But ecological succession must also occur on new land, in an area that has not supported life before. Primary succession (Figure below) is the type of ecological succession that happens in barren lands, such as those created by lava flow or retreating glaciers. Since the land that results from these processes is often completely new land, part of the primary succession process is soil formation.
Primary succession always starts with the establishment of a pioneer species, a species that first inhabits the disturbed area. In the case of barren rock, the pioneer species is lichen, a symbiotic relationship between a fungus and an algae or cyanobacteria. The fungus is able to absorb minerals and nutrients from the rock, and the algae or cyanobacteria is provides carbohydrates from photosynthesis. Since the lichen can photosynthesize and do not rely on soil, lichen can live in desolate environments. As the lichen grows, it breaks down the rock, which is the first step of soil formation.
Figure 24.12
Primary succession on a rock often begins with the growth of lichen.
The pioneer species is soon replaced by a series of other communities. Mosses and grasses will be able to grow in the newly created soil. During early succession, plant species like grasses that grow and reproduce quickly will be favored and take over the landscape. Over time, these plants improve the soil further and a few shrubs can begin to grow. Gradually the shrubs are then replaced by trees. Since trees are more successful competing for resources than shrubs and grasses, a forest will be the end result of primary succession if the climate supports that type of biome.
Secondary Succession
Sometimes ecological succession occurs in places where there is already soil, and that has previously supported life. Secondary succession is the type of ecological succession that happens after something destroys the community, but yet soil remains in the area. One event that can lead to secondary succession is the abandonment of a field that was once used for agriculture (Figure below). In this case, the pioneer species would be the grasses that first appear. Gradually the field would return to the natural state and look like it used to look before the influence of man.
Another event that results in secondary succession is a forest fire (Figure below and below). Although the area will look devastated at first, the seeds of new plants are underground and waiting for their chance to grow. Just like primary succession, the burned forest will go through a series of communities, starting with small grasses, then shrubs, and finally mature trees below. An orderly process of succession will always occur, whether a community is destroyed by man or the forces of nature.
Figure 24.13
This land was once used for growing crops. Now that the field is abandoned, secondary succession has begun. Pioneer species, such as the grasses, first appear and then shrubs begin to grow
Figure 24.14
The early stages of succession after a forest fire are shown in these pictures. Taken four years after the fire, they show the charred remains of the original forest as well as the small grasses and shrubs that are beginning to grow back in the area.
Figure 24.15
The early stages of succession after a forest fire are shown in these pictures. Taken four years after the fire, they show the charred remains of the original forest as well as the small grasses and shrubs that are beginning to grow back in the area.
Figure 24.16
In 1988, a forest fire destroyed much of Yellowstone National Park. This photo, taken 17 years later, shows that the forest is gradually growing back. Small grasses first grew here and are now being replaced by small trees and shrubs. This is an example of the later stages of secondary succession.
Climax Communities
Climax communities (Figure below) are the end result of ecological succession. In contrast with the series of changes that occur during ecological succession, the climax community is stable. The climax community will remain in equilibrium unless a disaster strikes and succession would have to start all over again.
Depending on the climate of the area, the composition of the climax community is different. In the tropics, the climax community might be a tropical rainforest. At the other extreme, in the northern parts of the world, the climax community might be a coniferous forest. The natural state of the biome defines the climax community.
Figure 24.17
These ancient redwood trees are part of a climax community, the end result of a series of community replacements during succession.
Lesson Summary
Ecological succession is the continual replacement of one community by another that occurs after some disturbance of the ecosystem.
Primary succession occurs in disturbed areas that have no or little soil, while secondary succession occurs in disturbed areas that previously supported life.
Climax communities are the end product of succession, when the ecosystem is again stable.
Review Questions
What is the term for a continuous replacement of one community by another following a disturbance?
What type of succession occurs in areas where there is no soil?
What type of succession occurs in areas where soil is present?
What is the term for the final stage of succession, when the community becomes stable?
Imagine a forest fire destroyed a forest. The forest will slowly re-establish itself, which is an example of what kind of succession?
A glacier slowly melts, leaving bare rock behind it. As life starts establishing itself on the newly available land, what kind of succession is this?
Does the climax community look the same in all parts of the world?
Further Reading / Supplemental Links
http://www.scribd.com/doc/529104/Ecological-Succession
http://www.biologycorner.com/worksheets/succession.html
http://ecolibrary.cs.brandeis.edu/general_search.php?id=CS_Succession@Secondary%20succession&page=links
http://www.nature.com/news/2010/100523/full/news.2010.258.html
http://en.wikipedia.org/wiki
Vocabulary
climax communities
A stable community that is the end product of succession.
ecological succession
The continual replacement of one community by another that occurs after some disturbance of the ecosystem.
primary succession
Ecological succession that occurs in disturbed areas that have no or little soil, i.e. after a glacier retreats.
pioneer species
The species that first inhabit a disturbed area.
secondary succession
Ecological succession that occurs in disturbed areas that have soil to begin with, i.e. after a forest fire.
