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2_SCIENCE, MATTER, ENERGY, AND ECOSYSTEMS: CONNECTIONS IN NATURE

When we try to pick out anything by itself, we find it hitched to everything else in the universe. JOHN MUIR

2-1 THE NATURE OF SCIENCE

What Is Science, and What Do Scientists Do?
Science is an attempt to discover order in nature and use that knowledge to make predictions about what is likely to happen in nature. Figure 2-1 summarizes the scientific process.

Figure 2.1 What scientists do.

The first thing scientists do is ask a question or identify a problem to be investigated. Then scientists working on this problem collect scientific data, or facts, by making observations and measurements. Repeated observations and measurements must confirm the resulting scientific data or facts, ideally by several different investigators.

The goal of science is not just data collection and classification of data [see pdf] but a new idea, principle, or model that
(1) connects and explains certain scientific data and
(2) leads to useful predictions about what is likely to happen in nature. Scientists working on a particular problem try to come up with a variety of possible or tentative explanations, or scientific hypotheses, of what they (or other scientists) observe in nature. One method scientists use to test a hypothesis is to develop a model, an approximate representation or simulation of a system being studied.

If repeated experiments or tests using models support a particular hypothesis or a group of related hypotheses, it becomes a scientific theory. In other words, a scientific theory is a verified, highly reliable, and widely accepted scientific hypothesis or a related group of scientific hypotheses.

To scientists, scientific theories are not to be taken lightly. They are not guesses, speculations, or suggestions. Instead, scientific theories are useful explanations of processes or natural phenomena that have a high degree of certainty because they are supported by extensive evidence.

A scientific theory is the closest thing to the "truth" or "absolute" proof that science can provide. New evidence or a better explanation may modify, or in rare cases overturn, a particular scientific theory. But unless or until this happens a scientific theory is the best and most reliable knowledge we have about how nature works.

Nonscientists often use the word theory incorrectly when they mean to refer to a scientific hypothesis, a tentative explanation that needs further evaluation. The statement, "Oh, that's just a theory," made in everyday conversation, implies a lack of knowledge and careful testing-the opposite of the scientific meaning of the word.

Another important result of science is a scientific, or natural, law: a description of what we find happening in nature over and over in the same way. For example, after making thousands of observations and measurements over many decades, scientists discovered the second law of thermodynamics. Simply stated, this law says that heat always flows spontaneously from hot to cold—something you learned the first time you touched a hot object.

A scientific law is no better than the accuracy of the observations or measurements upon which it is based. New or more accurate data may result in a scientific law being modified, or in rare cases overturned. However, scientific laws are highly reliable and well-tested descriptions of what we find occurring in nature.

How Do Scientists Learn About Nature?
We often hear about the scientific method. In reality, many scientific methods exist: they are ways scientists gather data and formulate and test scientific hypotheses, models, theories, and laws 

Here is an example of applying the scientific method to an everyday situation.

Observation:       You walk into your bedroom at night and flick on the light switch. The light does not come on.

Question:             Why did the light not come on? Hypothesis: Maybe the power for the house is out. Test the Hypothesis: If the power is out, the lights in other rooms should also be out.

Experiment:        To check this prediction, go to other rooms and click light switches.

Results:                Lights in other rooms come on when their switches are clicked.

Conclusion:         Power to whole house is not out.

New Hypothesis: Maybe the lightbulb is burned out.

Experiment:         Replace bulb with a new bulb.

Results:                Light comes on when switch is flicked.

Conclusion:         Second hypothesis is verified.

Situations in nature are usually much more complicated than this. A number of variables or factors influence most processes or parts of nature scientists seek to understand. Ideally, scientists conduct a controlled experiment to isolate and study the effect of a single variable. This single-variable analysis is done by setting up two groups:
(1) an experimental group, in which the chosen variable is changed in a known way, and
(2) a control group, in which the chosen variable is not changed. If the experiment is designed properly, any difference between the two groups should result from a variable that was changed in the experimental group.

A basic problem is that many of the phenomena environmental scientists investigate involve a huge number of interacting variables. This limitation is overcome in some cases by using multivariable analysis. This involves using mathematical models run on highspeed computers to analyze the interactions of many variables without having to carry out traditional controlled experiments.

How Valid Are the Results of Science?
Scientists can do two major things:
(1) disprove things and
(2) establish that a particular model, theory, or law has a very high probability or degree of certainty of being true. However, like scholars in any field, scientists cannot prove their theories, models, and laws are absolutely true.

When people say something has or has not been "scientifically proven," they can mislead us by falsely implying that science yields absolute proof or certainty. Although it may be extremely low, some degree of uncertainty is always involved in any scientific theory, model, or law.

How Does Frontier Science Differ from Consensus Science?
News reports often focus on
(1) new so-called scientific breakthroughs and
(2) disputes among scientists over the validity of preliminary (untested) data, hypotheses, and models. These preliminary results, called frontier science, are controversial because they have not been widely tested and accepted.

At the preliminary frontier stage, it is normal and healthy for reputable scientists in a field to disagree about
(1) the meaning and accuracy of scientific data and
(2) the validity of various hypotheses.

By contrast, consensus science consists of data, theories, and laws that scientists who are considered experts in the field involved widely accept. This aspect of science is very reliable but rarely considered newsworthy. One way to find out what scientists generally agree on is to seek out reports by scientific bodies such as the U.S. National Academy of Sciences and the British Royal Society that attempt to summarize consensus among experts in key areas of science.

2-2 MATTER AND ENERGY

What Are Nature's Building Blocks?
Matter is anything that has mass (the amount of material in an object) and takes up space. Scientists classify matter as existing in various levels of organization (Figure 2-2).

Matter is found in two chemical forms:

. Elements: the distinctive building blocks of matter that make up every material substance

. Compounds: two or more different elements held together in fixed proportions by attractive forces called chemical bonds

Various elements, compounds, or both can be found together in mixtures.

All matter is built from the 115 known chemical elements (92 of them occur naturally, and the other 23 have been synthesized in laboratories). To simplify, chemists represent each element by a one- or two-letter symbol. Examples used in this book are hydrogen (H), carbon (C), oxygen (O), nitrogen (N), phosphorus (P), sulfur (S), chlorine (Cl), fluorine (F), bromine (Br), sodium (Na), calcium (Ca), lead (Pb), mercury (Hg), arsenic (As), and uranium (U).

If you had a supermicroscope capable of looking at individual elements and compounds, you could see they are made up of three types of building blocks:

. Atoms: the smallest units of matter that are unique to a particular element.

. Ions: electrically charged atoms or combinations of atoms. Examples encountered in this book include (1) positive hydrogen ions (H +), sodium ions (Na +), calcium ions (Ca 2+), and ammonium ions (NH4 +) and (2) negative chloride ions(Cl -), nitrate ions (NO3 -), sulfate ions (SO4 2-), and phosphate ions (PO4 3-).

. Compounds: combinations of two or more atoms or ions of the same or different elements held together by chemical bonds. Chemists use a shorthand chemical formula to show the number of atoms (or ions) of each type in a compound. The formula (1) contains the symbols for each of the elements present and

(2) uses subscripts (but this scanned script does not show this) to represent the number of atoms or ions of each element in the compound's basic structural unit. Examples of compounds and their formulas encountered in this book are water (H2O, read as "H-two-O"), oxygen (O2), ozone (O3), nitrogen (N2), nitrous oxide (N2O), nitric oxide (NO), hydrogen sulfide (H2S), carbon monoxide (CO), carbon dioxide (CO2), nitrogen dioxide (NO2), sulfur dioxide (SO2), ammonia (NH3), sulfuric acid (H2SO4), nitric acid (HN03), methane (CH4), and glucose (C6H12O6)'

Table sugar, vitamins, plastics, aspirin, penicillin, and many other important materials have one thing in common: They are organic compounds, containing carbon atoms combined with each other and with atoms of one or more other elements such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, chlorine, and fluorine. All other compounds are called inorganic compounds.

The millions of known organic (carbon-based) compounds include the following:

Hydrocarbons: compounds of carbon and hydrogen atoms. An example is methane (CH4), the main component of natural gas.

Chlorinated hydrocarbons: compounds of carbon, hydrogen, and chlorine atoms. An example is the insecticide DDT (C14H9Cl5).

Simple carbohydrates (simple sugars): certain types of compounds of carbon, hydrogen, and oxygen atoms. An example is glucose (C6H1206)' which most plants and animals break down in their cells to obtain energy.

