Table of Contents

The Biosphere | Connectivity of the Spheres | Biogeochemical Cycles | Disruption of the Biosphere

Mass Extinctions | How to Reduce Species Extinction and Destruction of the Biosphere

Learning Objectives | Terms | Review Questions | Links

The Biosphere | Back to Top

The biosphere is the sum of all living matter on the Earth. Highly specialized organisms have adapted to the extreme boundaries of the uppermost atmosphere and lowermost ocean depths. The biosphere is interconnected with three other spheres of the physical environment: the lithosphere, the hydrosphere, and the atmosphere.

The lithosphere is the solid outer layer of the Earth's crust, including rocks, sand, and soil. The Earth's lithosphere is a dynamic area, with processes such as erosion, earthquakes, and plate tectonics (shown in Figure 1) constantly altering and forming/destroying the surface. The tectonic cycle describes the formation of new crust in some areas and its destruction in others.

Plate tectonics is the geological theory that proposes the lithosphere is composed of tectonic plates that are in constant motion relative to each other. New oceanic crust forms at divergent plate boundaries, which are expressed on the Earth's surface as midocean ridges (undersea) or rift valleys (on land). The mid-Atlantic Ridge and the East African Rift are examples of these features. Old crust is subducted and remelted at convergent plate boundaries, which are expressed on the Earth's surface as deep ocean trenches. The Marianas Trench and other trenches are examples of this type of boundary. The plates essentially "float" on a semi-molten lower crustal layer. Plate motion rates range between 2 and 15 centimeters per year. Tectonic processes have formed new continents, mountain ranges, and moved existing continents across the surface of the Earth, causing the evolution and extinction of species from resulting climate and environmental changes.

Figure 1. Types of plate tectonic boundaries. Image from; This Dynamic Earth, a website of the U.S. Geological Survey.

The hydrosphere includes all water at or near the Earth's surface. Water is very important to a number of biological and geological processes. The hydrologic cycle is the continuous recycling of water between the oceans and atmosphere. Evaporation is the movement of water from an ocean or a lake to the atmosphere. Transpiration by plants contributes to atmospheric water content. Precipitation (rainfall or snow) is the movement of water from the atmosphere to the land or ocean. Water on land can be either surface runoff or filter through soil to become groundwater.

The atmosphere is the envelope of gas that surrounds the Earth. Nitrogen and oxygen compose 99% of the modern atmosphere. The atmosphere becomes progressively thinner with increasing altitude.

Connectivity of the Spheres | Back to Top

The spheres are connected and relate to each other at a variety of interfaces. The geological process of volcanism transfers water from the lithosphere to the atmosphere, where it precipitates into the hydrosphere. The interface between water and air likewise allows transfer of material from one sphere to another.

Biogeochemical Cycles | Back to Top

More than thirty chemical elements are cycled through the environment by biogeochemical cycles. There are six important biogeochemical cycles that transport carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorous. Recall that these six elements comprise the bulk of atoms in living things. Carbon, the most abundant element in the human body, is not the most common element in the crust, silicon is.

The Hydrologic (Water) Cycle

Saltwater evaporates from sun's energy producing fresh water in clouds, leaving salts in the ocean. Water vapor cools and condenses to precipitation over oceans and land. Runoff forms freshwater lakes, streams, ponds, groundwater, and is held in plants and transpired. Some water infiltrates the ground, becoming part of the groundwater, returning very slowly to the oceans. Although the water cycle, Figure 2, shows water to be a renewable resource, only about 3% of that water is fresh and suitable for human use. Water may be polluted or inadequate for human populations concentrated in specific areas.

Figure 2. The hydrologic (water) cycle. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.

Cycling processes for other elements involve:

The Phosphorus Cycle

Weathering of rocks makes phosphate ions (PO4= and HPO4=) available to plants through uptake from the soil. The mineral apatite contains a small amount of phosphorous, although this is enough for all living things to utilize. Runoff returns phosphates to aquatic systems and sediment. Organisms use phosphate in phospholipids, ATP, teeth, bones, and shells. Phosphate is a limiting nutrient because most of it is being currently used in organisms. The inorganic source, apatite, is a rare mineral, further limiting the input of this essential nutrient. A specimen of this mineral is shown in Figure 3.

Figure 3. Apatite, the chief source of phosphate for living systems via the phosphorous cycle. Image from

Humans mine phosphate ores for use in fertilizer, as an animal feed supplement, and for detergents. Detergents, untreated human and animal wastes, and fertilizers from cropland add excess phosphate to water often causing population explosions (algal blooms) in lakes. The phosphate cycle is shown in Figure 4.

Figure 4. The phosphorous cycle. Image from

The Nitrogen Cycle

Atmospheric nitrogen gas (N2), the major portion of our modern atmosphere, is unfortunately in a form that is bnot usable by plants and most other organisms. Plants therefore depend on various types of nitrogen-fixing bacteria to take up nitrogen gas and make it available to them as some form of organic nitrogen. Nitrogen fixation occurs when nitrogen gas is chemically reduced and nitrogen is added to organic compounds. Atmospheric nitrogen is converted to ammonium (NH4+) by some cyanobacteria in aquatic ecosystems and by nitrogen-fixing bacteria in the nodules on roots of legume (beans, peas, clover, etc.) plants in terrestrial ecosystems. Plants take up both NH4+ and nitrate (NO3-) from soil. The nitrate (NO3-) is enzymatically reduced to ammonium (NH4+) and used in the production of both amino acids and nucleic acids. The nitrogen cycle is shown in Figure 5.

