spacer Introduction
>Home Page 
>About this Guide 
>Table of Contents

1.  What you can do
2.  Water
3.  Ecology
4.  Amphibians
5.  Environmental Issues
6.  Keystone species
7.  Get Wet!-
     Field Study Ideas

8.  The Zoo Experience
9.  Frogs & Friends
10. Case Studies
11. Resources
12. Glossary

Wetland Curriculum Resource
Unit 5. Environmental Issues - Background for Educators

Global Issues

For more than 350 million years, amphibians have survived on our earth. They have evolved as one of the most successful vertebrate groups. Today however, there is need for concern, as many species worldwide have become extinct, and the continued existence of amphibians remains in peril. Worldwide, sixteen countries across five different continents are experiencing massive declines.

It is very difficult to ascertain the cause or causes for such declines. Implicated as possible explanations for local declines are habitat loss, natural population fluctuations, pesticides, heavy-metal poisoning, competition from exotic species, the stocking of lakes and ponds with fish that feed on amphibian eggs, predation, increased demand on frog legs, or landscaping. While habitat destruction is suggested to be the leading cause for declines, many scientists disagree. Habitat destruction simply does not explain why so many species are disappearing in protected, or isolated areas. This has led scientists to believe that habitat destruction cannot be responsible for worldwide declines.

Even the impact of natural population fluctuations has been dismissed because the declines are occurring in many different species, all at the same time. The most baffling aspect of this crisis is, in areas where some species have declined dramatically, other species are thriving. Research has shown that in areas of the Rocky Mountains, thriving wood frog populations and declining northern leopard frog populations co-exist. Both are closely related species, belonging to the genus Rana. Although it is not known exactly why this is happening, it could be due to their tolerance to acidic conditions. Wood frogs can tolerate lower pH conditions. This variation of tolerance levels between closely related species makes them valuable indicators for detecting early signs of environmental degradation.

On a much broader scale, some scientists have concluded that various environmental factors are the cause for such worldwide declines. Amphibians are extremely sensitive to environmental contamination and can detect many changes. They are sensitive to environmental change because of the following reasons:

  • as an herbivorous aquatic larvae and as a carnivorous adult, amphibians are affected by environmental contaminants in any of their environments or trophic levels
  • they occupy low and high levels in food chains
  • they possess a permeable skin that can absorb contaminants

The main factors that scientists believe are contributing to worldwide amphibian declines are global warming, UV radiation, acid deposition, and chemical contamination.


Global warming is closely associated with the greenhouse effect. The greenhouse effect is a natural process which has been occurring on the earth for millions of years. It involves the interactions of carbon dioxide, sunlight, and other gases found in the atmosphere. The greenhouse effect is so-named because the process is similar to the warming process inside a greenhouse. When sunlight penetrates through the glass of a greenhouse, the light energy is converted to infrared energy which warms up the greenhouse. The greenhouse remains warm as heat cannot escape. In nature, carbon dioxide, water vapour, and other gases allow the short wavelengths of solar radiation to pass through to the earth. When the wavelengths hit the earth's surface, they are converted to infrared radiation. However, when a short wavelength hits the earth, the wavelength lengthens, causing less energy to escape (some is absorbed). The longer wavelengths cannot escape through the atmosphere. Warming occurs when more heat is retained than what has escaped. Global warming refers to an enhanced greenhouse effect. This is caused by increased levels of greenhouse gases in the atmosphere. Greenhouse gases are important because without them, the earth would be about 30 deg. C colder than what it already is. However, increased levels of greenhouse gases contributes to global warming.

Greenhouse Gases and their Sources

The five greenhouse gases contributing to global warming include:

carbon dioxide: It accounts for 50% of the gases contributing to global warming. The two major sources are the combustion of fossil fuels ( which adds about 5.6 billion tonnes of added carbon dioxide to the atmosphere each year), and the burning of the rainforest in Brazil.

