Amphibians are vertebrates that include frogs, toads, newts and salamanders. They are semi-aquatic, spending part of their time submerged in water and the rest on land. Their name, amphibian, comes from the Greek word amphibios, which means 'double life'. Most species require living near a water source their whole lives to prevent dehydration; since amphibians lack scales, a shell, or tough skin, they can lose moisture very quickly.
Amphibian Life Cycle
Amphibians have what is known as a biphasic life cycle, which means that as eggs and tadpoles, amphibian larvae live in water, and gradually change to suit a primarily terrestrial life. As a male and female mate, the female can lay as many as 4,000 eggs, generally within the shelter of plants; their eggs are sticky and can therefore stick to them. This begins the cycle of metamorphosis, where the eggs grow into their larval stage as tadpoles, and finally into adult amphibians. Upon hatching, the tadpoles will consume the soft egg jelly as nourishment. A huge percentage of the tadpoles will not survive into adulthood, as disease, predators, or poor environmental conditions claim a large number of them.
After their egg sac is fully consumed, they begin to eat drifting plant matter and breathe solely through their gills at this stage. As they grow, their lungs develop, and they begin making trips to the surface in order to breathe. Soon after, they begin to eat small invertebrates such as freshwater snais as their diet becomes increasingly carnivorous.
As they continue to grow, they begin to develop legs and their tails shrink. Frogs and toads lose the tails eventually, while newts and salamanders retain it, albeit shorter than in their larval stage.
Taxonomy
Class: Amphibia
Amphibians and the Environment
Since the 1970's, drastic losses in amphibian populations have been recorded across the world. Some species of amphibians have been reported to be extinct in recent years. Reasons for this decline haven't been fully researched, but evidence indicates that introduced predators, habitat destruction or alteration, acid rain and climate change may be the top causes of this sudden drop in amphibian numbers globally.
What does this mean? Amphibians live both on land and water; they need to be near a water source but are not strictly aquatic. They are not protected by the stable environment of the oceans as fish are. They are exposed to the harshness of terrestrial life and yet also have adapted to life submerged in water by developing very permeable skin. Since their skin is highly absorbative and they are semi-aquatic, they can indicate how well an environment can support life.
There are nearly 5,000 species of amphibians worldwide, with new species being discovered every year. Their diversity is apparent by their adaptation to nearly every region on earth excepting polar regions and the oceans. Their numbers are greatest in Central and South America as well as Madagascar.
Amphibian population decline has tended to be sharp in the areas where human beings are disrupting their habitats. The draining of wetlands and the damming of waterways has affected many amphibians' environments. Use of land for agricultural purposes and the clearing of naturally wooded areas also destroyes vital parts of these animals' habitats. All of these modifications affect local climate in small ways and yet this makes a big impact on how these creatures are able to survive.
Amphibian disease has been a significant factor in reducing wild populations. As with all factors affecting amphibian decline, much additional research is needed to solidify factual basis for many theories associated with it. That said, the major diseases thought to be causing population reductions are listed below.
- Chytridiomycosis: Chytridiomycosis is caused by the pathogen Batrachochytrium dendrobatidis. It is spread through the medium of water, and cannot likely withstand periods outside of it. It is able to multiply without having an amphibian as a host for it and therefore can cover a wide area. Exactly why Chytridiomycosis causes mortality is unknown, and environmental factors seem to affect it in that way; lower temperatures increase mortality rate in wild amphibians.
- Ranaviral Disease: This disease is caused by multiple species of viruses in the Ranavirus genus. Mortality rates in capitivity and in the wild are both high. Like Chytridiomycosis, low temperature seems to affect mortality rates.
Increased exposure to UVB rays have been known to be a factor in population fluctuation among amphibian species. Studies have shown that while not necessarily harmful to adult amphibians, it does hinder proper development of their eggs. Stratospheric ozone depletion has caused an increase in ultraviolet rays reaching the earth's surface. Combined with existant pathogens and increases in acidity of their aquatic environments, UVB exposure can have a more dramatic effect. Factors such as the introduction of species exotic to areas containing amphibian populations and intentional human predation for such things as the frog leg trade are also contributing to population declines in some areas.
The bottom line with regard to global amphibian population declines is what it says about our changing environment. Clearly many conditions are shifting, and considering what is happening to this delicate group of animals, it can make us reflect on what effects our actions are having on a global level. Many of the factors affecting amphibians in the wild are due in part to human modification of the environment. However, more research in this area is needed to properly determine what can be done about the many amphibian species in critical danger of extinction.
Amphibians as Pets
From the awesomely named waxy monkey tree frog (Pyllomedusa sauvagii) in the Order Anura to the shy red-eyed tree frog (Agalychnis callidryas) and the mascarene rocket frog (Ptychadena mascareniensis) that can leap over 17 feet (5 meters) in the air, amphibians can make fascinating pets if properly cared for.
As with any pet, it is important to find a good veterinarian that is experienced in handling and treating reptiles and amphibians. Many vets are not, so try asking around at local pet stores or herp breeders to get a good recommendation. A good online resource is http://www.herpvetconnection.com - they rely on pet owner's recommendations to list vets on their sites so take that into consideration if using the site.
If kept as a pet, a prospective herp keeper must be aware of the needs of these creatures. They must have a very stable environment that properly mimics what they would have in the wild. Amphibians, for the most part, are best housed in a fishtank so that an appropriate swimming area can be given to them. Most of the time, amphibians will need to be fed insects, worms, and fish.
A moss or soil substrate is recommended to cover the bottom of their tank excepting the swimming area. The substrate will help retain humidity in the tank as for many amphibians, it should be kept at a moderate to high level. The arboreal frogs are one of the few types of amphibians that should not have a large swimming area since there would be a risk of them drowning in such cases.
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Amphibians are some of the most fascinating creatures in the world; from their biphasic life cycle to their important position as a link in vertebrate evolution and their rapidly dwindling numbers, they truly are incredible animals.
Sources
http://www.usgs.gov/amphibians.html
http://www.geocities.com/darthdusan/amphibians.htm
http://www.jcu.edu.au/school/phtm/PHTM/frogs/formidable.htm
http://www.herpindex.com
http://www.open.ac.uk/daptf/index.htm
http://www.ucmp.berkeley.edu/vertebrates/tetrapods/amphibintro.html
http://wgbis.ces.iisc.ernet.in/biodiversity/amphibians/reproduction.htm