Figure 5. The nitrogen cycle. Image from

Nitrification is the inorganic production of nitrates. Nitrogen gas (N2) is converted to nitrate (NO3-) by cosmic radiation, meteor trails, and lightning in the atmosphere. Human technology can now manufacture nitrates for use in fertilizers. In soil, bacteria convert ammonium (NH4+) to nitrate (NO3-) by a two-step process shown in Figure 6. Nitrite-producing bacteria convert ammonia to nitrite (NO2-). Next, nitrate-producing bacteria convert nitrite to nitrate. These two groups of bacteria are called nitrifying bacteria.

Denitrification is conversion of nitrate to nitrous oxide and nitrogen gas back to atmosphere. This is done by denitrifying bacteria in both aquatic and terrestrial ecosystems. The process of denitrification almost, but not completely, counterbalances nitrogen fixation.

Figure 6. Development of a root nodule, a place in the roots of certain plants, most notably legumes (the pea family), where bacteria live symbiotically with the plant. This is one route by which inorganic nitrogen (N2) is converted into organic nitrogen that can then be used by other organisms. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.

The Carbon Cycle

There is a relationship between the two major metabolic processes of photosynthesis and cellular respiration. Cellular respiration releases carbon dioxide, which is used as a raw material in photosynthesis. Photosynthesis in turn releases oxygen used in respiration. Animals and other heterotrophs depend on green organisms for organic food, energy, and oxygen. In the carbon cycle, organisms exchange carbon dioxide with the atmosphere. On land, plants take up carbon dioxide via photosynthesis and incorporate it into food used by themselves and heterotrophs. When organisms respire, some of this carbon is returned to the atmosphere in the molecules of carbon dioxide. In aquatic ecosystems, carbon dioxide from air combines with water to give carbonic acid, which breaks down to bicarbonate ions. Bicarbonate ions are a source of carbon for algae. When aquatic organisms respire, they release carbon dioxide that becomes bicarbonate (HCO3). The amount of bicarbonate in water is in equilibrium with amount of carbon dioxide in air.

Living and dead organisms are reservoirs of carbon in carbon cycle, shown in Figure 7. More than 800 billion tons of carbon are in the world's biota, mainly in cells of trees. An additional 1,000 to 3,000 billion tons of carbon occurs in plant and animal remains in the soil. Fossil fuels, such as coal, petroleum, and natural gas, were formed during various times of the geologic past when exceptional amount of organic matter were rapidly buried in an environment that locally lacked biologic activity. Inorganic calcium carbonate (the minerals calcite and aragonite, CaCO3) accumulates in limestone and the calcite of shells.

Human burning of fossil fuels and wood has increased the amount of carbon dioxide released into atmosphere to 42 billion metric tons in only 22 years. Since human activity in 22 years probably released 78 billion metric tons, 36 billion metric tons were probably absorbed in oceans. Increased carbon dioxide levels may increase the greenhouse effect, where such gases allow the Sun's radiant energy to pass through to Earth where it is absorbed and reradiated as heat. Instead of radiating fromthe earth back into space, this heat is then trapped on Earth, perhaps causing or contributing to global warming.

Figure 7. The global carbon cycle. Image from

Excess output occurs when biomass is suddenly released, such as in slash and burn agriculture. Excess input commonly occurs from agricultural increase in organic fertilizer and other nutrients into an ecosystem. Generally speaking, the faster matter cycles, the faster it can recover from human intervention.

Recently, speculation has centered on the interconnection of biogeochemical cycles and their possible roles in a "superorganism" that James Lovelock has called Gaia, after the ancient Greek Earth goddess. While an intriguing notion, the degree of organization of these cycles is not as complex or integrated as the cells in a body. Gaia remains controversial hypothesis that continues to generate speculation and research both pro and con.

Disruption of the Biosphere | Back to Top

Human technology and population growth can directly and indirectly disturb the biosphere. They key question now is: can humans cause global climate change? The human population has experienced phenomenal exponential growth since the Industrial Revolution. Modern agriculture and medicine have increased growth rates for our population, resulting in over 90 million people added each year. United Nations estimates indicate a human population of 10 billion may exist by the end of the twenty-first century.

Human populations are in a growth phase, shown in Figure 8. Since evolving around 200,000 years ago, our species has proliferated and spread over the Earth. Beginning in 1650, the slow population increases of our species exponentially increased. New technologies for hunting and farming have facilitated this expansion. It took 1800 years to reach a total population of 1 billion, but only 130 years to reach 2 billion, and a mere 45 years to reach 4 billion.

Figure 8. Human population growth over the past 10,000 years. Note the effects of worldwide disease (the Black Death) and technological advances on the population size. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.

