What is a Frog
by Tessa Broholm
Amphibians Characteristics & Fun Facts for both adults and children. Amphibians are a group of animals that includes frogs, toads, newts and salamanders. They are sometimes confused with reptiles, a group that includes turtles, snakes and lizards, because they share some similarities. For example, both amphibians and reptiles are cold-blooded. This means that they rely on external conditions to keep them warm, unlike mammals (including humans) that are warm-blooded (“About Amphibians & Reptiles,” 2020; Spelman, 2012). There are some special characteristics of amphibians that set them apart from other animals.
Most amphibians have an egg-larva-adult life cycle (Spelman, 2012). Most adult amphibians lay eggs in water. When the eggs hatch, larvae emerge. Frogs and toads at this stage are commonly called tadpoles. The larvae may live in the water for a period of time before they undergo metamorphosis, meaning they lose their tails and gills, instead growing limbs and lungs that allow them to leave the water and live on land (see Figure 1).
Adult amphibians live on the land, although they still need to stay in a moist or wet environment. This is because another special feature of amphibians is their skin. Amphibians have delicate skin that allows them to breathe and absorb water (Spelman, 2012). Because of this special skin, amphibians have very strict requirements for where they live. Too much sun exposure can damage their cells, much like a human getting sunburnt. If there is too much wind, an amphibian may lose water through its skin and become dehydrated (Spelman, 2012).
Some unique species
The Oregon spotted frog (Rana pretiosa) is federally listed as threatened under the Endangered Species Act (“Oregon spotted frog,” 2020). A medium-sized frog covered with black spots, the Oregon spotted frog is native to the Pacific Northwest. To survive the winters, the Oregon spotted frog protects itself from freezing and predators by overwintering in flowing, oxygenated waters.
The eastern hellbender (Cryptobranchus alleganiensis alleganiensis) is the largest salamander found in the United States, growing up to 29 inches (more than 2 feet) in length! The eastern hellbender is found in streams from southern New York to Georgia, and as far west as Oklahoma and Kansas (“Eastern Hellbender Cryptobranchus alleganiensis alleganiensis,” 2019). People used to think that the hellbenders were venomous, but don’t worry, they are not.
The California red-legged frog (Rana draytonii) is federally listed as Threatened under the Endangered Species Act (“California red-legged frog,” 2011). Found throughout California, the California red-legged frog has lost an estimated 70% of its native habitat. One of the largest frogs found in the western United States, its diet has been found to include other small frogs and even mice, in addition to invertebrates.
An Early Warning System
Amphibian sensitivity to the environment has made them an indicator species, meaning the presence or absence of a species in an environment reflects the condition of that environment (Waddell, 2010). Another way to think of this is amphibians are the “canary in the coalmine.” Since amphibians are generally locally abundant, if there is a sudden reduction in their numbers or they become absent from an area, this suggests that a change has occurred in the environment. This is especially true for aquatic environments, upon which most amphibians rely for at least part of their life cycle.
In 1989, it was brought to the attention of scientists that amphibians worldwide were facing population declines (Márque & Alberch, 1991). A major cause of the decline was determined to be habitat destruction, a human-cause phenomenon that affects many different species (Márque & Alberch, 1991). However, amphibian population losses were also being seen in pristine and undisturbed habitats, which led researchers to study hydrological characteristics (i.e. water).
Water pollution and poor water quality are global concerns (Márque & Alberch, 1991). Chemicals used in agriculture and industries pose serious threats to species and may cause severe damage. Nitrogen-based fertilizers are some common pollutants that can pose serious threats to amphibians when found in high concentrations in waterways (Relyea, Schoeppner, & Hoverman, 2005).
The Oregon spotted frog, Rana pretiosa, is one such species that has disappeared from 78% of its historical range due to exposure to chemicals as a byproduct of intense agriculture (Marco, Quilchano, & Blaustein, 1999; “Oregon spotted frog,” 2020). While different species can survive different concentrations of toxicity in their environment, the most sensitive species are the early indicators that change is happening.