Larger and more complex organic compounds, called polymers, consist of a number of basic structural or molecular units (monomers) linked by chemical bonds, somewhat like cars linked in a freight train. The three major types of organic polymers are
(1) complex carbohydrates consisting of two or more monomers of simple sugars (such as glucose) linked together,
(2) proteins formed by linking together monomers of amino acids, and
(3) nucleic acids, such as DNA and RNA, made by linked sequences of monomers called nucleotides.

Genes consist of specific sequences of nucleotides in a DNA molecule. These coded units of genetic information about specific traits are passed on from parents to offspring during reproduction. Chromosomes are combinations of genes that make up a single DNA molecule, together with a number of proteins. Genetic information coded in your chromosomal DNA is what makes you different from an oak leaf, an alligator, or a flea and from your parents. The relationships of genetic material to cells are depicted in Figure 2-3.

Figure 2-3 Relationships among cells, nuclei, chromosomes, DNA, and genes.

The

human

body

contains

about 100

trillion

cells.

There is a

nucleus

inside

each

human cell

(except red

blood cells).

Each

nucleus

contains 46

chromosomes,

arranged

in 23

pairs.

One

chromosome

of every

pair

is from

each

parent.

The

chromosomes

are filled

with tightly

coiled

strands

of DNA.

Genes are segments of DNAthat contain instructions to make proteinsthe buildingblocks of life.

The genes in each cell are coded by sequences of nucleotides in its DNA molecules.

What Are Atoms Made Of?
If you increased the magnification of your supermicroscope, you would find that each different type of atom contains a certain number of subatomic particles. The main building blocks of an atom are
(1) positively charged protons (p),
(2) uncharged neutrons (n), and
(3) negatively charged electrons (e).

Each atom consists of
(1) an extremely small center, or nucleus, containing protons and neutrons and
(2) one or more electrons in rapid motion somewhere outside the nucleus.

Each atom has an equal number of positively charged protons (inside its nucleus) and negatively charged electrons (outside its nucleus). Because these electrical charges cancel one another, the atom as a whole has no net electrical charge.

Each element has its own specific atomic number, equal to the number of protons in the nucleus of each of its atoms. The simplest element, hydrogen (H), has only 1 proton in its nucleus, so its atomic number is 1. Carbon (C), with 6 protons, has an atomic number of 6, whereas uranium (U), a much larger atom, has 92 protons and an atomic number of 92.

Because electrons have so little mass compared with the mass of a proton or a neutron, most of an atom's mass is concentrated in its nucleus. The mass of an atom is described in terms of its mass number: the total number of neutrons and protons in its nucleus. For example,
(1) a hydrogen atom with 1 proton and no neutrons in its nucleus has a mass number of 1, and
(2) an atom of uranium with 92 protons and 143 neutrons in its nucleus has a mass number of 235 (92 + 143 = 235).

All atoms of an element have the same number of protons in their nuclei. However, they may have different numbers of uncharged neutrons in their nuclei, and thus may have different mass numbers. Various forms of an element having the same atomic number but a different mass number are called isotopes of that element. Attaching their mass numbers to the name or symbol of the element identifies isotopes. For example, hydrogen has three isotopes: hydrogen-l (1H), hydrogen-2 (2H, common name deuterium), and hydrogen-3 (3H, common name tritium). A natural sample of an element contains a mixture of its isotopes in a fixed proportion or percentage abundance by weight (Figure 2-4).

Figure 2-4 Isotopes of hydrogen and uranium. All isotopes of hydrogen have an atomic number of 1 because each has one proton in its nucleus: similarly, all uranium isotopes have an atomic number of 92. However, each isotope of these elements has a different mass number because its nucleus contains a different number of neutrons. Figures in parentheses indicate the percentage abundance by weight of each isotope in a natural sample of the element.

What Are Matter Quality and Material Efficiency?
Matter quality is a measure of how useful a form of matter is to us as a resource, based on its availability and concentration. High-quality matter
(1) is concentrated,
(2) usually is found near the earth's surface, and
(3) has great potential for use as a matter resource.

Low-quality matter
(1) is dilute,
(2) often is deep underground or dispersed in the ocean or the atmosphere, and
(3) usually has little potential for use as a matter resource (Figure 2-5, p. 20).

Figure 2.5 Examples of differences in matter quality. Highquality matter (left-hand column) is fairly easy to extract and is concentrated; low-quality matter (right-hand column) is more difficult to extract and is more dispersed than high-quality matter.

High Quality Low Quality

Solid

Salt

Coal

Aluminum can

Gas

Solution of salt in water

Coal-fired power plant emissions

Aluminum ore

An aluminum can is a more concentrated, higherquality form of aluminum than aluminum ore containing the same amount of aluminum. That is why it takes less energy, water, and money to recycle an aluminum can than to make a new can from aluminum ore.

Material efficiency, or resource productivity, is the total amount of material needed to produce each unit of goods or services. Although resource productivity has been improving, only about 2-6% of the matter resources flowing through the economies of developed countries ends up providing useful goods and services. Because of such waste, business expert Paul Hawken and physicist Amory Lovins contend that resource productivity in developed countries could be improved by 75-90% within two decades using existing technologies.

What Are Different Forms of Energy?
Energy is the capacity to do work and transfer heat. Work is performed when an object-be it a grain of sand, this book, or a giant boulder-is moved over some distance. Work, or matter movement, also is needed to boil water or to burn natural gas to heat a house or cook food. Energy is also the heat that flows automatically from a hot object to a cold object when they come in contact. Radiation is the transmission of energy through space as particles or waves.

What Is the Difference Between Kinetic Energy and Potential Energy?
Two major types of energy are:

. Kinetic energy, which matter has because of its mass and its speed or velocity. Examples of this energy in motion are
(1) wind (a moving mass of air),
(2) flowing streams,
(3) heat flowing from a body at a high temperature to one at a lower temperature, and
(4) electricity (flowing electrons).

. Potential energy, which is stored and potentially available for use. Examples of this stored energy are:
(1) a rock held in your hand,
(2) an unlit stick of dynamite,
(3) still water behind a dam,
(4) the chemical energy stored in gasoline molecules, and
(5) the nuclear energy stored in the nuclei of atoms.

Potential energy can be changed to kinetic energy.
When you drop a rock, its potential energy changes into kinetic energy.
When you bum gasoline in a car engine, the potential energy stored in the chemical bonds of its molecules changes into heat, light, and mechanical (kinetic) energy that propels the car.

What Is Electromagnetic Radiation?
Another type of energy is electromagnetic radiation: energy radiated in the form of a wave as a result of the changing electric and magnetic fields. There are many different forms of electromagnetic radiation, each with a different wavelength (distance between successive peaks or troughs in the wave) and energy content (Figure 2-6). Such radiation travels through space at the speed of light, which is about 300,000 kilometers per second (186,000 miles per second).

What Is the Difference Between Heat and Temperature?
Heat is the total kinetic energy of all the moving atoms, ions, or molecules within a given substance, excluding the overall motion of the whole object). Temperature is the average speed of motion of the atoms, ions, or molecules in a sample of matter at a given moment.

What Is Energy Quality?
Energy quality is a measure of an energy source's ability to do useful work (Figure 2-7).

Figure 2-7 Categories of the quality (usefulness for performing various energy tasks) of different sources of energy. High-quality energy is concentrated and has great ability to perform useful work. Low-quality energy is dispersed and has little ability to do useful work. To avoid unnecessary energy waste, it is best to match the quality of an energy source with the quality of energy needed to perform a task.

High-quality energy is concentrated and can perform much useful work. Examples are
(1) electricity,
(2) the chemical energy stored in coal and gasoline,
(3) concentrated sunlight, and
(4) nuclei of uranium235, used as fuel in nuclear power plants.

By contrast, low-quality energy is dispersed and has little ability to do useful work. An example is heat dispersed in the moving molecules of a large amount of matter (such as the atmosphere or a large body of water) so that its temperature is low.

For example, the total amount of heat stored in the Atlantic Ocean is greater than the amount of highquality chemical energy stored in all the oil deposits of Saudi Arabia. Yet the ocean's heat is so widely dispersed, it cannot be used to move things or to heat things to high temperatures. It makes sense to match the quality of an energy source with the quality of energy needed to perform a particular task (Figure 2-7) because doing so saves energy and usually money (unless government subsidies or taxes have distorted the energy marketplace).