CFCs (chlorofluorocarbons): CFCs account for 23% of the gases contributing to global warming. Sources include cooling devices, the electronics industry, and spray cans. One CFC molecule is 10,000 times more capable of trapping and holding heat than one molecule of carbon dioxide.

methane: It accounts for 11% of the gases contributing to global warming. Sources include landfill sites, cattle, termites, rice paddies, and places where anaerobic decomposition occurs (wetlands). It is a powerful greenhouse gas, capable of trapping and holding heat 40 times better than carbon dioxide.

ground level ozone: It accounts for 9% of the gases contributing to global warming. A major source is nitrogen dioxide, which is emitted from the tailpipes of automobiles. Other sources include electrical motors, and high voltage equipment.

nitrous oxide: It accounts for 7% of the gases contributing to global warming. Natural sources include the breakdown of proteins (i.e. plants). Almost half comes from the chemical industry in the production of fertilizers.

A Cause For Concern

Throughout the history of the earth, past climatic changes have occurred. The various ice ages were the result of drastic climatic changes. It has even been speculated that a climatic change was responsible for the demise of the dinosaurs. The cause for concern now, is that it is occurring at a time when the earth is heavily populated. Also, it is occurring at a rate 100 times faster than the last climatic change which followed the last ice age. Scientists have estimated that the average global temperature will rise 4 deg. C by the year 2070. Although this seems like a small amount, the difference between the average global temperature during the last ice age when most of Canada was under a 2 kilometre sheet of ice 10,000 years ago, and today's average is about 4 deg. C.

Results of Global Warming

The following are the changes that are predicted to occur as a result of global warming:

  • the earth's mean temperature will rise, causing deserts to expand
  • wind systems and precipitation patterns will be affected, resulting in more rain and much more dramatic weather disturbances (i.e. tornados)
  • climates will be warmer and wetter
  • ocean levels will rise by 30 metres
  • ecosystems will be disturbed
  • plants and animals who normally require hundreds or thousands of years to adapt to changed conditions under normal circumstances will undergo massive setbacks

The Effects Of Global Warming On Amphibians

Many scientists believe that increased warming of the earth's surface will have major effects on amphibians. The concern is the effect that the warming will contribute to the changing of weather patterns. Even the slightest changes in precipitation can affect the reproduction and survival of amphibians. Because of their dependency on water for survival, if there is little rainfall, or if ponds dry out too quickly, there will be a decreased number of young emerging that year. Amphibians are adapted to coping with this problem on a yearly basis in one of two ways. Firstly, they produce and lay large numbers of eggs, which will therefore increase the chances of survival for some eggs to make it to adulthood. Secondly, amphibians live long lives, so if there is a high mortality rate in one season, there is probable opportunity to breed again the following season. If the changing climatic patterns are contributing to some of the population declines, it would be difficult for populations to re-establish themselves over a longer periods of time.

As a result of global warming, climatologists predict that continental interiors will become drier, and droughts more severe. Today, scientists believe the effects of global warming are causing plants and animals to migrate northward at a rate of 300 to 600 miles per century.


In the early 1900s, scientists discovered a thin layer of ozone about 15 to 50 km thick high in the atmosphere, in what is known as the stratosphere. Ozone itself, is a pale blue, poisonous gas which shields the earth from harmful UV radiation emitted from the sun. About 90% of the earth's ozone is high in the atmosphere, where it benefits life on earth. Although most UV radiation is absorbed by the ozone layer, some does pass through the atmosphere reaching the earth's surface. Too much exposure to UV radiation causes skin cancer, cataracts, weakening of the immune system, damage to food crops, and the killing of phytoplankton in aquatic ecosystems.

The remaining 10% is ground level ozone, a highly toxic substance. The major source of ground level ozone is in the combustion of automobile engines. Another source is industrial, specifically sterilizing and bleaching. Ground level ozone traps heat near the earth's surface.

