Key Concepts

After completing this chapter, you will be able to

1. Describe matter and its forms.
2. Describe the nature of energy, its various forms, and the laws that govern its transformations.
3. Explain how Earth is a flow-through system for solar energy.
4. Identify the three major components of Earth’s energy budget.
5. Describe energy relationships within ecosystems, including the fixation of solar energy by primary producers and the passage of that fixed energy through other components of the ecosystem.
6. Explain why the trophic structure of ecological productivity is pyramid-shaped and why ecosystems cannot support many top predators.
7. Compare the feeding strategies of humans living a hunting and gathering lifestyle with those of modern urban people.

Introduction

None of planet Earth, its biosphere, or ecosystems at any scale are self-sustaining with respect to energy. In fact, without continuous access to an external source of energy, all of these entities would quickly deplete their quantities of stored energy and would rapidly cool, and in the case of the biosphere and ecosystems, would cease to function in ways that support life. The external source of energy to those systems is solar energy, which is stored mainly as heat and biomass. In effect, solar energy is absorbed by green plants and algae and is utilized to fix carbon dioxide and water into simple sugars through a process known as photosynthesis. This biological fixation of solar energy provides the energetic basis for almost all organisms and ecosystems (the few exceptions are described later). Energy is critical to the functioning of physical processes throughout the universe, and of ecological processes in the biosphere of Earth. Matter, on the other hand, is conserved on Earth, and largely exists in fixed quantities in the form of different chemical elements and compounds. In this chapter we will examine the physical nature of matter and energy, the laws that govern their behavior and transformations, and their roles in ecosystems.

Matter

At its most fundamental level, life is made up of matter. Matter is any substance that occupies space and has mass and can exist in three physical states—solid, liquid, and gas. Matter consisting of every element can exist in each of the three physical states, it is just a matter of subjecting it to enough freezing and high pressure (to solidify gases, even helium) or enough heat and low pressure (to vaporize metals, even tungsten). Though related, mass is not the same as weight. For example, the same object on the Moon would weigh less than it does on Earth because of differences in gravity, but it would still have the same mass. The important distinction between mass and weight is that weight is a force, but mass is the property of the object that (along with gravity) determines the strength of this force.

Elements are unique forms of matter with specific chemical and physical properties that cannot be broken down into smaller substances by ordinary chemical reactions. There are 118 elements, but only 92 occur naturally. The remaining elements are synthesized in laboratories and are unstable. Each element is designated by its chemical symbol, which is a single capital letter or, when the first letter is already “taken” by another element, a combination of two letters. Some elements follow the English term for the element, such as C for carbon and Ca for calcium. Other elements’ chemical symbols derive from their Latin names; for example, the symbol for sodium is Na, referring to natrium, the Latin word for sodium.

About 25 elements are essential to life, though six elements constitute most of the biomass of all living organisms: oxygen (O), carbon (C), hydrogen (H), phosphorus (P), sulfur (S), and nitrogen (N). In the non-living world, elements are found in different proportions, and some elements common to living organisms are relatively rare on the earth, as shown in Table 3.1. For example, the atmosphere is rich in nitrogen and oxygen but contains little carbon and hydrogen, while the earth’s crust, although it contains oxygen and a small amount of hydrogen, has little nitrogen and carbon. Despite their differences in abundance, all elements, and the chemical reactions between them, obey the same chemical and physical laws regardless of whether they are a part of the living or non-living world.

Table 3.1. Approximate Percentage of Elements in Living Organisms (Humans) Compared to the Non-living World.

Element	Life (Humans)	Atmosphere	Earth’s Crust
Oxygen (O)	65%	21%	46%
Carbon (C)	18%	trace	trace
Hydrogen (H)	10%	trace	0.1%
Nitrogen (N)	3%	78%	trace

The Structure of the Atom

To understand how elements come together, we must first discuss the smallest component or building block of an element, the atom. An atom is the smallest unit of matter that retains all of the chemical properties of an element. For example, one gold atom has all of the properties of gold in that it is a solid metal at room temperature. A gold coin is simply a very large number of gold atoms molded into the shape of a coin and containing small amounts of other elements known as impurities. Gold atoms cannot be broken down into anything smaller while still retaining the properties of gold.