What Is the Difference Between a Physical and a Chemical Change?
A physical change involves no change in chemical composition. Cutting a piece of aluminum foil into small pieces is one example. Changing a substance from one physical state to another is a second example. When solid water (ice) is melted or liquid water is boiled, none of the H2O molecules involved are altered; instead, the molecules are organized in different spatial (physical) patterns.

In a chemical change, or chemical reaction, in contrast, the chemical compositions of the elements or compounds are altered. Chemists use shorthand chemical equations to represent what happens in a chemical reaction. For example, when coal bums completely, the solid carbon (C) it contains combines with oxygen gas (O2) from the atmosphere to form the gaseous compound carbon dioxide (CO2), We can represent this chemical reaction in shorthand form as C + O2 >> CO2 + energy.

Energy is given off in this reaction, making coal a useful fuel. The reaction also shows how the complete burning of coal (or any of the carbon-containing compounds in wood, natural gas, oil, and gasoline) gives off carbon dioxide gas, which is a key gas that can lead to warming of the lower atmosphere (troposphere).

The Law of Conservation of Matter: Why Is There No "Away"?
We may change various elements and compounds from one physical or chemical form to another, but in no physical and chemical change can we create or destroy any of the atoms involved. All we can do is rearrange them into different spatial patterns (physical changes) or different combinations (chemical changes). The italicized statement, based on many thousands of measurements, is known as the law of conservation of matter.

The law of conservation of matter means there is no "away" in "to throwaway." Everything we think we have thrown away is still here with us in one form or another. We can make the environment cleaner and convert some potentially harmful chemicals into less harmful physical or chemical forms. However, the law of conservation of matter means we will always face the problem of what to do with some quantity of wastes and pollutants.

What Is the First Law of Thermodynamics?
You Cannot Get Something for Nothing

Scientists have observed energy being changed from one form to another in millions of physical and chemical changes, but they have never been able to detect the creation or destruction of any energy (except in nuclear changes). The results of their experiments have been summarized in the law of conservation of energy, also known as the first law of thermodynamics: In all physical and chemical changes, energy is neither created nor destroyed, but it may be converted from one form to another.

This scientific law tells us that when one form of energy is converted to another form in any physical or chemical change, energy input always equals energy output. No matter how hard we try or how clever we are, we cannot get more energy out of a system than we put in; in other words, we cannot get something for nothing in terms of energy quantity.

What Is the Second Law of Thermodynamics?
You Cannot Even Break Even

Because the first law of thermodynamics states that energy can be neither created nor destroyed, we may be tempted to think there will always be enough energy. Yet if we fill a car's tank with gasoline and drive around, or use a flashlight battery until it is dead, something has been lost. If it is not energy, what is it? The answer is energy quality (Figure 2-7), the amount of energy available that can perform useful work.

Countless experiments have shown that when energy is changed from one form to another, a decrease in energy quality always occurs. The results of these experiments have been summarized in what is called the the second law of thermodynamics: When energy is changed from one form to another, some of the useful energy is always degraded to lower-quality, more dispersed, less useful energy. This degraded energy usually takes the form of heat given off at a low temperature to the surroundings (environment). There it is dispersed by the random motion of air or water molecules and becomes even more disorderly and less useful.

Basically, this law says that in any energy conversion, we always end up with less usable energy than we started with. So not only can we not get something for nothing in terms of energy quantity, we cannot even break even in terms of energy quality because energy always goes from a more useful to a less useful form when energy is changed from one form to another. No one has ever found a violation of this fundamental scientific law.

Here are three examples of the second law of thermodynamics in action:

. When a car is driven, only about 10% of the highquality chemical energy available in its gasoline fuel is converted into mechanical energy (to propel the vehicle) and electrical energy (to run its electrical systems). The remaining 90% is degraded to low-quality heat that is released into the environment and eventually lost into space.

. When electrical energy flows through filament wires in an incandescent lightbulb, it is changed into about 5% useful light and 95% low-quality heat that flows into the environment. In other words, this socalled lightbulb is really a heatbulb.

. In living systems, solar energy is converted into chemical energy (food molecules) and then mto mechanical energy (moving, thinking, and living). During each of these conversions, high-quality energy is degraded and flows into the environment as lowquality heat (Figure 2-8).

Figure 2-8 The second law of thermodynamics in action in living systems. Each time energy is changed from one form to another, some of the initial input of high-quality energy is degraded, usually to low-quality heat that disperses into the environment.

The second law of thermodynamics also means we can never recycle or reuse high-quality energy to perform useful work. Once the concentrated energy in a serving of food, a liter of gasoline, a lump of coal, or a chunk of uranium is released, it is degraded to low-quality heat that is dispersed into the environment. We can heat air or water at a low temperature and upgrade it to highquality energy, but the second law of thermodynamics tells us that it will take more high-quality energy to do this than we get in return.

Energy efficiency, or energy productivity, is a measure of how much useful work is accomplished by a particular input of energy into a system. As with material efficiency (p. 19), there is plenty of room for improvement. For example, scientists estimate that only about 16% of the energy used in the United States ends up performing useful work. The remaining 84% is either unavoidably wasted because of the second law of thermodynamics (41 %) or unnecessarily wasted (43%).

2-3 EARTH'S LIFE-SUPPORT SYSTEMS: FROM ORGANISMS TO THE BIOSPHERE

What Is Ecology?
Ecology (from the Greek wordsoikos, "house" or "place to live," and logos, "study of") is the study of how organisms interact with one another and with their nonliving environment. In effect, it is a study of connections in nature.

What Are Organisms and Species?
Ecologists focus on trying to understand the interactions among organisms, populations, communities, ecosystems, and the biosphere (Figure 2-2).

An organism is any form of life. The cell is the basic unit of life in organisms. Organisms may consist of a single cell (bacteria, for instance) or many cells.

Organisms can be classified into species, or groups of organisms that resemble one another in appearance, behavior, chemistry, and genetic makeup. Organisms that reproduce sexually are classified in the same species if, under natural conditions, they can
(1) actually [potentially is excluded in the modern classification] breed with one another and
(2) produce live, fertile offspring.

We do not know how many species exist on the earth. Estimates range from 3.6 million to 100 million. Most are insects (Spotlight, p. 24) and microorganisms too small to be seen with the naked eye (Connections, p. 25). Excluding hordes of bacterial species, a best guess of the number of species is about 10-14 million.

So far biologists have identified and named about 1.5-1.8 million species, not including bacteria. Biologists know a fair amount about roughly one-third of the known species but understand the detailed roles and interactions of only a few.


Have you Thanked an Insect Today?

Insects have a bad reputation. We classify many insect species as pests because they
(1) compete with us for food,
(2) spread human diseases (such as malaria), and
(3) invade our lawns, gardens, and houses. Some people have "bugitis," fear all insects, and think the only good bug is a dead bug. However, this view fails to recognize the vital roles insects play in helping sustain life on earth.

A large proportion of the earth's plant species depend on insects to pollinate their flowers. In turn, we and other land-dwelling animals depend on plants for food, either by eating them or by consuming animals that eat them. Without pollinating insects, very few fruits and vegetables would be available for us and plant-eating animals to eat.

Insects which eat other insects help control the populations of at least half the species of insects we call pests. This free pest control service is an important part of the natural resources or ecological services (Figure 1-2, p. 3) that help sustain us.

Suppose all insects disappeared today. Within a year most of the earth's animals would become extinct because of the disappearance of so much plant life. The earth would be covered with rotting vegetation and animal carcasses being decomposed by unimaginably huge hordes of bacteria and fungi.

Fortunately, this is not a realistic scenario because insects, which have been around for at least 400 million years, are phenomenally successful forms of life. They were the first animals to invade the land and, later, the air. Today they are by far the planet's most diverse, abundant, and successful animals.

The environmental lesson is that although insects can thrive without newcomers such as us, we and most other land organisms would perish quickly without them.


2_a Critical Thinking
Are you afraid of most insects? If so, try to trace where this fear might have originated.


Microbes: the Invisible Rulers of Earth

They are everywhere and there are trillions of them. Billions are found inside your body, on your body, in a handful of soil, and in a cup of river water.

These mostly invisible rulers of the earth are microbes, a catchall term for many thousands of species of bacteria, protozoa, fungi, and yeasts, most of which are too small for us to see with the naked eye.

Most microbes do not get the respect they deserve. Most of us think of them as threats to our health in the form of
(1) infectious bacteria or "germs,"
(2) fungi that cause athlete's foot and other skin diseases, and
(3) protozoa that cause killer diseases such as malaria. However, these potentially harmful microbes are in the minority.