Research has indicated that the protective ozone layer is thinning (not an actual "hole"). The cause of the thinning is the result of CFC molecules that are released into the atmosphere. CFCs are commonly used in the electronics industry as a cleaning solvent, as a foaming agent to make furniture, insulation, and styrofoam, in any cooling device such as air-conditioners or refrigerators, and as a propellant to push sprays out of cans. Every time one CFC molecule is released into the atmosphere, it destroys 100,000 ozone molecules. It takes 20 years for that one CFC molecule to reach the ozone layer. That is why the concern for ozone depletion has suddenly been brought to our attention. We are now starting to see the effects of CFCs on the ozone over the past years.

Ultraviolet rays are classified into three categories; UVa, UVb, and UVc. UVA rays penetrate deeply into your skin and cause premature aging and wrinkling of the skin. UVb rays attack the surface of your skin, producing painful burns and contributing to skin cancer. UVC rays are completely filtered by the earth's ozone layer and have no harmful effect. UVb radiation has the greatest affect on amphibians.

The Effects of UV Radiation On Amphibians

From existing research and data collected, there is evidence to suggest that UV radiation causes mortality or reproductive failure. The strongest evidence to support this theory is with declines in species that live in higher elevations and latitudes than those in lower altitudes and the tropics. It is particularly evident in species with dark pigmentations, which are more inclined to absorb such radiation. It has been found that amphibians that lay their eggs in open water under direct sunlight, as compared to those who lay eggs in the shade or under water, suffer less hatching success. Studies are being conducted on comparing the development of tadpoles in a laboratory environment and in a natural setting. The raising of tadpoles in a laboratory away from direct sunlight indicated a higher success rate. Those raised in a natural setting under direct sunlight suffered high mortality. Further research is being conducted to ascertain if amphibian declines are related to UV radiation, as it is not easy to determine.


During the 1870s in England, the first effects of acid deposition were being observed by a British chemist named Robert Smith. He discovered that smoke and fumes released by industries reacted with precipitation and thereby lowering the pH. Within a century, the same effects were being noticed in the freshwater ecosystems of North America, as fish stocks began to decline in many lakes.

The largest contributor to acid deposition is the burning of fossil fuels. Today, the average pH of precipitation is 5.6. About two-thirds of the acidity in acid deposition in North America comes from sulphur dioxide emissions. Natural sources of sulphur dioxide include volcanoes, geysers, sea spray (sulphur is released from the decay of life in the ocean), forest fires (wood has a high concentration of sulphur), and the decomposition of organic matter. Human-made sources include fossil fuel combustion, and the smelting of sulphur-rich ore. It is estimated that 60 million tons of sulphur dioxide is released into the atmosphere each year. Sulphur dioxide is a very dense gas (twice the density of air) and therefore falls and settles readily on the ground. It is a pungent, colourless, irritating gas, and becomes very acidic when it combines with oxygen to form sulphuric acid.

Ninety-three percent of the sulphur dioxide in the atmosphere is produced in the western hemisphere. That amounts to 100 million tons annually. Sudbury is the largest source of sulphur dioxide in Ontario through smelting processes at Inco. In the smelting process, heat is added to the ore, and then oxygen is passed over it releasing the sulphur (eg. NiS + O2 -> Ni + SO2). The sulphur dioxide emissions enter the atmosphere from the tall smokestacks.

Because of its serious effects on life forms, it is the most thoroughly investigated pollutant. With aquatic organisms, it has its greatest effect in the spring. Highly acidic meltwater released into aquatic ecosystems causes acid shock which inhibits spawning and hatching of the organisms (fish and amphibians).

The other one-third of acidity in acid deposition comes from nitrogen oxide. Natural sources of nitrogen oxides are volcanoes, lightning, and bacteria. Human-made sources are fossil fuel combustion, and the internal combustion engine of automobiles. Nitrogen dioxide is also a very dense gas (twice the density of air). It is a powerful oxidizing agent, and carcinogenic. When it combines with water, it is very acidic producing nitric acid. Both sulphur dioxide and nitrogen oxide emissions are the leading contributors to acid rain.

Acid deposition can be found in two forms; as wet deposition, or as dry deposition. Wet deposition is also referred to as acid precipitation, which includes rain, snow, mist, hail, sleet, and fog. Dry deposition occurs in two forms; as particulates (solid particles in the atmosphere), or as gases. When either of these two forms of dry deposition comes in contact with water, wet deposition is formed.

