptible to pollution. Due to their double nature as land and aquatic animals, air and water pollution can easily affect them. Sometimes this can kill frogs directly, while other times it harms them in other ways. For example, atrazine, one of the most common herbicides, has been known to castrate many male frogs, making them infertile, and even turn them female in certain cases. Other pollutants, such as DDT, PCBs, and other chemicals can have similar effects. It is possible that pollutants also cause frog mutations, which prevent frogs from moving easily and is usually fatal, though many believe that mutations are caused by a parasite called Ribeiroia ondatrae instead.
Overexploitation
Frogs and toads are commonly taken out of the wild, often illegally, and sent across the world for the pet, food, and bait trades. Not only are many of these frog species already endangered, but the few regulations on amphibian trade are usually poorly enforced. Therefore, amphibians are rarely tested to see if they are infected, and they can easily escape into the wild. By spreading infected frogs around the world, we are invariably spreading the deadly disease as well, to species much less capable of surviving it. According to SAVE THE FROGS! founder and executive director Dr. Kerry Kriger, “If humans continue to spread chytridiomycosis through the international trade and transport of amphibians, many of which are sick, we can expect many more frog species to go extinct.” Other consequences of overexploitation include the spread of invasive species which can negatively impact amphibians, and the more direct depletion of wild amphibian populations.
Infectious Diseases
A third threat to amphibian species is infectious diseases. The most dangerous of them all is a deadly fungus is Batrachochytrium dendrobatidis (Bd), which causes chytridiomycosis, a lethal disease that has contributed greatly to the extinction of many amphibian species. Bd, a fungus usually found in cooler, high elevation climates, harms amphibians when too many of its zoospores (about 10,000 or so) enter their skin. This causes amphibians to tighten the pores of their skin in response, and shed their skin excessively. Unfortunately, this response only serves to harm the amphibians further, making it more difficult to breathe and drink while doing nothing to stop the zoospores from entering their skin. Eventually, the amphibians become lethargic, lose their righting reflex, and die of dehydration or suffocation. Furthermore, while scientists have been working assiduously to discover vaccines or other preventative methods that could save frogs and toads from chytridiomycosis, no effective solutions have been found. The biggest driver for the spread of this disease is the human trade of amphibians, which has few restrictions and few tests for infectious diseases.
Invasive Species
While the trade of amphibians themselves is extremely harmful to frogs, so can the trade of other organisms. Invasive species can be a huge threat to amphibians. For instance, fish are often used to stock ponds, but if they are not native species, then they may eat either frogs or their young at dangerous rates. Even other amphibians that can compete for resources or spread diseases, like the cane toads in Australia, which were introduced to eat the cane beetles that were eating crops, to replace pesticides and other chemicals. However, the cane toads thrived, spreading rapidly. Not only did they compete with native amphibians, sometimes even feeding on them, they also had no predators, since they can secrete a deadly poison that killed any animal that tried to eat them. Even now, they continue to threaten Australian wildlife, and are a major threat to other amphibians. Something as simple as even releasing a pet into the wild can have devastating consequences on surrounding environments, so it is important to keep organisms in their natural environments.
Climate Change
Even in pristine, disease free national parks and wilderness areas, amphibians are vanishing, and it is likely that they are declining due to climate change and global warming. Frogs and other amphibians are more sensitive to temperature changes than most organisms for two reasons. One, warming atmospheres greatly increase likelihood of amphibian dehydration, because of their permeable skin. Two, because it affects amphibian breeding phenology. Amphibians typically breed in the spring and summer after hibernating in the winter. This cycle is triggered by annual changes in temperature and moisture. However, as temperatures increase, amphibians emerge from dormancy and breed sooner than they normally do. For example, FrogWatch citizen scientists recently discovered certain frog populations in coastal South Carolina breeding 72 days earlier than in previous years. This can adversely affect amphibian populations, because by breeding earlier, amphibians are more vulnerable to early freezes and snowmelt floods.
UV Radiation
Frogs and other amphibians are also declining due to UV, particularly UV-B. Unfortunately, this radiation is increasing in our atmosphere as chlorofluorocarbons dissipate the ozone layer that traps them. According to the Earth Observatory, “UV-B causes damage at the molecular level to the fundamental building block of life— deoxyribonucleic acid (DNA).” Not only do adult frogs and salamanders have thin, sensitive skin, but so do juveniles and eggs, so amphibians of all ages can be negatively affected by increasing levels in UV-B radiation. However, these effects can vary greatly, with some species being affected greatly, and others not at all. For example, UV-B radiation can cause the skin of Gray Tree frogs to darken, and cause them to swim less. For others, like the Pacific Tree frog, this radiation can reduce larval survival and increase abnormalities in behavior and development. Other common effects include increased embryo mortality and behavioral avoidance. Interestingly, different populations of amphibian species can be differently affected by UV-B radiation as well, possibly because certain regions have more UV-B radiation than others. Regardless, the fact that UV-B radiation has been increasing in our atmosphere is alarming.
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Threat Synergism
While all of these factors are detrimental towards the health of frogs and other amphibians, they are not necessarily fatal. If amphibians were only facing one of these threats, then it is doubtful they would be declining as steeply as they are. The true danger comes in the form of synergistic effects. Synergy is a word that describes how, by working together, separate entities can produce a greater effect than the sum of the individuals. In other words, the whole is greater than the sum of its parts. This is exactly the case with the threats amphibians face. A frog species dealing with, say, an invasive, introduced predatory fish, would be harmed, but might not go extinct. However, if this species was also losing habitat, or dehydrating due to global warming, it would be far less effective at coping with the fish, and much more likely to die out. For example, in a study from the University of Pittsburgh, 42% of Gray Tree frog tadpoles exposed to 5mg/L of malathion, a harmful pesticide, died, and when they were also exposed to predatory cues from an adult eastern newt, 82% died. The pesticide itself was harmful, but the synergistic effects of it with predators hurt the tadpoles further.
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