Most of the earth's hordes of microbes not only are harmless but also make the rest of life possible. Some of them playa vital role in producing foods such as bread, cheese, yogurt, vinegar, tofu, soy sauce, beer, and wine. Others provide us with food by converting nitrogen gas in the atmosphere into forms that plants can take up from the soil as nutrients.

Bacteria and fungi in the soil decompose organic wastes into nutrients that can be taken up by plants. Bacteria in your intestinal tract break down the food you eat. Some microbes in your nose prevent harmful bacteria from reaching your lungs. Other microbes have been the source of disease-fighting antibiotics, including penicillin, erythromycin, and streptomycin.

Another vital ecological service that some microbes provide is controlling some plant diseases and populations of insect species that attack food crops. Enlisting some of these microbes for pest control can reduce the use of potentially harmful chemical pesticides.

In addition, genetic engineers are developing microbes than can
(1) extract metals from ores,
(2) break down various pollutants, and
(3) help clean up toxic waste sites.

Harvard biologist Edward O. Wilson, who has developed many important ecological theories and is one of the world's experts on ants, says that if he were starting over he would study microbes.


2_b Critical Thinking
A bumper sticker reads, "Have You Thanked Microbes Today?" Give reasons for doing so, and explain why microbes are the real rulers of the earth.


What Is a Population?
A population consists of a group of interacting individuals of the same species that occupy a specific area at the same time. Examples are all
(1) sunfish in a pond,
(2) white oak trees in a forest, and
(3) people in a country.

In most natural populations, individuals vary slightly in their genetic makeup, which is why they do not all look or behave exactly alike-a phenomenon called genetic diversity.

The place where a population (or an individual organism) normally lives is its habitat. It may be as large as an ocean or prairie or as small as the underside of a rotting log or the intestine of a termite.

What Are Communities, Ecosystems, and the Biosphere?
Populations of the different species occupying a particular place make up a community, or biological community. It is a complex interacting network of plants, animals, and microorganisms.

An ecosystem is a community of different species interacting with one another and with their nonliving environment of matter and, energy. Ecosystems can range in size from a puddle of water to a stream, a patch of woods, an entire forest, or a desert. Ecosystems can be natural or artificial (human created). Examples of human-created ecosystems are crop fields, farm ponds, and reservoirs. All of the earth's ecosystems together make up what we call the biosphere.

What Are the Major Parts of the Earth's Life-Support Systems?

We can think of the earth as being made up of several spherical layers (Figure 2-9).

Figure 2-9 The general structure of the earth.

The atmosphere is a thin envelope or membrane of air around the planet. Its inner layer, the troposphere, extends only about 17 kilometers (11 miles) above sea level but contains most of the planet's air, mostly nitrogen (78%) and oxygen (21%). The next layer, stretching 17-48 kilometers (11-30 miles) above the earth's surface, is the stratosphere. Its lower portion contains enough ozone (03) to filter out most of the sun's harmful ultraviolet radiation, thus allowing life to exist on land and in the surface layers of bodies of water.

The hydrosphere consists of the earth's
(1) liquid water (both surface and underground),
(2) ice (polar ice, icebergs, and ice in frozen soil layers, or permafrost), and
(3) water vapor in the atmosphere. The lithosphere is the earth's crust and upper mantle; the crust contains nonrenewable fossil fuels and minerals we use as well as renewable soil chemicals (nutrients) needed for plant life.

The biosphere is the portion of the earth in which living (biotic) organisms exist and interact with one another and with their nonliving (abiotic) environment. The biosphere includes most of the hydrosphere and parts of the lower atmosphere and upper lithosphere. It reaches from the deepest ocean floor, 20 kilometers (12 miles) below sea level, to the tops of the highest mountains. If the earth were an apple, the biosphere would be no thicker than the apple's skin. The goal of ecology is to understand the interactions in this thin, life-supporting global skin or membrane of air, water, soil, and organisms.

What Sustains Life on Earth?
Life on the earth depends on three interconnected factors (Figure 2-10):

Figure 2-10  Life on the earth depends on
(1) the oneway flow of energy (dashed lines) from the sun through the biosphere,
(2) the cycling of crucial elements (solid lines around circles), and
(3) gravity, which keeps atmospheric gases from escaping into space and draws chemicals downward in the matter cycles. This simplified model depicts only a few of the many cycling elements.

. The one-way flow of high-quality energy from the sun
(a) through materials and living things in their feeding interactions,
(b) into the environment as low-quality energy (mostly heat dispersed into air or water molecules at a low temperature), and
(c) eventually back into space as heat.

. The cycling of matter (the atoms, ions, or compounds needed for survival by living organisms) through parts of the biosphere. The earth is closed to significant inputs of matter from space. Thus essentially all the nutrients used by organisms are already present on earth and must be recycled again and again for life to continue.

. Gravity, which
(a) allows the planet to hold on to its atmosphere and
(b) causes the downward movement of chemicals in the matter cycles.

What Happens to Solar Energy Reaching the Earth?
Because the earth is a tiny sphere in the vastness of space, it receives only about one-billionth of the sun's output of energy-mostly as visible light (Figure 2-6). Much of this energy is either reflected away or absorbed by chemicals in its atmosphere (Figure 2-11).

Figure 2-11 The flow of energy to and from the earth.

Most unreflected solar radiation is degraded into infrared radiation (which we experience as heat) as it interacts with the earth. Greenhouse gases (such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone) in the atmosphere reduce this flow of heat back into space. This helps warm the earth by acting somewhat like the glass in a greenhouse or the windows in a closed car, which allow a buildup of heat. Without this natural greenhouse effect (Figure 2-12), the earth would be too cold for life as we know it to exist.

(a) Rays of sunlight penetrate the lower atmosphere and warm the earth's surface.

(b) The earth's surface absorbs much of the incoming solar radiation and degrades it to longer-wavelength infrared radiation (heat), which rises into the lower atmosphere. Some of this heat escapes into space and some is absorbed by molecules of greenhouse gases and emitted as infrared radiation, which warms the lower atmosphere.

(c) As concentrations of greenhouse gases rise, their molecules absorb and emit more infrared radiation, which adds more heat to the lower atmosphere.

Figure 2-12 The greenhouse effect. Without the atmospheric warming provided by this natural effect, the earth would be a cold and mostly lifeless planet. According to the widely accepted greenhouse theory, when concentrations of greenhouse gases in the atmosphere rise, the average temperature of the troposphere also rises. (Modified by permission from Cecie Starr, Biology: Concepts and Principles, 4th ed., Pacific Grove, Calif.: Brooks/Cole, 2000)

Why Is the Earth So Favorable for Life?
Life on the earth as we know it needs a certain temperature range: Venus is much too hot and Mars is much too cold, but the earth is just right. (Otherwise, you would not be reading these words.)

Life as we know it depends on the liquid water that dominates the earth's surface. Again, temperature is crucial; life on the earth needs average temperatures between the freezing and boiling points of water, between 0Cand 100C (32F and 212F) at the earth's range of atmospheric pressures.

The earth's orbit is the right distance from the sun to provide these conditions. If the earth were much closer, it would be too hot-like Venus-for water vapor to condense to form rain. If it were much farther away, its surface would be so cold-like Mars-that its water would exist only as ice. The earth also spins; if it did not, the side facing the sun would be too hot and the other side too cold for water-based life to exist.

The earth is also the right size; that is, it has enough gravitational mass to keep its iron and nickel core molten and to keep the gaseous molecules in its atmosphere from flying off into space. (A much smaller earth would be unable to hold on to an atmosphere consisting of such light molecules as N2, O2, CO2, and H2O.)

The slow transfer of its internal heat (geothermal energy) to the surface also helps keep the planet at the right temperature for life. And thanks to the development of photosynthesizing bacteria more than 2 billion years ago, an ozone sunscreen protects us and many other forms of life from an overdose of ultraviolet radiation.

On a time scale of millions of years, the earth is enormously resilient and adaptive. During the 3.7 billion years since life arose, the average surface temperature of the earth has remained within the narrow range of 10-20C (50-68F), even with a 30-40% increase in the sun's energy output. In short, the earth is just right for life as we know it.

What Are the Major Components of Ecosystems?
The biosphere and its ecosystems can be separated into two parts:
(1) abiotic, or nonliving, components (water, air, nutrients, and solar energy) and
(2) biotic, or living, components (plants, animals, and microorganisms, sometimes called biota). Figures 2-13 and 2-14 (p. 28) are greatly simplified diagrams of some of the biotic and abiotic components in a freshwater aquatic ecosystem and a terrestrial ecosystem.