Long Range Transport of Air Pollutants (LRTAP)

Pollutants may be carried hundreds or even thousands of kilometres by prevailing winds. This phenomenon is known as the Long Range Transport of Airborne Pollutants (LRTAP). Prolonged transport of sulphur and nitrogen compounds allows time for the chemical and physical change of sulphur dioxide and nitrogen oxides into acidic compounds.

In order to reduce local air pollution and to meet government standards, the most inexpensive method for power plants and industries to accomplish this was to build taller smokestacks to spew pollutants above the inversion layer (a layer of dense cool air trapped under a layer of less dense warm air which prevents the upward flow of air currents to release air pollution). Although this solved the local air pollution problem, it created greater pollution problems in areas downwind. It is estimated that more than 50% of the acid rain that falls in Canada comes from the U.S., and more than 10% of the acid rain that falls in northeastern U.S. comes from Canadian sources. Much of the acid rain that is carried into Ontario originates from the moist air over the Gulf of Mexico. This air collects pollution as it moves north over the coal and oil burning power and industrial plants from the central and upper midwestern states - Ohio, Indiana, Pennsylvania, Illinois, Missouri, West Virginia, and Tennessee. The concern between Canada and the U.S. over the LRTAP led to the development of the Clean Air Act of 1990. Both countries have agreed to make significant reductions in sulphur dioxide and nitrogen oxide emissions by the year 2000.

The Major Effects of Acid Deposition On Aquatic Ecosystems

  • decreased neutralizing capacity of surface water (the neutral ground water is constantly neutralizing the acidity of the precipitation).
  • decreased pH
  • increased concentration of toxic metals, such as aluminum, lead, mercury, and zinc which interrupt metabolic processes of aquatic organisms
  • decreased fish and amphibian populations
  • decreased diversity of aquatic organisms
  • dominance of acid-tolerant species, such as blue-green algae
  • reduced productivity in ecosystems

The Effects of Acid Deposition On Amphibians

Since 1957, scientists have known the impact of acidity on certain species of amphibians, particularly frogs. Scientists have learned that species vary considerably with respect to their tolerance to acidity. The limit for breeding amphibians is pH 4. Eggs will hatch between pH 4 to 5, however larvae tend to develop abnormalities, normally resulting in death or predation. Research has shown that increased acidity levels slows the rate at which tadpoles develop. This ultimately affects species who breed in temporary ponds, as they may not complete development before the pond dries out. On the other hand, some species show little effect on exposure to acidic conditions. Some species, such as the wood frog, prefer an acidic environment.

Scientists theorize that this could be a genetic adaptation, evolved from long-term exposure to an acidic environment, possibly to decrease competition from other species. The theory to this adaptation does not recognize the sudden impact of increased acidity on a species.

Another cause for concern of acid rain is the synergistic effect that it has on heavy metals in soils. Heavy metals in soils react with acid rain, causing the metals to breakdown into a soluble form. These soluble metals are then exposed to and absorbed by amphibian skin which weakens the immunity system, leaving amphibians more susceptible to other contaminants or disease.


We have already introduced some of the chemical contaminants that affect amphibians, such as carbon dioxide, CFCs and other greenhouse gases that contribute to global warming, and other compounds, such as sulphur dioxide, and nitrogen dioxide that are leading factors in the creation of acid deposition. However, many other chemicals have been released into the environment that affect Ontario's wetlands.

Where do these chemicals come from?

These chemicals are produced by many sources including industrial processes (everything from poisonous heavy metals produced in manufacturing cars to toxic dry cleaning solvents used to clean clothes), from agriculture (fertilizers and pesticides), and from home use (in 1991 it was estimated that each Canadian household discarded approximately 41 litres of hazardous waste per year in such forms as used motor oil, paint stripper, solvents, household cleaners, bleaches, garden supplies and so on).