Points to Consider
Think about what would happen if dangerous toxins were illegally dumped near a river?
Discuss why it is important to seek alternative energy sources.
Do we have an infinite supply of fossil fuels, or can we run out some day?
Chapter 25: Environmental Problems
Lesson 25.1: Air Pollution
Lesson Objectives
Discuss the types of outdoor pollution and what causes them.
Describe the effects of outdoor pollution on the environment.
Discuss where indoor air pollutants come from and what they are.
Describe the health hazards of both indoor and outdoor pollutants.
Discuss how you can protect yourself from air pollution.
Check your Understanding
Describe the five layers of the Earth’s atmosphere (See Figure below).
Exosphere: from 300-600 mi up to 6,000 mi
Thermosphere: from 265,000 – 285,000 ft to 400+ mi
Mesosphere: from about 160,000 ft to the range of 265,000 – 285,000 ft
Stratosphere: from 23,000 – 60,000 ft range to about 160,000 ft; contains most of the ozone layer (with relatively high [a few parts per million] concentrations of ozone – the ozone layer is mainly located from approxim
ately 50,000 to 115,000 ft above Earth’s surface)
Troposphere: from the Earth’s surface to between 23,000 ft at the poles and 60,000 ft at the equator
Describe the chemical composition of the atmosphere.
Explain the significance of the atmosphere.
Figure 25.1
The layers of the atmosphere with altitude.
Introduction
Air is all around us and is everywhere and its mix of gases is essential for life. Despite the atmosphere’s vastness, human activities, like the emission of chemical substances, particulate matter (smoke and dust), and even biological materials, cause air pollution. This pollution affects entire ecosystems, worldwide. Pollution is also a big problem indoors. Pollution, both the outdoor and indoor varieties, cause many health problems as well as deaths. In spite of all the dangers to human health from pollutants, there are ways for you to protect yourself.
Pollution of Outdoor Air
Air is so easy to take for granted. In its unpolluted state, it cannot be seen, smelled, tasted, felt, or heard, except when it blows or during cloud formation. Yet its gases are very important for life: nitrogen helps build proteins and nucleic acids, oxygen helps to power life, carbon dioxide provides the carbon to build bodies, and water has many unique properties which most forms of life depend on.
Outdoor air pollution consists of either chemical, physical (e.g. particulate matter), or biological agents that modify the natural characteristics of the atmosphere and cause unwanted changes to the environment and to human health. Primary pollutants are added directly to the atmosphere by such processes as fires (Figure below) or combustion of fossil fuels (Figure below), such as oil, coal, or natural gas (Figure below). Secondary pollutants are formed when primary pollutants interact with sunlight, air, or each other. Both types are equally damaging.
Figure 25.2
Wildfires, either natural- or human-caused, release particulate matter into the air, one of the many causes of air pollution.
Figure 25.3
A major source of air pollution is the burning of fossil fuels from factories, power plants, and motor vehicles. Photo was taken prior to installation of emission controls equipment for removal of sulfur dioxide and particulate matter.
Figure 25.4
The majority of air pollutants can be found in the burning of fossil fuels for heat, electricity, industry, waste disposal, and transportation, the latter seen here on a busy highway.
Most air pollutants can be traced to the burning of fossil fuels. These include the burning of fuels in power plants to generate electricity, in factories to make machinery run, in stoves and furnaces for heating, in various modes of transportation, and in waste facilities to burn waste. Even before the use of fossil fuels since the Industrial Revolution, wood was burned for heat and cooking in fireplaces and campfires, and vegetation was burned for agriculture and land management.
In addition to the burning of fossil fuels, other sources of human-caused (anthropogenic) air pollution are agriculture, such as cattle ranching, fertilizers, herbicides and pesticides, and erosion; industry, such as production of solvents, plastics, refrigerants, and aerosols; nuclear power and defense; landfills; mining; and biological warfare.
Environmental Effects of Outdoor Air Pollution
Many outdoor air pollutants may impair the health of plants and animals (including humans). There are many specific problems caused by the burning of fossil fuels. For example, sulfur oxides from coal-fired power plants and nitrogen oxides from motor vehicle exhaust cause acid rain (Figure below) (precipitation or deposits with a low pH). This has adverse effects on forests, freshwater habitats, and soils, killing insects and aquatic life.
Figure 25.5
A forest in the Jizera Mountains of the Czech Republic shows effects attributed to acid rain. At higher altitudes, effects of acid rain on soils combines with increased precipitation and fog to directly affect foliage.