Figure 2-13 Major components of a freshwater ecosystem.

Figure 2-14 Major components of an ecosystem in a field.

What Are the Major Nonliving Components of Ecosystems?
The nonliving, or abiotic, components of an ecosystem are the physical and chemical factors that influence living organisms in land (terrestrial) ecosystems and aquatic life zones (Figure 2-15, p. 28).

Terrestrial Ecosystems Aquatic Life Zones
. Sunlight
. Temperature
. Precipitation
.Wind
. Latitude
  (distance from equator)
. Altitude
  (distance above sea level) 
. Fire frequency
.Soil
. Light penetration . Water currents
. Dissolved nutrient concentrations
  (especially Nand P)
. Suspended solids
. Salinity

Figure 2-15 Key physical and chemical or abiotic factors affecting terrestrial ecosystems (left) and aquatic life zones (right).

Different species thrive under different physical conditions. Some need bright sunlight, and others thrive better in shade. Some need a hot environment and others a cool or cold one. Some do best under wet conditions and others under dry conditions.

Each population in an ecosystem has a range of tolerance to variations in its physical and chemical environment (Figure 2-16). Individuals within a population may also have slightly different tolerance ranges for temperature or other factors because of small differences in genetic makeup, health, and age. Thus, although a trout population may do best within a narrow band of temperatures (optimum level or range), a few individuals can survive above and below that band. As Figure 2-16 shows, tolerance has its limits, beyond which none of the trout can survive.

Figure 2-16 Range of tolerance for a population of organisms to an abiotic environmental factor-in this case, temperature.

These observations are summarized in the law of tolerance: The existence, abundance, and distribution of a species in an ecosystem are determined by whether the levels of one or more physical or chemical factors fall within the range tolerated by that species. A species may have a wide range of tolerance to some factors and a narrow range of tolerance to others. Most organisms are least tolerant during juvenile or reproductive stages of their life cycles. Highly tolerant species can live in a variety of habitats with widely different conditions.

A variety of factors can affect the number of organisms in a population. However, sometimes one factor, known as a limiting factor, is more important in regulating population growth than other factors. This ecological principle, related to the law of tolerance, is called the limiting factor principle: Too much or too little of any abiotic factor can limit or prevent growth of a population, even if all other factors are at or near the optimum range of tolerance.

On land, precipitation often is the limiting factor. Lack of water in a desert limits plant growth. Soil nutrients also can act as a limiting factor on land. Suppose a farmer plants corn in phosphorus-poor soil. Even if water, nitrogen, potassium, and other nutrients are at optimum levels, the corn will stop growing when it uses up the available phosphorus.

Too much of an abiotic factor can also be limiting. For example, too much water or too much fertilizer can kill plants, a common mistake of many beginning gardeners.

Important limiting factors for aquatic life zones include
(1) temperature,
(2) sunlight,
(3) dissolved oxygen (DO) content (the amount of oxygen gas dissolved in a given volume of water at a particular temperature and pressure), and (4) nutrient availability. Another limiting factor in aquatic life zones is salinity (the amounts of various inorganic minerals or salts dissolved in a given volume of water).

What Are the Major Living Components of Ecosystems?
Living organisms in ecosystems usually are classified as either producers or consumers, based on how they get food. Producers, sometimes called autotrophs (self-feeders), make their own food from compounds obtained from their environment. All other organisms are consumers that depend directly or indirectly on food provided by producers.

On land, most producers are green plants. In freshwater and marine life zones, algae and plants are the major producers near shorelines. In open water, the dominant producers are phytoplankton (most of them microscopic) that float or drift in the water.

Most producers capture sunlight to make complex compounds ( such as glucose, C6H1206 ) by photosynthesis. Although hundreds of chemical changes take place during photosynthesis, the overall reaction can be summarized as follows:

        carbon dioxide + water + solar energy  >> glucose + oxygen

A few producers, mostly specialized bacteria, can convert simple compounds from their environment into more complex nutrient compounds without sunlight, a process called chemosynthesis.

. All other organisms in an ecosystem are consumers, or heterotrophs ("other-feeders"), which get their energy and nutrients by feeding on other organisms or their remains. Based on their primary source of food, consumers are classified as

. Herbivores (plant eaters), or primary consumers, which feed directly on producers.

. Carnivores (meat eaters), which feed on other consumers. Those feeding only on primary consumers are called secondary consumers; those feeding on other carnivores are called tertiary (higher-level) consumers.

. Omnivores (such as pigs, rats, foxes, bears, cockroaches, and humans), which eat plants and animals. .

. Scavengers (such as vultures, flies, hyenas, and some species of sharks and ants), which feed on dead organisms.

. Detritivores (detritus feeders and decomposers), which feed on detritus (/di-TRI-tus/), or parts of dead organisms and cast-off fragments and wastes of living organisms (Figure 2-17).

. Detritus feeders (such as crabs, carpenter ants, termites, and earthworms), which extract nutrients from partly decomposed organic matter in leaf litter, plant debris, and animal dung.

. Decomposers (mostly certain types of bacteria and fungi), which recycle organic matter in ecosystems. They do this by (1) breaking down (biodegrading) dead organic material (detritus) to get nutrients and

(2) releasing the resulting simpler inorganic compounds into the soil and water, where they can be taken up as nutrients by producers.

Both producers and consumers use the chemical energy stored in glucose and other organic compounds to fuel their life processes. In most cells, this energy is released by aerobic respiration, which uses oxygen to convert organic nutrients back into carbon dioxide and water. The net effect of the hundreds of steps in this complex process is represented by the following reaction:

glucose + oxygen  >> carbon dioxide + water + energy

Although the detailed steps differ, the net chemical change for aerobic respiration is the opposite of that for photosynthesis.

The survival of any individual organism depends on the flow of matter and energy through its body. However, an ecosystem as a whole survives primarily through a combination of matter recycling (rather than one-way flow) and one-way energy flow (Figure 2-18).

Decomposers complete the cycle of matter by breaking down detritus into inorganic nutrients that are used by producers. Without decomposers,
(1) the entire world would soon be knee deep in plant litter, dead animal bodies, animal wastes, and garbage, and
(2) most life as we know it would no longer exist.

Figure 2-17 Some detritivores, called detritus feeders, directly consume tiny fragments of this log. Other detritivores, called decomposers (mostly fungi and bacteria), digest complex organic chemicals in fragments of the log into simpler inorganic nutrients. These nutrients can be used again by producers if they are not washed away or otherwise removed from the system.

What Is Biodiversity, and Why Is It Important?
One important renewable resource is biological divery, or biodiversity: the different forms of life and life;taining processes that can best survive the variety conditions currently found on the earth. Kinds of )diversity include the following:

Genetic diversity (variety in the genetic makeup long individuals within a species).

Species diversity (variety among the species or ;tinct types of living organisms found in different bitats of the planet).

Ecological diversity (variety of forests, deserts, lsslands, streams, lakes, oceans, coral reefs, wetids, and other biological communities). Functional diversity (biological and chemical Jcesses or functions such as energy flow and matter ding needed for the survival of species and biologil communities; Figure 2-18).

This rich variety of genes, species, biological com nities, and life-sustaining biological and chemical ocesses

Gives us food, wood, fibers, energy, raw materials, :lustrial chemicals, and medicines, all of which pour mdreds of billions of dollars into the global econo.y each year.

Provides us with free recycling, purification, and ,tural pest control services.

Figure 2-18 The main structural components (energy, chemicals, and organisms) of an ecosystem are linked by matter recycling and the one way flow of high-quality energy from the sun, through organisms, and then into the environment as low-quality heat.

Every species here today
(1) contains genetic information that represents thousands to millions of years of adaptation to the earth's changing environmental conditions and
(2) is the raw material for future adaptations.

Loss of biodiversity
(1) reduces the availability of ecosystem services (Figure 1-2, p. 3) and
(2) decreases the ability of species, communities, and ecosystems to adapt to changing environmental conditions. Biodiversity is nature's insurance policy against disasters.

Some people also include human cultural diversity as part of the earth's biodiversity. The variety of human cultures represents numerous social and technological solutions to changing environmental conditions.

