Amphibian ecology and evolution
Amphibians are found in ponds, streams, wetlands of all types, under rotten logs, in leaf litter, in trees, underground, even in pools of rain water inside large leaves. However, they are not able to osmoregulate in salt water and, therefore, are not found in the ocean. Although some amphibians defy the rules and thrive in cold or dry conditions, the group reaches its highest diversity and numbers in warm, humid climates.
In the wet tropics, amphibians remain active all year around, but in the temperate zone, winter temperatures cool their bodies, forcing them to become inactive.
In the autumn, environmental cues direct amphibians to find moist, sheltered places like muddy pond bottoms or deep leaf litter to hibernate.
The wood frog (Rana sylvatica) has the most northerly range of any amphibian, crossing the Arctic circle, into the Mackenzie River valley in the Northwest Territories. Antifreeze in the frog's cells prevents the formation of ice crystals and keeps it alive until spring.
In warm regions where rainfall is unpredictable or seasonal, resident amphibians have some innovative ways of avoiding dessication. Phyllomedusa sauvagei is a large South American tree frog which spends much of its time in tree tops where wind and heat can cause rapid water loss from evaporation. When this frog rests, special glands in its skin secrete a waxy substance which the frog then rubs all over its body. This coat provides enough waterproofing to lower the frog's rate of water loss close to that of a desert iguana!
Several frog and salamander species take another approach to protecting themselves from drying out. Some species of Australian frogs in the genera Cyclorana and Limnodynastes as well as the salamander Siren intermedia of North America, react to arid conditions by burying themselves in soil and forming a watertight envelope from sloughed-off outer layers of skin.
One of the first sounds to be heard on cool spring evenings after the snow melt is the high pitched calling of the spring peepers (Pseudacris crucifer). At the same time, often in the same pond, salamanders are also moving about in the chilled waters, silently breeding.
Peepers are among the first frogs to chorus and breed, but as their reproductive period ends, other species take their turn so that summer evenings in a wetland are rarely quiet. Only male frogs call, advertising to females that they are receptive to mating, and guiding them by sound to their perches. A female will listen carefully, selecting her mate based on the quality of his call. For example, female grey tree frogs prefer males who can hold a note for a long time.
Salamanders are lead to each other by chemical cues and often use complex courtship behaviours to ensure that mating occurs with the right species. A male Salamandra hugs the female from beneath, wrapping his front legs around hers. He then moves to the side, directing the female's cloaca over his spermatophore.
Whereas a tadpole is little more than a swimming filter with a gut, an adult amphibian will typically consumet anything that it can stuff into its mouth. Some Amphibia are slow deliberate feeders, while others can quickly flip their special, elongate tongues out to glom onto prey at some distance. Frogs are well known for this, often grabbing prey in mid-leap.
The hyoid apparatus is part of the breathing mechanism in most amphibians. However, in the lungless salamander (Plethodontidae) it is freed-up to perform another function, and helps to project the tongue further and faster, sometimes extending it to more than the length of the salamander's body!
Without the ability to chew their food, swallowing can be difficult for amphibians. Many resort to using their front feet to stuff uncooperative meals down their throats. Toads even pull their eye-balls into their skulls to help force down resisting morsels.
Although most adult amphibians feed largely on invertebrates, some species have unique dietary preferences. The Surinam toad (Pipa pipa) is an aquatic piscivore which has long fingers covered in tiny spines for grasping slippery fish. Another specialized carnivore, the ornate horned frog of South America, is less discriminating. It sits buried in leaf litter on the forest floor, waiting to make a meal of any small vertebrate that happens by, whether it be another amphibian or a small mammal or bird! By contrast, a species of Brazilian tree frog is a strict herbivore, eating only berries.
Despite clever cryptic colouration and poisonous glands, amphibians are food for many animals, including humans. The well muscled legs of some jumping frogs are as often served as a restaurant delicacy as skewered over a campfire.
Some birds and mammals have even learned to peel away the unpleasant tasting skin of the more poisonous species, to get at the muscular flesh underneath. Eggs and tadpoles are even more vulnerable and are eaten by everything from insects to fish and other amphibians, including other tadpoles. Egg-masses can also be fed upon by much smaller predators like molds and bacteria.
In the late 1980s, amphibian researchers noticed that amphibian populations, especially frogs and toads, were declining in numbers, and in some cases, to the point of extinction. The fact that reports of declining amphibian populations were coming from around the world suggested that some large-scale, far-reaching factor was producing these declines. It is possible that these amphibian declines are a reflection of a serious ecological problem and that amphibians, like a canary in a coal mine, could be indicators to humans of environmental quality.
