>About this Guide
>Table of Contents
1. What you can do
5. Environmental Issues
6. Keystone species
7. Get Wet!-
Field Study Ideas
8. The Zoo Experience
9. Frogs & Friends
10. Case Studies
CASE STUDY 1: GLOBAL AMPHIBIAN DECLINE(Level: 7 : 10 acad : 10 appl : 11 appl : 12 acad )
Amphibians are perhaps one of the most successful groups of wild life on the earth. They evolved before the reign of the dinosaur, and continued to thrive through climatic changes which resulted in the demise of other species. During the 1989 First International Herpetological Congress, the future success of amphibians was questioned. Researchers from all over the world reported declines in amphibian populations. In some cases, extinction of species had already occurred. Despite speculation and numerous theories, the reasons for the decline are still a mystery.
Throughout the world, the disappearance of many amphibians has been recorded. In Australia, the once common Gastric Brooding Frog (Rheobatrachus silus) disappeared within its protected habitat. The peculiar reproduction of the frog sparked the interest of both naturalists and scientists. Following fertilization, the mother swallowed the eggs, and fully formed frogs emerged from her mouth. The Gastric Brooding Frog was able to suppress the acids in its stomach while the frogs developed. This unique behaviour intrigued scientists who were interested in a treatment for stomach ulcers. The information may be lost forever.
The Golden Toad in Costa Rica has also disappeared from a protected habitat. The toad lived in a square kilometre region of Costa Rica's Monteverde Cloud Forest Reserve. The brilliant orange colour of the male easily distinguished it from other toad species. Within two years, the toads disappeared from the regular breeding areas.
The decline of amphibian populations has even occurred in our own backyards. The Cricket Frog has disappeared from Pelee Island in Lake Erie, and from adjacent mainland areas in the United States. Canada has witnessed the decline of the Leopard Frog and Bullfrog. In Quebec, a study determined the Chorus Frog has disappeared or become rare in areas where it was abundant. Pollution in once pristine waterways threatens many amphibian species such as the Mudpuppy.
Even more perplexing is the presence of a declining species and a stable one in the same habitat. In the North American Rocky mountains, the Leopard Frog, Rana pipiens, has declined, while the Wood frog, Rana sylvatica, has been unaffected. Physiological, or ecological differences between the two species may play a role in the situation.
Although many species are declining, some are actually thriving. The Giant Cane Toad, Bufo marinus, native to South America and Mexico, was introduced to Australia in the 1930s to control the Grey-back Beetle. The Cane Toad controlled the beetle and then spread into the surrounding wetlands. The Toad is now considered to be a pest, and its distribution is spreading every year. (See also Case Study 3: The Introduction of the Cane Toad into Australia.),/P>
The unique biology of amphibians has been cited as one possible explanation for the current declines. Amphibians are intrinsically connected to water at one or more stage in their life cycle. The life history begins as the tadpole emerges from the egg, breathing oxygen through gills and consuming plant material. Most amphibians change into terrestrial adults. The carnivorous adult consumes insects, and breaths through lungs and the moist outer layer of skin. Every spring, amphibians emerge from hibernation to breed in wetlands. The biphasic lifestyle of amphibians makes them especially vulnerable to changes in the air, water, or land. The presence of amphibians in an ecosystem is generally considered to be an indicator of environmental health. The sensitive skin and complex life cycle of amphibians makes them susceptible to changes in environmental quality.
The greatest threat to the survival of amphibians is the destruction or fragmentation of wetland habitats. In Ontario alone, over 75% of the pre-settlement wetlands have been altered or drained. In the South American rainforests, wetlands are being destroyed before the amphibians can be identified.
Although habitat destruction has affected many amphibians, it can not explain why amphibians like the Golden Toad have disappeared. Although amphibians populations undergo cycles of increase and decrease, the current decline is too widespread to be considered natural. Anthropogenic factors such as acid precipitation, increased levels of ultraviolet radiation, and pesticides could be affecting amphibians. One study, performed by John Harte of the University of California and colleagues, determined acid precipitation affected the development of Tiger Salamander, Ambystoma tigrinum, eggs.
The increased levels of ultraviolet radiation may suppress the immune system and development of amphibians living at higher elevations. Many amphibians are exposed to sunlight during the day as they bask to maintain body temperature. Exposure also occurs during the spring when amphibians breed in sun-warmed waters.
Pesticides and other forms of pollution can affect amphibians on both land and water. Amphibians do not drink. They absorb water and much of the oxygen they need through the skin. The stress caused by long periods of sub-lethal exposure to low levels of environmental contaminants (that may be well within allowable toxicity guidelines) may make amphibians more vulnerable to viral and bacterial infections.
Humans, either directly or indirectly, have contributed to the decline of amphibians. Harvesting for fish bait, food, or student dissections has increased the stress on many struggling populations. Exotic introductions have spread lethal viruses to amphibians and increased predation pressures. The introduction of the Cane Toad into Australia is an example of an introduction which has caused a decline in the native White's Tree Frog population.
The worldwide decline of amphibians is too widespread to be considered a natural downturn in the population. A crash is occurring. Despite the vulnerability of amphibians to pollution and habitat destruction, they have survived drastic environmental changes in the past. The present decline may indicate a dangerous level of environmental degradation.
The disappearance of amphibians is a very real problem. Although the exact cause of the decline is not known, we do not have to be passive observers. The destruction of amphibian habitats, specifically wetlands, is a problem which can be solved by students of all ages. Wetlands can be restored, protected from future destruction, or created where none exist. Habitat creation to link existing wetlands can improve reproductive success in amphibians, and allow populations to move throughout their range. The future of amphibians is dependant on how we manage our backyards and beyond.
Answer: Some natural causes that may account for changes in amphibian population size and distribution include:
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