2-4 CONNECTIONS: FOOD WEBS AND ENERGY FLOW IN ECOSYSTEMS

What Are Food Chains and Food Webs?
All organisms, whether dead or alive, are potential sources of food for other organisms. A caterpillar eats a leaf, a robin eats the caterpillar, and a hawk eats the robin. Decomposers consume the leaf, caterpillar, robin, and hawk after they die. As a result, there is little waste in natural ecosystems.

The sequence of organisms, each of which is a source of food for the next, is called a food chain. It determines how energy and nutrients move from one organism to another through the ecosystem (Figure 2-19, p. 32).

Figure 2-19 A food chain. The arrows show how chemical energy in food flows through various trophic levels, or energy transfers. Most of the energy flowing through the chain is degraded to heat, in accordance with the second law of thermodynamics. Food chains rarely have more than four trophic levels. Can you explain why?

Ecologists assign each organism in an ecosystem to a feeding level, or trophic level (from the Greek word trophos, "nourishment"), depending on whether it is a producer or a consumer and on what it eats or decomposes. Producers belong to the first trophic level, primary consumers to the second trophic level, secondary (decomposers and detritus feeders) consumers to the third, and so on. Detritivores and decomposers process detritus from all trophic levels.

Real ecosystems are more complex than this. Most consumers feed on more than one type of organism, and most organisms are eaten by more than one type of consumer. Because most species participate in several different food chains, the organisms in most ecosystems form a complex network of interconnected food chains called a food web (Figure 2-20). Trophic levels can be assigned in food webs just as in food chains.

How Can We Represent the Energy Flow in an Ecosystem?
Pyramids of Energy Flow Each trophic level in a food chain or web contains a certain amount of biomass, the dry weight of all organic matter contained in its organisms. In a food chain or web, chemical energy stored in biomass is transferred from one trophic level to another.

The percentage of usable energy transferred as biomass from one trophic level to the next is called ecological efficiency. It ranges from 5% to 20% (that is, a loss of 80-95%) depending on the types of species and the ecosystem involved, but 10% is typical.

Assuming 10% ecological efficiency (90% loss) at each trophic transfer, if green plants in an area manage to capture 10,000 units of energy from the sun, then only about 1,000 units of energy will be available to support herbivores and only about 100 units to support carnivores.

The more trophic levels or steps in a food chain or web, the greater the cumulative loss of usable energy as energy flows through the various trophic levels. The pyramid of energy flow in Figure 2-21 (p. 34) illustrates this energy loss for a simple food chain, assuming a 90% energy loss with each transfer. Figure 2-22 (p. 34) shows the pyramid of energy flow during 1 year for an aquatic ecosystem in Silver Springs, Florida.

Figure 2-21 Generalized pyramid of energy flow showing the decrease in usable energy available at each succeeding trophic level in a food chain or web. In nature, ecological efficiency varies from 5% to 20%, with 10% efficiency being common. This model assumes a 10% ecological efficiency (90% loss in usable energy to the environment, in the form of lowquality heat) with each transfer from one trophic level to another.

        Top carnivores                                        Decomposers/detritivore
        Carnivores
        Herbivores
        Producers (cells, leaves, roots, and stems).
Figure 2-22 Annual pyramid of energy flow (in kilocalories per square meter per year) for an aquatic ecosystem in Silver Springs, Florida. (Adapted from Cecie Starr, Biology: Concepts and Applications, 4th ed., Pacific Grove, Calif.: Brooks/Cole, 2000)

Type of Ecosystem
Estuaries,   Swamps and marshes,   Tropical rain forest                                 (greatest)

Temperate forest
Northern coniferous forest (taiga)
Savanna
Agricultural land
Woodland and shrubland
Temperate grassland
Lakes and streams
Continental shelf
Open ocean
Tundra (arctic and alpine)
Desert scrub
Extreme desert                                     (least)

Figure 2-23 Estimated annual average net primary productivity (NPP) per unit of area in major life zones and ecosystems, expressed as kilocalories of energy produced per square meter per year (kcal/m2jyr). (Data from R. H. Whittaker, Communities and Ecosystems, 2nd ed., New York: Macmillan, 1975)

Energy flow pyramids explain why the earth can support more people if they eat at lower trophic levels by consuming grains, vegetables, and fruits directly (for example, grain human) rather than passing such crops through another trophic level and eating grain eaters (grain steer human).

The large loss in energy between successive trophic levels also explains why food chains and webs rarely have more than four or five trophic levels. In most cases, too little energy is left after four or five transfers to support organisms feeding at these high trophic levels. This explains why
(1) there are so few top carnivores such as eagles, hawks, tigers, and white sharks,
(2) such species usually are the first to suffer when the ecosystems that support them are disrupted, and
(3) these species are so vulnerable to extinction.

How Rapidly Do Producers in Different Ecosystems Produce Biomass?
The rate at which an ecosystem's producers convert solar energy into chemical energy as biomass is the ecosystem's gross primary productivity (GPP). However, to stay alive, grow, and reproduce, an ecosystem's producers must use some of the total biomass they produce for their own respiration. Only what is left, called net primary productivity (NPP), is available for use as food by other organisms (consumers) in an ecosystem:

Net primary productivity Rate at which producers storechemical energy as biomass (produced by photosynthesis) Rate at which producers use chemical energy stored as biomass (through aerobicrespiration)

Net primary productivity is the- rate at which energy for use by consumers is stored in new biomass (cells, leaves, roots, and stems).Various ecosystems and life zones differ in their NPP (Figure 2-23).

The most productive are (1) estuaries, (2) swamps and marshes, and (3) tropical rain forests. The least productive are (1) open ocean, (2) tundra (arctic and alpine grasslands), and (3) desert. Despite its low NPP, there is so much open ocean that it produces more of the earth's NPP per year than any of the other ecosystems and life zones shown in Figure 2-23. The earth's NPP is the upper limit determining the planet's carrying capacity for all consumer species.

How Much of the World's Net Rate of Biomass Production Do We Use?
Peter Vitousek and other ecologists estimate that humans now use, waste, or destroy about (1) 27% of the earth's total potential NPP and (2) 40% of the NPP of the planet's terrestrial ecosystems.

This is the main reason why we are crowding out or eliminating the habitats and food supplies of a growing number of other species. What might happen to us and to other consumer species if (1) the human population doubles over the next 40-50 years and (2) per capita consumption of resources such as food, timber, and grassland rises sharply?

2-5 CONNECTIONS: MATTER CYCLING IN ECOSYSTEMS

What Are Biogeochemical Cycles?
The nutrient atoms, ions, and compounds that organisms need to live, grow, and reproduce are cycled continuously from the nonliving environment (air, water, soil, and rock) to living organisms (biota) and then back again in what are called biogeochemical cycles (literally, life-earth-chemical cycles). These cycles, driven directly or indirectly by incoming solar energy and gravity, include the carbon, oxygen, nitrogen, phosphorus, and hydrologic (water) cycles (Figure 2-10).

The earth's chemical cycles also connect past, present, and future forms of life. Some of the carbon atoms in your skin may once have been part of a leaf, a dinosaur's skin, or a layer of limestone rock. Your grandmother, Plato, or a hunter-gatherer who lived 25,000 years ago may have inhaled some of the oxygen molecules you just inhaled.purifies, and distributes the earth's fixed supply of water, is shown in simplified form in Figure 2-24.

Figure 2.24 Simplified model of the hydrologic cycle.

How Is Water Cycled in the Biosphere?
The hydrologic cycle, or water cycle, which collects, purifies, and distributes the earth's fixed supply of water, is shown in simplified form in Figure 2-24.

The water cycle is powered by energy from the sun and by gravity. Incoming solar energy evaporates water from oceans, streams, lakes, soil, and vegetation. About 84% of water vapor in the atmosphere comes from the oceans, and the rest comes from land.

Some of the fresh water returning to the earth's surface as precipitation in this cycle becomes locked in glaciers. Most of the precipitation falling on terrestrial ecosystems becomes surface runoff flowing into streams and lakes, which eventually carry water back to the oceans, where it can be evaporated to cycle again.

Besides replenishing streams and lakes, surface runoff also causes soil erosion, which moves soil and weathered rock fragments from one place to another. Water is thus the primary sculptor of the earth's landscape. Because water dissolves many nutrient compounds, it is a major medium for transporting nutrients within and between ecosystems.

Throughout the hydrologic cycle, many natural processes act to purify water. Evaporation and subsequent precipitation act as a natural distillation process that removes impurities dissolved in water. Water flowing above ground through streams and lakes and

below ground in aquifers is naturally filtered and purified by chemical and biological processes. Thus the hydrologic cycle also can be viewed as a cycle of natural renewal of water quality.