Some examples of amphibian declines from around the world and close to home:
The Mountain Yellow-legged Frog (Rana muscosa) - These frogs were abundant in the Sierra Nevada mountains as late as the 1970s. A number of surveys in the 1990s found this species to be present at about half of the historical localities in U.S.National Parks, where they supposedly are protected from human impacts.
The Golden Toad (Bufo periglenes) - The Golden Toad lives in the cloud forest in Costa Rica. It was never very common, but in the late 1980s all known populations disappeared and this species has not been found in the wild since 1994.
The Stomach-brooding Frog (Rheobatrachus silus) - This frog was once so common in Australia that a good observer could find 100 frogs in a single night. The frog underwent a decline around 1980 and now appears to be extinct.
The Northern Leopard Frog (Rana pipiens) - Populations of Leopard Frogs in central Alberta, north of 51° N latitude, became extirpated in 1979. Declines, and some recoveries, of Leopard Frog populations have occurred across North America since the 1960s.
These are just a few of the well known declining species. There are many more species under investigation!
Loss of Habitat
Habitat destruction, such as logging, draining swamps, and urbanization, are clearly causing the decline of amphibians. Furthermore, roads create direct mortality (roadkill) and fragment populations. However, the same is true for almost all wildlife and although it explains why amphibians are declining, it doesn't explain why they're declining faster than other wildlife.
Agricultural runoff, loaded with pesticides, herbicides and chemical fertilizers, can kill frogs and toads directly, or impair their larval development. Water pollution in the form of phosphates can cause algal blooms, which clog waterways, preventing other plants and animals from living.
Introduction of Predators
When humans introduce alien species into an area, either by accident or intentionally, there are often unpredictable repercussions on entire ecosystems. As an example, the introduction of trout into high altitude lakes in the Sierra Nevada mountains has drastically reduced populations of the Mountain Yellow-legged Frog and other species of amphibians.
Ultra-Violet (UV) Radiation
The thinning of the ozone layer due to the use of chloroflourocarbons has led to an increase in the amount of UV light which reaches the earth. Amphibians generally have thin skin, and their eggs are usually exposed to sunlight, so this increased UV-B radiation may cause increased mortality and abnormalities in wild populations.
A combination of these factors may be involved. One factor may cause stress, allowing another factor to become deadly.
Almost 400 million years ago, a sturdy-finned fish clambered onto land to explore the possibilities of terrestrial life. Not only did it find safety from its predators, but a feast of insects as well. This first fish out of water is thought to have been a sarcopterygian (lobe-finned fish) and it gave rise to a fish-like creature with stouter limbs: the first amphibian.
Three genera of early Amphibia are known, ironically, from fossils found in Greenland and Russia (deposited before those two land masses drifted to their modern northerly positions). Although these first groups were still very aquatic, they quickly gave rise to more terrestrial forms.
For a time, the terrestrial field was wide open and amphibians emerged to occupy every available damp habitat. Over the next 100 million years, amphibians radiated, reaching much greater diversity than they have achieved since. They ranged hugely in size, from a few centimeters to the giant Eryops which was more than two meters long.
One group of amphibians drifted away from aquatic habitats toward drier terrain, becoming better adapted to life on land and spawning the first reptiles.
The decline in amphibian diversity began approximately 300 million years ago and coincided with the rise of the reptiles. By the Triassic period (220 million years before present), not only had reptiles largely supplanted amphibians but their domination had been bolstered by a general trend toward a more arid climate. A few amphibian groups persisted through the "age of reptiles" and gave rise to the three modern groups about 120 million years ago, in the middle of the Cretaceous period.
- P.E.Ahlberg and A.R. Milner. 1994. The origin and early diversification of tetrapods. Nature. No. 368
- Robert L.Carroll, Robert L. 2009. The Rise of Amphibians: 365 Million Years of Evolution. Baltimore: The Johns Hopkins University Press
- A.J.Clack. 2002: Gaining ground: the origin and evolution of tetrapods. Indiana University Press, Bloomington, Indiana. 369 pp
- Diego San Mauro, Miguel Vences, Marina Alcobendas, Rafael Zardoya and Axel Meyer. 2005. Initial diversification of living amphibians predated the breakup of Pangaea. American Naturalist 165 (5): 590–599