Two Classes of Countries

The developed countries are those that industrialized first, such as the United States, Canada, Japan, Russia, Australia, New Zealand, and all of Europe. These countries have twenty-two percent of the world's population, but consume nearly eighty percent of the energy and resources. Waste and pollution are also greater in proportion to use of resources.

The developing countries have seventy-eight percent of the population, but use only twenty percent of the Earth's resources. These countries include most of Africa, South America, India, and China. Most people in these coluntries have a lifestyle far below that of the inhabitants of the developed countries. As developing contries expand their economies, their people will strive to achieve the lifestyle of consumption and waste that we in developed countries take for granted. For instance, in China the bicycle remains the method of transportation for most people, but more and more motorcycles and cars are owned by the increasing affluent entrepenuers and government officials. When I visited the faculty and students at Wuyi University (China) in 1998, only a few of the faculty had motor bikes. A colleague of mine visited the same university in the summer of 2002 and reported that almost all faculty had motor bikes.

Industrialization is driven by energy consumption from coal, petroleum, and natural gas, otherwise known as fossil fuels. Fossil fuels formed by decomposition and alteration of the remains of plants, protists, and animals over a time span of several million years. These sources are considered nonrenewable since they are in limited supply in terms of human-scale time. Oil (petroleum) is the fuel most widely used, both as starting material for making gasoline and for other products. Historically coal was the first fossil fuel used in the European/American Industrial Revolution of the 1800s. Petroleum soon replaced coal, and has remained the mainstay of the energy fix needed by industrialized societies.

Pollution is any environmental change that adversely affects the lives and health of living things. Burning fossil fuels results in hydrocarbons, carbon, nitrogen and sulfur oxides, and particulates. Automobiles consume one-third of the world's production of oil and are the chief source of air pollution. Some scientists estimate there will be four times more automobiles in the world by 2025. Industrially produced halogens and the use of nitrogen fertilizers also influence the atmosphere in a negative manner.

Sources of energy that are relatively nonpolluting exist. However, these nonpolluting energy sources are not as commonly used as fossil fuels. Solar energy does not add additional heat to the atmosphere, although the production of solar panels may contribute to pollution. Solar panels, often mounted on rooftops, absorb and move rooftop heat into water circulating within the panels. Photovoltaic (solar) cells produce energy directly from sunlight, although the cost of this energy is several times what conventional (fossil) fuel-generated electricity costs.

Falling water is used to produce electricity in hydroelectric dams, asshown in Figure 9. However, most dams would not be compliant with newer, more strict environmental regulations. The damming of rivers to generate power (and hold water for irrigation and other purposes) can lead to local extinctions or declines in native fish populations. Water that id dammed up no longer freely flows, placing wetland habitats downstream from the dam at risk. The numerous dams built along the Salt and Verde Rivers in Phoenx, Arizona (USA) have led to a reduction of downstream biodiversity as once perenniel streams now flow only seasonally. Construction of the Glen Canyon Dam on the Colorado River in northern Arizona (shown in Figure 10) has led to vegetaion and other changes inside Grand Canyon Nartional Park. The Three Gorges Dam being currently built along the banks of the Yangtze River in China will displace over two million people, flood areas that have been inhabited for over 10,000 years, as well as disrupt the breeding patterns of the Yangtze sturgeon. When complete, this dam will generate enough power every day to light a city the size of Los Angeles, adding new power to the Chinese economic expansion.

Figure 9. Schematic view of a hydroelectric dam. Image from

Figure 10. Glen Canyon Dam, near Page Arizona, went online in the early 1960s. Water falling through the dam runs turbines, generating electrical power. Images from

Geothermal energy is derived from heat in the earth's core heats water to heat buildings and generate electricity. This source of energy is usually not located near enough to population centers to be economically feasible to exploit. One notable exception is in Iceland, which sits astride the midAtlantic Ridge, where the capitol city is heated exclusively by geothermal power.

Wind power can pump water or generate electricity. Windmills have for long times been used for water pumping or t he grinding of corn or other grains. One problem with wind power is that in most p;aces the winds do not blow at the same velocity all year round, and the power generated when the wind does blow is not effectively stored in batteries. That aside, there are several places in Oklahoma and california where wind power is cost-effective.

Solar, wind, hydro, and geothermal power are all considered renewable sources of energy since they are replenished by physical means. However, none of them have replaced fossil fuel in widespread use in the industrialized countries. Generally, European countries make better use of these forms of energy than does the United States.

Atmospheric Pollution

The one percent of the atmosphere (trace gases) that is neither nitrogen or oxygen, plays an important role in global climate and in shielding the Earth's surface from solar radiation. Agricultural and industrial gases may affect the atmosphere's ability to protect as well as alter the world's climate.

Carbon dioxide has many sources (cellular respiration, and the burning of wood or fossil fuels such as coal or petroleum). There are two main sinks for carbon dioxide: plants and the oceans. Plants convert carbon dioxide into organic molecules by photosynthesis. Oceans form calcium carbonate and over long periods of time, store it as limestone. Since the Industrial Revolution, carbon dioxide levels in the atmosphere have increased. This increase has rapidly accelerated during the past forty years. Both trends are shown in Figure 11.

Figure 11. Top image: Increase in carbon dioxide levels between 1960 and 1990. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission. Bottom image: Increase in the emissions of carbon during the interval from 1860-1995. Image from Images of Biology.