How Are Human Activities Affecting the Water Cycle? We have been intervening in the earth's current water cycle by:

. Withdrawing large quantities of fresh water from streams, lakes, and underground sources.

. Clearing vegetation from land for agriculture, mining, road and building construction, and other activities. This (1) increases runoff, (2) reduces infiltration that recharges groundwater supplies, (3) increases the risk of flooding, and (4) accelerates soil erosion and landslides.

. Modifying water quality by (1) adding nutrients (such as phosphates and nitrates found in fertilizers) and other pollutants and (2) changing ecological processes that purify water naturally.

How Is Carbon Cycled in the Biosphere?
The carbon cycle (Figure 2-25) is based on carbon dioxide gas, which makes up 0.036% of the volume of the troposphere and is also dissolved in water. Carbon dioxide is a key component of nature's thermostat. If the carbon cycle removes too much CO2 from the atmosphere, the atmosphere will cool; if the cycle generates too much, the atmosphere will get warmer. Thus even slight changes in the carbon cycle can affect climate and ultimately the types of life that can exist on various parts of the planet.

Terrestrial producers remove CO2 from the atmosphere, and aquatic producers remove it from the water. They then use photosynthesis to convert CO2 into complex carbohydrates such as glucose (C6H1206)

The cells in oxygen-consuming producers, consumers, and decomposers then carry out aerobic respiration. This breaks down glucose and other complex organic compounds and converts the carbon back to CO2 in the atmosphere or water for reuse by producers. This linkage between photosynthesis in producers and aerobic respiration in producers, consumers, and decomposers circulates carbon in the biosphere and is a major part of the global carbon cycle. Oxygen and hydrogen, the other elements in carbohydrates, cycle almost in step with carbon.

Over millions of years, buried deposits of dead plant matter and bacteria are compressed between layers of sediment, where they form carbon-containing

fossil fuels such as coal and oil (Figure 2-25). This carbon is not released to the atmosphere as CO2 for recycling until (1) these fuels are extracted and burned, or (2) long-term geological processes expose these deposits to air. In only a few hundred years, we have extracted and burned fossil fuels that took millions of years to form. This is why fossil fuels are nonrenewable resources on a human time scale.

Figure 2-25 Simplified model of the global carbon cycle. The left portion shows the movement of carbon through marine ecosystems, and the right portion shows its movement through terrestrial ecosystems. Carbon reservoirs are shown as boxes; processes that change one form of carbon to another are shown in unboxed print. (From Cecie Starr, Biology: Concepts and Applications, 4th ed., Pacific Grove, Calif.: Brooks/Cole, 2000)

How Are Human Activities Affecting the Carbon Cycle?
We have been intervening in the earth's carbon cycle in two ways that add carbon dioxide to the atmosphere:

. Clearing trees and other plants that absorb CO2 through photosynthesis

. Adding large amounts of CO2 by burning fossil fuels and wood

Computer mo<;lels of the earth's climate systems suggest that increased concentrations of atmospheric CO2 and other gases we are adding to the atmosphere could enhance the planet's natural greenhouse effect that helps warm the lower atmosphere (troposphere) and the earth's surface (Figure 2-12). The resulting global warming could (1) disrupt global food production and wildlife habitats and (2) raise the average sea level in various parts of the world.

How Is Nitrogen Cycled in the Biosphere?
Bacteria in Action
Nitrogen is the atmosphere's most abundant element, with chemically unreactive nitrogen gas (N2) making up 78% of the volume of the troposphere. However, N2 cannot be absorbed and used directly as a nutrient by multicellular plants or animals.

Fortunately,
(1) atmospheric electrical discharges in the form of lightning and
(2) certain bacteria in the soil and aquatic systems convert nitrogen gas into compounds that can enter food webs as part of the nitrogen cycle (Figure 2-26).

The nitrogen cycle consists of several major steps:

. Nitrogen fixation, in which specialized bacteria convert gaseous nitrogen (N2) to ammonia (NH3) that can be used by plants. This is done mostly by
(1) cyanobacteria in soil and water and
(2) Rhizobium bacteria living in small nodules (swellings) on the root systems of a wide variety of plant species, including soybeans and alfalfa.

. Nitrification, a two-step process in which most of the ammonia in soil is converted by specialized aerobic bacteria to (1) nitrite ions (NO2 -), which are toxic to plants and (2) nitrate ions (NO3 -), which are easily taken up by plants as a nutrient. Animals in turn get their nitrogen by eating plants or plant-eating animals.

. Ammonification, in which vast armies of specialized decomposer bacteria convert the nitrogen-rich organic compounds, wastes, cast-off particles, and dead bodies of organisms into (1) simpler nitrogencontaining inorganic compounds such as ammonia (NH3) and (2) water-soluble salts containing ammonium ions (NH4 +).

Figure 2.26 Greatly simplified model of the nitrogen cycle in a terrestrial ecosystem. Nitrogen reservoirs are shown as boxes; processes changing one form of nitrogen to another are shown in circles. (Adapted from Cecie Starr and Ralph Taggart, Biology: The Unity and Diversity of Life, 9th ed., Pacific Grove, Calif.: Brooks/Cole, 2001)

. Denitrification, in which other specialized bacteria convert NH3 and NH4 + back into nitrite (NO2 -) and nitrate (NO3 -) ions and then into nitrogen gas (N2) and nitrous oxide gas (N2O). These are then released to the atmosphere to begin the cycle again.

How Are Human Activities Affecting the Nitrogen Cycle?
We intervene in the nitrogen cycle in several ways:

. Adding large amounts of nitric oxide (NO) into the atmosphere when we burn any fuel. In the atmosphere, this nitric oxide combines with oxygen to form nitrogen dioxide gas (NO2), which can react with water vapor to form nitric acid (HNO3). Droplets of HNO3 dissolved in rain or snow are components of acid deposition, commonly called acid rain. Nitric acid, along with other air pollutants, can
(1) damage and weaken trees,
(2) upset aquatic ecosystems,
(3) corrode metals, and
(4) damage marble, stone, and other building materials.

. Adding nitrous oxide (N2O) to the atmosphere through the action of anaerobic bacteria on livestock wastes and commercial inorganic fertilizers applied to the soil. When N2O reaches the stratosphere, it can
(1) help warm the atmosphere by enhancing the natural greenhouse effect and
(2) contribute to depletion of the earth's ozone shield, which filters out harmful ultraviolet radiation from the sun.

. Removing nitrogen from topsoil when we
(1) harvest nitrogen-rich crops,
(2) irrigate crops, and
(3) burn or clear grasslands and forests before planting crops.

. Adding nitrogen compounds to aquatic ecosystems in agricultural runoff and discharge of municipal sewage. This excess of plant nutrients stimulates rapid growth of photosynthesizing algae and other aquatic plants. The subsequent breakdown of dead algae by aerobic decomposers can
(1) deplete the water of dissolved oxygen and
(2) disrupt aquatic ecosystems by killing some types of fish and other oxygen-using (aerobic) organisms.

. Accelerating the deposition of acidic nitrogen compounds (such as NO2 and HNO3) from the atmosphere onto terrestrial ecosystems. This excessive input of nitrogen can stimulate the growth of weedy plant species, which can outgrow and perhaps eliminate other plant species that cannot take up nitrogen as efficiently.

How Is Phosphorus Cycled in the Biosphere? Phosphorus circulates through water, the earth's crust, and living organisms in the phosphorus cycle (Figure 2-27). Bacteria are less important here

Figure 2-27 Simplified model of the phosphorus cycle. Phosphorus reservoirs are shown as boxes; processes that change one form of phosphorus to another are shown in unboxed print. (From Cecie Starr and Ralph Taggart, Biology: The Unity and Diversity of Life, 9th ed., Pacific Grove, Calif.: Brooks/Cole, 2001)

than in the nitrogen cycle. Unlike carbon and nitrogen, very little phosphorus circulates in the atmosphere because at the earth's normal temperatures and pressures, phosphorus and its compounds are not gases.

Phosphorous typically is found as phosphate salts containing phosphate ions (POl-) in terrestrial rock formations and ocean bottom sediments. Because most soils contain little phosphate, it is often the limiting factor for plant growth on land unless phosphorus (as phosphate salts mined from the earth) is applied to the soil as a fertilizer. Phosphorus also limits the growth of producer populations in many freshwater streams and lakes because phosphate salts are only slightly soluble in water.