Saltwater Amphibians
What about Amphibians and the Ocean? Generally, amphibians are not associated with salty water and oceans. So why are we writing about them here? Amphibians can act as an important early warning system for our waterways.
The water cycle begins with evaporation (see Figure 4). Water evaporates from large bodies of water, like lakes and oceans, where it is then transported into the atmosphere. From the atmosphere, water returns to earth as precipitation (rain or snow). Once on land, water finds its way back to the ocean by seeping slowly as ground water or joining up in streams and rivers on its return. Any pollution or contamination that occurs in water while it is on land eventually makes its way back to the ocean.
Let’s stick with our previous example of nitrogen-based fertilizers. Nitrogen is an important nutrient for living organisms. When too much nitrogen is added to an environment, it can increase the growth of plants and algae. A lot of plants may seem like a good thing, but when this happens in the water, the plants can use up all of the oxygen from the water causing organisms such as fish, to die. This process is called eutrophication.
This can happen in lakes and ponds, but it can also happen on a very large scale. Perhaps the most famous example of eutrophication is in the Gulf of Mexico, where the aptly named “dead zone” encompasses hundreds of square miles. (If you are interested in learning more on the Gulf of Mexico and the Dead Zone, you can visit here:
In the United States, heavy uses of animal manure and commercial fertilizers in agriculture are the main contributors to eutrophication. Runoff from agricultural fields enters the waterways, which studies have shown have immediate impact on organisms such as amphibians (Marco et al., 1999). As these pollutants continue to move downstream, their concentration increases as more sources of nutrients and fertilizers enter the system before reaching the ocean (Relyea et al., 2005).
What can you do?
The sensitivity of amphibians to their environment can act as an important early warning system. If you notice a decrease or change in population of amphibians in your locality, you may want to ask yourself, what has changed in my environment recently? By recognizing changes in your waterways and upstream environments, you can help bring awareness to damage that may be incurred in your local environment, but also damage that may occur in your downstream communities.
If you are interested in additional steps and you have the means, eating locally and organically contributes greatly not only to your local community and economy, but it has the additional benefit of removing your contribution to large-scale agriculture that uses commercial fertilizers.
Sources:
About Amphibians & Reptiles. (2020). Retrieved from https://www.massaudubon.org/learn/nature-wildlife/reptiles-amphibians/about-amphibians-reptiles
California red-legged frog. (2011).
Eastern Hellbender Cryptobranchus alleganiensis alleganiensis. (2019). Retrieved from https://www.fws.gov/southeast/wildlife/amphibians/eastern-hellbender/
Marco, A., Quilchano, C., & Blaustein, A. R. (1999). Sensitivity to nitrate and nitrite in pond-breeding amphibians from the Pacific Northwest, USA. Environmental Toxicology and Chemistry, 18(12), 2836–2839. https://doi.org/10.1002/etc.5620181225
Márque, R., & Alberch, P. (1991). Monitoring Environmental Change Through Amphibian Populations. In Global Change and Mediterranean-Type Ecosystems. Ecological Studies (Analysis and Synthesis) (pp. 471–481).
Oregon spotted frog. (2020). Retrieved from https://www.fws.gov/oregonfwo/articles.cfm?id=149489458
Relyea, R. A., Schoeppner, N. M., & Hoverman, J. T. (2005). Pesticides and amphibians: The importance of community context. Ecological Applications, 15(4), 1125–1134. https://doi.org/10.1890/04-0559
Spelman, L. (2012). National Geographic Animal Enclyopedia.
Waddell, E. (2010). Amphibian diversity in Bornean forests : a comparison of habitats using different sampling methods and richness estimators, 1–59. Retrieved from http://www.outrop.com/uploads/7/2/4/9/7249041/waddel_11_amphib_div_sab_and_bawan.pdf