Carbon dioxide and other gases allow light to pass, but trap reradiated heat from the Earth in the atmosphere much like glass in a greenhouse traps heat. Increased carbon dioxide levels lead to more and more heat trapped. This greenhouse effect is thought to be responsible for global warming, a phenomenon that has been going on for the past 10,000 years (Figure 12), but which seems to have accelerated during the past 150 years. Carbon dioxide is not the only gas that can cause a greenhouse effect. Carbon dioxide contributes to only 56% of greenhouse heating. Methane (CH4) is expelled in great quantities by cows, and as cattle production has increased so has their methane production (at a rate of about 1% per year).

Temperatures, Greenhouses, and Climate Changes

The average temperature of the Earth has risen by 0.5 degrees C over the past one hundred years. Although a long-term rise of two degrees would seem minor, this is thought sufficient to completely melt the glacial ice caps in Antarctica and Greenland, causing global sea-levels to rise 100 meters. This can alter climate patterns such as rainfall, ocean currents, and climate zones. Climate changes can have biological (such as causing migrations) as well as geopolitical and economic consequences.

Earth's climate fluctuates on both short-term and long-term time scales. There have been periods of Earth history with higher average annual temperatures than we have today, as well as the reverse condition, when glaciers covered extensive expanses of the Northern and/or Southern hemispheres. We are currently between ice ages, the last of which ended nearly 10,000 years ago. Climate fluctuations have left evidence in the distribution of fossils, living forms and their close relatives, and locations of certain types of sedimentary rocks. Studies of fossils and the sedimentary rocks they occur in have led to estimates of temperature. Paleoecology is the branch of science that deals with such data in an attempt to reconstruct the evironments of the distant (and not so distant) past.

Figure 12. Fluctuations in carbon dioxide concentration related to ice ages (glacials) and warm periods (interglacials). Image courtesy of Prof. Dennis L. Hartmann, Department of Atmospheric Sciences, University of Washington,, used by permission.

Some scientists are concerned that global climate will warm at a rate ten times faster than it has in the past. Composition of the minor components of the atmosphere, such as carbon dioxide, provide clues about thepossible rapidity of climatic changes. In 1850, atmospheric carbon dioxide was approximately 280 parts per million (ppm). Today, it is about 350 ppm. This increase is due largely to burning of fossil fuels and clearing of forests. Oceans (and photosynthetic organisms) currently absorb half of the carbon dioxide emitted. Methane is another atmospheric pollutant produced by by oil and gas wells, rice paddies, cows, etc. This gas is increasing by one percent per year.

The greenhouse effect, shown in Figure 13, is a warming of the lower atmosphere caused by accumulation of certain greenhouse gases (notably carbon dioxide and methane) that allow rays of the sun to pass through, but then reflect or reradiate heat to the Earth. In this way heat is trapped on Earth much the same way heat is trapped behind the glass panels of a greenhouse.

Figure 13. Diagram of the Earth's heat budget and the effects oif greenhoiuse gases in causing the greenhouse effect and global warming. Image courtesy of Prof. Dennis L. Hartmann, Department of Atmospheric Sciences, University of Washington,, used by permission.

Greenhouse gases include a diverse variety of atmospheric gases, as shown in Figure 14. Carbon dioxide is a product of burning fossil fuel and wood. Nitrous oxide (NO2), produced by fertilizer use and released from decomposition of animal wastes is another prominent greenhouse gas. Methane (CH4) is produced by bacteria (especially in animal intestines), sediments, swamps, certain types of landfills, and in flooded rice paddies. In some cases methane can be collected and used to generate a small amount of electricity. Chlorofluorocarbons (CFCs), in particular Freon (a refrigerant) are greenhouse gases thought responsible to depletion of the planetary ozone layer in the upper atmosphere. Halons, such as halocarbons; CxFxBrx), are released from fire extinguishers. Water vapor is a greenhouse gas since clouds reradiate heat back to Earth.

Figure 14. Increasing concentrations of various greenhouse gases over the past 200 years. Note the vertical scale changes to parts per billion (ppb) for the nitrogen oxides and the CFC-11 graphs. The CFC graph begins in the 1950s since this type of pollutant did not exist before that date. Images from Images of Biology (out of print).

Analysis of gas trapped in Arctic ice (and glaciers in other parts of the world) shows earth's temperature fluctuated in the past, according to the levels of carbon dioxide and methane.

Greenhouse gases differ in their ability to absorb specific wavelengths of infrared radiation. The heat retention capacity of methane is about 25 times greater than that of carbon dioxide. Nitrous oxide is about 200 times more effective than carbon dioxide. The global climate appears to have risen about 0.5 C since the Industrial Revolution. With that revolution came an increase in the use of fossil fuels, as well as production of certain greenhouse gases that were extremely rare before industrialization. Some computer models predict a rise of from 1.5 degrees C to 4.5 degrees C by 2060.