How Are Human Activities Affecting the Phosphorus Cycle?
We intervene in the earth's phosphorus cycle by

. Mining large quantities of phosphate rock for use in commercial inorganic fertilizers and detergents.

. Reducing the available phosphate in tropical forests by removing trees. When such forests are cut and burned, most remaining phosphorus and other soil nutrients are washed away by heavy rains, and the land becomes unproductive.

. Adding excess phosphate to aquatic ecosystems in
(1) runoff of animal wastes from livestock feedlots,
(2) runoff of commercial phosphate fertilizers from cropland, and
(3) discharge of municipal sewage. Too much of this nutrient causes explosive growth of cyanobacteria, algae, and aquatic plants. When these plants die and are decomposed, they use up dissolved oxygen and thus disrupt aquatic ecosystems.

Figure 2-28 The rock cycle, the slowest of the earth's cyclic processes. The earth's materials are recycled over millions of years by three processes: melting, erosion, and metamorphism, which produce igneous, sedimentary, and metamorphic rocks. Rock of any of the three classes can be converted to rock of either of the other two classes (or can even be recycled within its own class)

What Is the Rock Cycle?
Rock is any material that makes up a large, natural, continuous part of the earth's crust. Based on the way it forms, rock is placed in three broad classes:

. Igneous rock formed below or on the earth's surface when molten rock material (magma)
(1) wells up from the earth's upper mantle or deep crust,
(2) cools, and
(3) hardens into rock. Examples of igneous rock are
(a) granite (formed underground) and
(b) lava rock (formed above ground when molten lava cools and hardens).

Sedimentary rock formed from sediment when preexisting rocks are
(1) weathered and eroded into small pieces,
(2) transported from their sources, and
(3) deposited in a body of surface water. As deposited layers from weathering and erosion become buried and compacted, the resulting pressure causes their particles to bond together to form sedimentary rocks such as sandstone and shale.

Metamorphic rock produced when a preexisting rock is subjected to
(1) high temperatures (which may cause it to melt partially),
(2) high pressures,
(3) chemically active fluids, or
(4) a combination of these agents. Examples are anthracite (a form of coal), slate, and marble.

Rocks are constantly exposed to various physical and chemical conditions that can change them over time. The interaction of processes that change rocks from one type to another is called the rock cycle (Figure 2-28). The slowest of the earth's cyclic processes, the rock cycle recycles material over millions of years. It concentrates the planet's nonrenewable mineral resources on which we depend.


All things come from earth, and to earth they all return. MENANDER (342-290 B.C.)

2_REVIEW QUESTIONS

1. Define the boldfaced terms in this chapter.

2. Define science and explain how it works. Distinguish among scientific data, scientific hypothesis, scientific model,

scientific theory, and scientific law. Explain why a scientific theory should be taken seriously.

3. What is a controlled experiment? What is multivariable analysis?

4. What does it mean to say that something has been scientifically proven? If scientists cannot establish absolute proof, what do they establish?

5. Distinguish between frontier science and consensus science.

6. Distinguish among matter, elements, compounds, and mixtures.

7. Distinguish among atoms, ions, and compounds, and give an example of each. What three major types of subatomic particles are found in atoms? Which two of these particles are found in the nucleus, and which is found outside the nucleus?

8. Distinguish between atomic number and mass number. What is an isotope of an atom?

9. Distinguish between high-quality matter and low-quality matter, and give an example of each. What is material efficiency?

10. What is energy? Distinguish between kinetic energy and potential energy, and give an example of each.

11. Distinguish between high-quality energy and lowquality energy, and give an example of each. What is energy efficiency?

12. Distinguish between a physical change and a chemical change, and give an example of each.

13. What is the law of conservation of matter? Explain why there is no "away" as a repository for pollution.

14. Distinguish between the first law of thermodynamics and the second law of thermodynamics, and give an example of each law in action. Use the second law of thermodynamics to explain why energy cannot be recycled.

15. What is ecology? What five levels of organization of matter are the main focus of ecology? Distinguish among organism, cell, species, population, genetic diversity, habitat, community, ecosystem, and biosphere.

16. Why are insects important for many forms of life and for you and your lifestyle?

17. Explain why microbes (microorganisms) are so important.

18. Distinguish among the atmosphere, troposphere, stratosphere, hydrosphere, lithosphere, and biosphere.

19. What three processes sustain life on earth? What is the natural greenhouse effect?

20. Distinguish between the abiotic and biotic components of ecosystems, and give three examples of each component.

21. Distinguish between range of tolerance for a population in an ecosystem and the law of tolerance. How does each of these factors affect the composition (structure) of ecosystems?

22. What is a limiting factor, and how do such factors affect the composition of ecosystems? What are two important limiting factors for terrestrial ecosystems and for aquatic ecosystems?

23. Distinguish between producers and consumers in ecosystems, and give three examples of each type. What is photosynthesis, and why is it important to both producers and consumers? What is chemosynthesis? What is aerobic respiration?

24. Distinguish among primary consumers (herbivores), secondary consumers (carnivores), tertiary consumers, omnivores, scavengers, detritivores, detritus feeders, and decomposers. Why are decomposers important, and what would happen without them?

25. What are the four components of biodiversity? Why is biodiversity important to (a) the earth's life-support systems and (b) the economy?

26. Distinguish among a food chain, trophic level, biomass, ecological efficiency, pyramid of energy flow, and food web.

27. Distinguish between gross primary productivity (GPP) and net primary productivity (NPP). Explain how NPP affects the number of consumers in an ecosystem and on the earth. List two of the most productive ecosystems and two of the least productive ecosystems.

28. About what percentages of total potential NPP of (a) the entire earth and (b) the earth's terrestrial ecosystems are used, wasted, or destroyed by humans?

29. What is a biogeochemical cycle? Describe the water, carbon, nitrogen, and phosphorus biogeochemical cycles. Describe how human activities are affecting each of these cycles.

30. Distinguish among igneous, sedimentary, and metamorphic rock, and describe how they are connected in the rock cycle.


CRITICAL THINKING

1. Respond to the following statements:
(a)  Scientists have not absolutely proven that anyone has ever died from smoking cigarettes.
(b)  The greenhouse theory—that certain gases (such as water vapor and carbon dioxide) warm the atmosphere—is not a reliable idea because it is only a scientific theory.

2. See whether you can find an advertisement or article describing some aspect of science in which
(a) the concept of scientific proof is misused,
(b) the term theory is used when it should have been hypothesis, and
(c) a consensus scientific finding is dismissed or downplayed because it is "only a theory."

3. A tree grows and increases its mass. Explain why this is not a violation of the law of conservation of matter.

4. If there is no "away," why is the world not filled with waste matter?

5. Use the second law of thermodynamics to explain why a barrel of oil can be used only once as a fuel.

6.
(a) Imagine you have the power to violate the law of conservation of energy (the first law of thermodynamics) for 1 day. What are the three most important things you would do with this power?
(b) Repeat this process, imagining you have the power to violate the second law of thermodynamics for 1 day.

7.
(a) A bumper sticker asks, "Have you thanked a green plant today?" Give two reasons for appreciating a green plant.
(b) Trace the sources of the materials that make up the bumper sticker, and decide whether the sticker itself is a sound application of the slogan.
(c) Explain how decomposers help keep you alive.

8. Using the second law of thermodynamics, explain why
(a) there is such a sharp decrease in usable energy as energy flows through a food chain or web and
(b) many poor people in developing countries live on a mostly vegetarian diet.

9. What would happen to an ecosystem if
(a) all its decomposers and detritus feeders were eliminated or
(b) all its producers were eliminated?


INFOTRAC COLLEGE EDITION

1. Pauly, D., V. Christensen, R. Froese, and M. L. Palomares. 2000. Fishing down aquatic food webs. American Scientist 88: 46. Keywords: "fishing" and "aquatic food webs." As pressure on fishing becomes more intense, humans are catching and keeping ever smaller fish. This has implications for the entire aquatic food web, including the recovery of higher-trophic-Ievel species. This work presents a new way of looking at fisheries science using basic ecological principles.

2. Lockaby, B. G., and W. H. Conner. 1999. N:P balance in wetland forests: Productivity across a biogeochemical continuum. The Botanical Review 65: 171. Keywords; "wetland forests" and "productivity." This paper integrates the concepts of biogeochemical cycling, limiting factors, and productivity by looking at differences among wetland forests.


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