Global warming (an increase in the earth’s temperature) is thought to be caused mostly by the increase of greenhouse gases (water vapor, carbon dioxide, methane, ozone, chlorofluorocarbons (CFCs), nitrous oxide, hydrofluorocarbons, and perfluorocarbons) via the greenhouse effect (the atmosphere’s trapping of heat energy radiated from the Earth’s surface).
Water vapor causes about 36-70% of the greenhouse effect and carbon dioxide causes 9-26%. Fossil fuel burning has produced approximately three-quarters of the carbon dioxide from human activity over the past 20 years, while most of the rest is due to land-use change, particularly deforestation (Figure below). Methane causes 4-9% of the greenhouse effect and ozone causes 3-7%. Some other naturally occurring gases contribute very little to the greenhouse effect; one of these, nitrous oxide, is increasing in concentration due to an increase in such human activities as agriculture.
Figure 25.6
Deforestation, shown here as a result of burning for agriculture in southern Mexico, has resulted in a significant increase in atmospheric carbon dioxide over the past 20 years.
The effect of global warming is to increase the average temperature of the Earth’s near-surface air and oceans. This increase in global temperature will cause the sea level to rise and is expected to cause an increase in intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, and species extinctions.
Other environmental problems caused by human-caused air pollution include global dimming (a reduction in the amount of radiation reaching the Earth’s surface) and ozone depletion (the latter being two related declines in stratospheric ozone). Particulate matter from the burning of wood and coal and aerosols (airborne solid particles or liquid droplets) cause global dimming, by absorbing solar energy and reflecting sunlight back into space. Environmental effects of global dimming include less photosynthesis, resulting in less food for all trophic levels; less energy to drive evaporation and the hydrologic cycle; and cooler ocean temperatures, which may lead to changes in rainfall and drought.
Ozone is both a benefit and detriment. As a component of the upper atmosphere, it has shielded all life from as much as 97-99% of the lethal solar ultraviolet (UV) radiation. However, as a ground-level product of the interaction between pollutants and sunlight, ozone itself is considered a pollutant which is toxic to animals’ respiratory systems.
Ozone depletion consists of both losses in the total amount of ozone in the Earth’s stratosphere – about 4% per year from 1980 to 2001, and the much larger loss, the ozone hole, a seasonal decline over Antarctica. A secondary effect of ozone depletion is a decline in stratospheric temperatures. The pollutants that are responsible for ozone depletion are CFCs, from the use of aerosol sprays, refrigerants (Freon), cleaning solvents, and fire extinguishers.
Ozone depletion and the resulting increase in levels of UV radiation reaching Earth could result in the reduced abundance of UV-sensitive nitrogen-fixing bacteria, which cause a disruption of nitrogen cycles, and a loss of plankton, causing a disruption of ocean food chains.
Pollution of Indoor Air
Lack of indoor ventilation and circulation concentrates air pollution in places where people often spend a majority of their time, and allows them to accumulate more than they would otherwise occur in nature. Some of these indoor pollutants include radon gas, released from the Earth in certain locations and then trapped inside buildings; formaldehyde gas, emitted from building materials, such as carpeting and plywood; volatile organic compounds (VOCs) are given off by paint and solvents as they dry; and lead paint, which can degenerate into dust.
Other air pollutants are caused by the use of air fresheners, incense, and other scented items. Wood fires in stoves and fireplaces can produce significant amounts of smoke particulates into the air. Use of pesticides and other chemical sprays indoors, without proper ventilation, can be another source of indoor pollution.
Carbon monoxide (CO) is often released by fa
ulty vents and chimneys, poorly adjusted pilot lights, or by the burning of charcoal indoors. Flaws (non-functioning built-in traps) in domestic plumbing can result in emission of sewer gas and hydrogen sulfide. Dry cleaning fluids, such as tetrachloroethylene, can be emitted from clothing, days after dry cleaning. The extensive use of asbestos in industrial and domestic environments in the past has left a potentially very dangerous material in many localities (Figure below).
Figure 25.7
The extensive use of asbestos in industrial (as pictured here, asbestos-covered pipes in an oil-refining plant) and domestic environments in the past has left a potentially very dangerous material in many localities.
Biological sources of air pollution, such as gases and airborne particulates, are also found indoors. These are produced from pet dander; dust from minute skin flakes and decomposed hair; dust mites (which produce enzymes and micrometer-sized fecal droppings) from bedding, carpeting, and furniture; methane from the inhabitants; mold (which generates mycotoxins and spores) from walls, ceilings, and other structures; air conditioning systems, can incubate certain bacteria and mold; and pollen, dust, and mold from houseplants, soil, and surrounding gardens.
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