The ecological effects of such a sudden rise in global temperature would be noticeable. From studies of fossils we can estimate how long it would naturally take for such rises in temerature. The effects of human activity on the atmosphere will accelerate this rise from a span of centuries to just a few decades. As oceans warm, temperatures in polar regions would likely rise to a greater degree than other areas. Glaciers would melt and sea levels would rise due to melting and expansion of warm water. Water evaporation would increase with increased rainfall along coasts and dry conditions inland. Droughts would reduce agricultural productivity and trees would die off. Expansion of forests into Arctic areas would not likely offset loss of forests in temperate zones. Coastal agricultural lands and deltas in Bangladesh, India, and China would be inundated. In China two-thirds of the 1.2 billion people live in low-lying coastal areas. Many of these areas would be flooded by rising sea levels.

The Ozone Layer

Earth's atmosphere conswists of a number of different layers. The troposphere is the lower atmospheric layer. It surrounds us at ground level. The stratosphere is often referred to as the upper atmosphere. The stratosphere contains the ozone shield, a layer of ozone (O3) in the stratosphere, 50 km above the ground. Ozone is produced in the upper atmosphere when sunlight strikes oxygen atoms and causes them to temporarily combine. Stratospheric ozone helps filter most of the high-energy ultraviolet radiation that causes cancer and mutations. The development of atmospheric ozone shield is one of the necessary events in the history of life that permitted life to exist on land. Aquatic organisms, including all known early life forms, are shielded by water. Known hazards of UV radiation include increased mutation rate, which can lead to skin cancer and cataracts, depression of the immune system, impaired crop and tree growth, and the death of plankton. Each 1% drop in ozone is thought to increase human skin cancer rates by 4-6%. The United Nations Environment Program predicts a 26 percent rise in cataracts and nonmelanoma skin cancers for every 10% drop in ozone. This translates to 1.75 million cases of cataracts and 300,000 more cases of skin cancer every year. The incidences of certain skin cancers is shown in Figure 15.

Figure 15. Incidence of skin cancer (black) and squamous cancer (gray) for selected cities in the United States during July 1992. Image modified by M.J. Farabee from data obtained from NASA.

During the 1980s scientists discovered a "hole" in the ozone over Antarctica, and that some depletion of worldwide ozone had taken place. By the 1990s atmospheric scientists had detected an annual loss of 40-50% of the ozone above Antarctica, which produced an ozone hole every spring. The development of ozone holes depends on complex atmospheric conditions.

Ozone is being destroyed by the release of gases, such as chlorofluorocarbons (CFCs), containing chlorine (Cl-) atoms in the stratosphere. CFCs are used in refrigerators, air conditioners, and solvents. Chlorine atoms come from breakdown of CFCs, which were in heavy human use from 1950 to 1990. One CFC molecule can destroy 100,000 ozone molecules. International agreements were developed to phase out the use of CFCs by the year 2000. However, since it takes 20-30 years for CFCs to rise to the upper atmosphere, and another 100 years for their destruction, ozone destruction will continue for some time to come.

Acid Deposition

Humans also alter their local atmosphere by pollution and acid rain. Burning of fossil fuels releases carbon dioxide and nitrogen and sulfur oxides. Sulfur combines with atmospheric water vapor to form sulfuric acid. Forests and lakes suffer from the pH and soil acidity changes resulting from acid rain. Changes in the pH of precipitation between 1955 and 1980 are shown in Figure 16.

Figure 16. Increases in acid precipitation between 1955 and 1980. Numbers are pH values. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (, used with permission.

Pure water has a pH of 7, neutral on the pH scale. In such a solurtion, the dissociation of H2O produces equal numbers of H+ and OH- ions. Atmospheric CO2 combines with water to produce a weak carbonic acid (H2CO3) and an increased number of H+ ions. Rainwater normally has a pH of 5.6 rather than 7.0. However, near industrialized or urban areas, rainfall pH is nearer 4.0 and some fog clouds drop to a pH as acidic as 1.7. Living vegetation and limestone used for monuments and buildings rapidly deteriorate under such "acid rains."

Coal and oil routinely burned by power plants emit sulfur dioxide (SO2) into the air. Oil from Kuwait has a naturally high sulfur content. Oil well fires, some set on purpose during the Gulf War of the early 1990s, released much sulfur dioxide into the atmosphere. Most of the commercially exploitable coals in the US have high sulfur content. Automobile exhaust contributes nitrogen oxides to the air. Both sulfur dioxide and nitrogen oxides are converted to acids when they combine with water vapor in the air. Sulfur and nitrogen oxides are emitted in one locale while deposition occurs in another location across boundaries.

Acid deposition is responsible for the following:


Smog is an urban problem caused by combustion of fuels. Pollutants react with sunlight to cause more than 100 secondary pollutants that can cause respiratory problems (asthma and such) in humans. Most problems with acid rain and smog are caused by the use of fossil fuels. The U.S. gets 90% of its energy from fossil fuels, more than half of which is wasted. Conservation measures, more fuel efficient vehicles, mass transit, and alternative energy sources are possible measures to be taken. World supplies of petroleum are estimated to run out in 50-100 years.

Photochemical smog is air pollution that contains nitrogen oxides (NOx) [where the x is a 2 or 3] and hydrocarbons (HC), that react together in the presence of sunlight to produce ozone (O3) and peroxylacetyl nitrate (PAN). Both NOx and hydrocarbons result from the burning (or combustion) of fossil fuel. Additional hydrocarbons come from various other sources as well, including paint solvents and pesticides.