In addition to chemicals that are recognised as toxic, scientists have expressed increasing concerns about certain synthetic chemicals. More than 100,000 chemical compounds have been created in the past fifty years and released into the environment and 1,000 new ones are manufactured annually . Some of these chemicals are thought to mimic naturally occurring hormones and are thought to have serious effects on all life on earth including humans. Researchers are investigating the link between these hormone mimics and a range of disorders including cancer, infertility, birth deformities, and neurological disorders.

The Effects of Chemical Contamination On Amphibians

Animals that live in wetlands, near industrialized areas such as the Great Lakes or near most human settlements, can be exposed to chemical contaminants through all stages of their life. They may take in chemicals in the food they eat, the water they live in, and the air they breathe. There are a wide range of chemicals that affect wildlife but of particular concern are fat-soluble contaminants that build up in the body. These include polychlorinated biphenyls (PCBs), dioxins and furans, and organochlorine pesticides.

Studies of wetland animals, particularly mudpuppies and snapping turtles, have shown that animals exposed to these chemicals in the wetlands around the Great Lakes and St. Lawrence River show abnormal development. These abnormal developments include a high number of unhatched eggs (twice that of areas without these chemicals) and abnormal developments of tails, legs, shells, eyes and yolk sacs. For example, mudpuppies from contaminated areas show extra or fewer toes than normal, or toes that were fused together with a much greater frequency (60% occurrence) than those from non-contaminated sites (8% occurrence). Findings like these suggest that toxic chemicals may be causing poor survival of the eggs and the production of fewer, healthy young animals entering the population. (For further information see the enclosed Great Lakes Fact Sheet - "Amphibians and Reptiles in Great Lakes Wetlands: Threats and Conservation")

Another impact comes from chemical fertilizers, (and, for that matter, from organic fertilizers like manure). Fertilizers make plants grow. When excess fertilizer reach an aquatic ecosystem, they artificially increase the nutrient levels resulting in increased primary production (i.e. production of algae, plants, etc.). A large quantity of oxygen is used to achieve this high level of production and, as the plant life is increased, sunlight is blocked out, oxygen reduced. This reduced oxygen is followed by a large die off of the plant life. This die-off then increases oxygen demand for the decaying process and the cycle continues.

Some Good News

While many chemicals pose a serious threat to our environment and the buildup of chemicals from a range of sources continues to be a serious problem, changes are occurring. During the last 20 years the level of contamination in the Great Lakes has decreased, certain chemicals have been banned from use, and the disposal of chemicals is being handled with greater care by many users because of tougher legislation and increased awareness of the serious consequences of improper use.


Although there is a noticeable decline of amphibians in some areas of the world, other areas show stability or even increases. While population growths are encouraging, declines are a critical issue because of the rate at which they are occurring. Scientists are at a stand-still because of the lack of long-term population data to compare current declines with. Because of common cyclic fluctuations common to amphibians, and their long life spans, several decades of monitoring must be completed to examine population fluctuations and determine possible explanations. Unfortunately, during this time frame, several other species may become extinct. If there is something in the environment that is causing amphibian declines, then we need to show concern and to take action.

You Can Help....

Your class can contribute to our understanding of the distribution and biology of amphibians by keeping field notes of amphibian activity and distribution. Have your class contribute notes and reports for a student/educator newsletter which is sent to all participating schools across Ontario. See Exercise 5.11, "Look What I Found" for further details. Also, a Wetland Monitoring Sheet and Amphibian Observation Sheet are contained in Exercise 2.9, Sally Mander's Favourite Place.

"Marsh Monitoring Program"
Long Point Bird Observatory
P.O. Box 160
Port Rowan, ON N0E 1MO
Phone: (519) 586-3531

Information about "Backyard Frog Surveys" and "Amphibian Road Call Counts" can be obtained by contacting:

Frog Surveys, Canadian Wildlife Service
Environment Canada,
Canada Centre for Inland Waters,
867 Lakeshore Rd., P.O. Box 5050
Burlington, ON L7R 4A6
Phone: (905) 336-4968

:: Top ::


Copyright 2013 Adopt-A-Pond - Toronto Zoo       All Rights Reserved