Breathing O3 affects both the respiratory and nervous systems, resulting in respiratory distress, headache, and exhaustion. Ozone is damaging to plants, resulting in leaf mottling and reduced growth.

Carbon monoxide (CO) is a gas that comes from burning of fossil fuels in the industrial regions. High levels of CO increase the formation of ozone (O3). CO combines preferentially with hemoglobin and prevents hemoglobin from carrying oxygen. The amount of CO over the Southern Hemisphere, produced by the burning of tropical forests, is equal to that over the Northern Hemisphere, produced by industrial activity..

Thermal inversions are local occurrences of polluted air being trapped close to the surface. This is a major problem in cities located in a valley, like the Phoenix (Arizona) metropolitan area. Warm air near the ground usually rises and dissipates into the upper atmosphere. Air pollutants, including smog and soot, can be trapped near ground due to a thermal inversion. A thermal inversion occurs when a layer of dense cold air is trapped under a layer of warm air. Areas around hills are susceptible because air stagnates, with little mixing. At certain times of the year, usually in winter, thermal inversions in Phoenix can cause difficulty in breating for some individuals who has asthma and other respiratory diseases.

Water Pollution Is Altering the Hydrosphere

Air pollutants will eventually precipitate into the hydrosphere. During cooler periods, ice acts as a water reservoir, forming glaciers that lower sea-level and affect climate. Global warming will melt this reservoir, raising sea level (or sinking coastlines). The hydrosphere can be directly altered by water polluted by human wastes. There are three basic sources of water pollutants: municipal sewage, industrial discharges, and agriculture/mining/logging discharges of sediment.

Freshwater is required for domestic purposes, including drinking, crop irrigation, industrial use, and energy production. Freshwater resources include surface water from lakes and rivers, and underground aquifers. Pollution contributes to the shortage of freshwater. Solid wastes include household trash, sewage sludge, agricultural residue, mining refuse, and industrial waste. Pollution comes from either a point source, an identifiable source of the pollution, or a nonpoint source, a broad area of pollution with no single idetifiable source, such as runoff. The United States spends more than $9 billion a year on cleanup (including the Superfund) compared to a mere $200 million per year to prevent pollution. Recycling may save industry money, extend the life of increasingly scarce resources, and prevent or lessen pollution.

Sewage treatment plants degrade organic wastes, which would otherwise cause oxygen depletion in lakes. Human feces contain pathogens (viruses, bacteria) that cause cholera, typhoid fever, and dysentery. Sewage treatment plants use bacteria to break down organic matter into inorganic nutrients. The treated water can then be used for various purposes, depending on state and local laws governing sewage treated water. During the spring of 2000, Los Angeles made national news because of the city's plans to reintroduce treated sewage water to the home water supply.

Agricultural and industrial wastes present a number of water pollution problems. Intensive animal farming or the presence of many septic tanks releases ammonium (NH4+) from wastes. This ammonium is converted by bacteria to soluble nitrate that moves through the soil to water supplies. Between 5-10% of all wells in the United States have nitrate levels higher than the recommended maximum. Industrial wastes include heavy metals and organochlorides, such as some pesticides. These are not degraded in nature or in normal sewage treatment, and accumulate in deltas. When these wastes enter water, they are subject to biological magnification. Decomposers are unable to break down these wastes, and they are not excreted. The molecules, therefore, remain in tissues and are passed up the food chain to the next consumer. They become more concentrated at each level in the food chain. Since aquatic food chains have more links, biological magnification is greater. Where humans are the final consumers, human milk can contain detectable amounts of DDT and PCBs.

Aquifer pollution is an increasingly significant health threat. Prior to environmental regulations enacted during the 1970s, many industries ran wastewater into a pit from which pollutants could seep into the ground Much of this material eventually made its way to the groundwater in various aquifers below the surface. Wastewater and chemical wastes were also injected into deep wells. Both practices are being phased out; there are few alternatives for industry to dispose of wastes, other than to reduce the volume or develop a long-term containment facility.

The oceans are the final recipients of wastes deposited in rivers and along the coasts. Waste dumping occurs at sea, and ocean currents sometimes transport both trash and pollutants back to shore. Solid pollutants cause death of birds, fish, and marine mammals that mistake them for food and get entangled. Offshore mining and shipping add pollutants to the oceans. Five million metric tons of oil a year, over one gram per 100 square meters of ocean surface, ends up in oceans. Large oil spills kill plankton, fish larvae, and shellfishes, as well as birds and marine mammals. The Exxon Valdez oil spill in Alaska's Prince William Sound leaked 44 million liters of crude oil. During the Gulf War, 120 million liters were released from damaged onshore storage tanks in the Persian Gulf. Petroleum is biodegradable; takes a long time because low-nutrient content does not support bacteria. Some species of fish are in dramatic decline from combined effects of pollution and overfishing.

Desertification and Deforestation

In 1950, 20 % of the world's population lived in cities. Predictions of this number rising to 60% by 2000 have been made. The trend of growth of urban areas began with the Industrial revolution of the 1800s, but greatly accelerated after World War II. Buidling new housing around new (or enlarging) cities, removes land from agricultural uses. Change of land use can alter heat distribution patterns and surface water runoff. Expanded urbanization also degrades the environment. Estrella Mountain Commmunity College, my college, was built in 1991 (opened in 1992) in an old cotton field. When we opened our campus the nearest houses were over one mile away. New houses and development are now surrounding our once isolated campus shown in Figure 17.

Figure 17. Estrella Mountain Community College as it appeared in May 1996. The section of land on the eastern border of the photograph (right side) is currently (6/2001) being developed into tract houses. Image from the USGS terrabyte server,

In agricultural areas, wind and rain carry away about 25 billion tons of top soil yearly, worldwide. At such a rate, it is estimated that practically all top soil will be lost by the middle of the next century. Soil erosion causes a loss of productivity; it is compensated for by fertilizers, pesticides, and fossil fuel energy. One solution is to employ strip-cropping and contour farming to control soil erosion. Desertification is transformation of marginal lands to desert conditions due to overgrazing and overfarming. This is most evident along the southern edge of the Sahara Desert in Africa. Over 240,000 square miles of once-productive grazing land has become desert in the last fifty years. A similar process can occur if U.S. rangeland is overgrazed. The Dust Bowl, shown in Figure 18, was an area of the southern Great Plains that experienced several years of drought, coupled with poor farming techniques and economic hard times. Was the area becoming a desert before soil conservation and improved farming methods halted the trend?

Figure 18. Image of the dust bowl. Image from, original source The US National Archives.

Canada has seen immense stands of trees cut down for paper, wood products and particleboard. Tropical rain forests are more biologically diverse than temperate forests. U.S. temperate forests contain about 400 species of trees; a typical 10-hectare rain forest contains 750 species. The loss of U.S. forests is shown in Figure 19. South American streams contain about twice the species found in all of the U.S. and Canada. A N.A.S. study estimates a million species are in danger of disappearing in 20 years due to deforestation. Lost species that have never been studied may have been sources of food or medicine. Logging in tropical forests meets the demand for furniture and also the desire of local people to farm the land. Slash-and-burn agriculture also contributes to the destruction of tropical rain forests. The ashes provide temporary nutrients to raise crops. After a few years, the fertility of the land is insufficient to raise crops and farmers move on. Cattle ranching usually takes over from farming. Pig-iron industry in Brazil also requires wood charcoal to smelt the pig iron.

Figure 19. Deforestation of North America between 1620 and 1926. Forested areas are shown as black or gray. Image from, originally from National Geography Standards.

Mass Extinctions | Back to Top

Extinctions occur when environments change too fast. Local extinctions can occur, as can mass extinctions: they differ in scale, scope and the numbers of species involved. There have been five environmental changes (mostly cooling) of global proportions that resulted in the five mass extinctions in Earth history. Recovery from these extinctions took millions of years. A spectacular exception was the large meteorite strike sixty-six million years ago near the Yucatán peninsula in Mexico that either caused the extinction of dinosaurs and 75% of all marine species, or was the nail in the coffin of the dinosaurs.

There have been several natural mass extinctions in the history of earth followed by recovery. Human activities that reduce biodiversity began about 30,000 years ago with development of social and language skills to apply increasingly better stone tool technology to trap and kill the largher animals. Hunting contributes to the estimated extinction of 15,000 to 30,000 species a year. Fish stocks are being depleted by overfishing. Commercial trade causes exploitation of tigers, cheetahs, leopards, jaguars, etc. for furs; sharks for fins; rhinoceros for rhino horn powder; elephants for tusks for ivory; and cacti for gardeners.

A major cause of extinction is the loss of habitat to support a species. The habitat for a species may be totally destroyed through natural events or human activities. Habitats may be fragmented into small pieces that cannot support the population. By 2010, very little undisturbed rain forest will exist outside of national parks.

Accidental or purposeful introduction of new species can cause extinction of endemic species. Introduction of brown tree snake to Guam has resulted in extinction of nine of eleven native bird species. The carp, an Asian fish that can tolerate polluted waters, is now more prevalent than our native fish. Kudzu, a plant native to Asia, has beome a major pest throughout the south. Water hyacinth escaped from captivity and has become a hazard to navigation, as well as disrupting the local environment.

Global climate change may be so rapid that many species cannot adjust. Biological magnification of pesticides has reduced predatory bird populations. Acid deposition is implicated in the worldwide decline in amphibian populations.

Conservation biology is a discipline that brings together many fields to attempt to solve biodiversity problems. It attempts to develop practical approaches to preventing extinction of species and destruction of ecosystems. Most conservation biologists believe biological diversity is good and each species has a value all its own. Sustainability is concept that it is possible to meet economic needs while protecting environment. Some economists argue that as per capita income increases, environmental degradation first increases, then decreases as people become affluent enough to begin to protect the environment.

Development in Tropical Regions

Human-caused environmental changes and extinctions are accelerating in the tropics. If this pace continues nearly half all species could be extinct by 2050. Development in tropical areas causes more extinctions due to the great diversity found in tropical rain forests (half all species on Earth). Tropical reefs are also under siege by water pollution, leading to even more extinctions. Nearly half the rain forests are gone already. By as early as 2010 (less than 15 years!) all rain forests will be gone if present trends of exploitation and human population growth continue.

Species are not equally likely to suffer extinction. Cockroaches hav e been around for 300 million years essentially unchanged and probably will be around for millions more. Island species, species with limited habitats, low reproductive rates, high territory requirements, susceptibility to pollution, predators, and having economic value: all make species susceptible to extinction.

What Does Extinction Mean to Me?

We migh ask ourselves, why should we try to save species from extinction? One answer is that wildlife is a curiosity for which humans have no need. Others seek to preserve nature for its own sake. There are some economic reasons to save species from extinction: food and non-food uses.

Approximately 7000 plant species have historically been used by humans as food. Today, thirty of these species provide 95% of all human food. Just four -wheat, corn, rice, and potatoes- provide most of the world's food. Nearly 75,000 edible plants species exist, many superior in nutrition and quality to the 30 we favor today. Low genetic diversity, resulting from centuries of selective inbreeding, make crops especially susceptible to pests and parasites. During the 1970s the U.S. corn crop was almost completely wiped out by a leaf fungus. The corn crop was saved by interbreeding it with a rare species of wild corn from Mexico. Genetic engineering may also offer some hope by facilitating transfer of genes between species. This increases the value of wild strains which can be used as sources for new traits to be introduced into crops.

At current rates, 25,000 plant species will become extinct by the year 2000, before we have a chance to study them. Gene banks to save seeds, spores and genetic material have been proposed as a solution. Currently they contain only a small fraction of all wild plant species. Hybridization is also a potential method to improve domesticated animal stocks.

Many wild plants species provide economically important products, such as rubber. Originally many modern medicines started out as plants used by premodern societies, such as aspirin. Forty-seven major drugs have been produced from flowering plants taken from the rain forests. An estimated 320 valuable drugs remain undiscovered in the forests Taxol, an extract from the rare Pacific yew tree (Taxus), is a potent anticancer drug. Some products, like oils, exotic fruits, and rubber are known as forest-sustainable resources since the forest does not have to be cut down to harvest them, and in most cases bring in more money than the wood from the trees is worth.

Ecosystems as Environmental Support Systems

Species diversity is important to preserve because ecosystems are composed of species and provide us with so many of life's essentials, dismantle them and we have a real big problem:

How to Reduce Species Extinction and Destruction of the Biosphere | Back to Top

Conferences in 1992 and 1994 resulted in binding international treaties to control management of finite resources and population growth along with the resulting human impact on the biosphere.

Identification, Description, and Research on Value of Species

One of the first steps is to measure how much biodiversity there is. Diversity can be measured at different levels: genetic diversity, species diversity, and ecosystem diversity. Only 1.7 million species have been described, estimates place the total number of species at 100,000,000. Biologist E.O. Wilson has estimated that it would take the life's work on 25,000 specialists to completely study and describe the mostly undescribed tropical species. There are currently only 1500 specialists in tropical biology.

Establishment of Preserves

It is essential to preserve the habitat of endangered species. Once a species habitat has been destroyed, it is difficult to successfully reestablish the species; no ecosystem has ever been completely rebuilt. Preserves are well established in Africa, Asia, and the developed countries, although they are not well established in Central and South America. Only 1% of the Earth's land surface has been set aside as preserves.

Development of Laws and Regulations to Protect Endangered Species

Poachers are difficult to catch and punish, especially in under developed countries. In 1975 a treaty made it illegal to trade or sell products from endangered species. The 1989 ban on sale of elephant ivory has been successful in slowing the decline in elephant populations.

The Endangered Species Act

Nearly 4000 species in the U.S. are in danger of extinction by the year 2000. The Endangered Species Act of 1973 directs governmental agencies to maintain a list of threatened or endangered species. The Act has been controversial and was recently challenged by the Republican's Contract with America. The spotted owl became a major issue in the 1992 Presidential election. Most species on the list are closer to extinction now than when they were put on the list.

Captive Breeding

One last resort is to establish breeding programs in captivity. Many species do not do well in such programs, so alternatives like gene banks must be tried. Breeding programs are costly and the young from such programs cannot be released into the wild, even if their habitat still remains.

Reduction of the Socioeconomic Causes of Extinction

Basic social and economic issues that drive explosive population growth and increased exploitation of natural areas must be addressed. Estimate predict a doubling of the 5.4 billion human population by 2050, most of that growth is expected in developing countries. Slowing such growth will take decades at best. Removing economic incentives that cause many species extinctions is a more immediate measure.

Learning Objectives | Back to Top

Under Construction

Terms | Back to Top

acid rain


biogeochemical cycles



chlorofluorocarbons (CFCs)

convergent plate boundaries



developed countries

divergent plate boundaries

Dust Bowl

endangered species


fossil fuels

greenhouse effect

greenhouse gases





plate tectonics



soil conservation




Review Questions | Back to Top

Under Construction

Links | Back to Top

All text contents ©1995, 2000, 2001, 2002, 2007, by M.J. Farabee. Use for educational purposes is encouraged.

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