An atom is composed of two regions: the nucleus, which is in the center of the atom and contains protons and neutrons, and the outermost region of the atom which holds its electrons in orbit around the nucleus (Figure 3.1). Atoms contain protons, electrons, and neutrons, among other subatomic particles. The only exception is hydrogen (H), which is made of one proton and one electron with no neutrons.

Figure 3.1. The Atom. Elements, such as helium, depicted here, are made up of atoms. Atoms are made up of protons and neutrons located within the nucleus, with electrons in orbitals surrounding the nucleus.

Protons and neutrons have approximately the same mass, about 1.67 × 10^-24 grams. Scientists arbitrarily define this amount of mass as one atomic mass unit (amu) or one Dalton, as shown in Table 3.2. Although similar in mass, protons and neutrons differ in their electric charge. A proton is positively charged whereas a neutron is uncharged. Therefore, the number of neutrons in an atom contributes significantly to its mass, but not to its charge. Electrons are much smaller in mass than protons, weighing only 9.11 × 10^-28 grams, or about 1/1800 of an atomic mass unit. Hence, they do not contribute much to an element’s overall atomic mass. Therefore, when considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom’s mass based on the number of protons and neutrons alone. Although not significant contributors to mass, electrons do contribute greatly to the atom’s charge, as each electron has a negative charge equal to the positive charge of a proton. In uncharged, neutral atoms, the number of electrons orbiting the nucleus is equal to the number of protons inside the nucleus. In these atoms, the positive and negative charges cancel each other out, leading to an atom with no net charge.

Accounting for the sizes of protons, neutrons, and electrons, most of the volume of an atom—greater than 99 percent—is, in fact, empty space. With all this empty space, one might ask why so-called solid objects do not just pass through one another. The reason they do not is that the electrons that surround all atoms are negatively charged and negative charges repel each other.

Table 3.2. Protons, Neutrons, and Electrons.

	Charge	Mass (amu)	Location
Proton	+1	1	nucleus
Neutron	0	1	nucleus
Electron	–1	0	orbitals

Atomic Number and Mass

Atoms of each element contain a characteristic number of protons and electrons. The number of protons determines an element’s atomic number and is used to distinguish one element from another. The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess. Together, the number of protons and the number of neutrons determine an element’s mass number. Note that the small contribution of mass from electrons is disregarded in calculating the mass number. This approximation of mass can be used to easily calculate how many neutrons an element has by simply subtracting the number of protons from the mass number. Since an element’s isotopes will have slightly different mass numbers, scientists also determine the atomic mass, which is the calculated mean of the mass number for its naturally occurring isotopes. Often, the resulting number contains a fraction. For example, the atomic mass of chlorine (Cl) is 35.45 because chlorine is composed of several isotopes, some (the majority) with atomic mass 35 (17 protons and 18 neutrons) and some with atomic mass 37 (17 protons and 20 neutrons).

Isotopes

Isotopes are different forms of an element that have the same number of protons but a different number of neutrons. Some elements—such as carbon, potassium, and uranium—have naturally occurring isotopes. For example, the three most common isotopes of carbon are carbon-12 (with six protons and six neutrons), carbon-13 (with six protons and seven neutrons), and carbon-14 (with six protons and eight neutrons). Carbon-12 is by far the most abundant isotope of carbon and makes up about 98.9% of all naturally occurring carbon. Some isotopes may emit neutrons, protons, and electrons, and attain a more stable atomic configuration (lower level of potential energy); these are radioactive isotopes, or radioisotopes. Radioactive decay (e.g., carbon-14 losing neutrons to eventually become carbon-12) describes the energy loss that occurs when an unstable atom’s nucleus releases radiation (Figure 3.2).

Figure 3.2. Radioactive Decay. Schematic of ¹⁴C production and decay in the atmosphere. ¹⁴C is produced in the atmosphere by cosmic neutrons colliding with nitrogen atoms. The newly formed ¹⁴C is oxidized to ¹⁴CO₂ where it then enters the biosphere. Following an organism’s death, radioactive decay occurs converting the ¹⁴C back to ¹⁴N. Source: used with permission from Australian National University by Dr. Stewart Fallon.

The Periodic Table

The different elements are organized and displayed in the periodic table (Figure 3.3). Devised by Russian chemist Dmitri Mendeleev (1834–1907) in 1869, the table groups elements that, although unique, share certain chemical properties with other elements. Each horizontal row in the table is called a period, and each vertical column, which lists elements with similar chemical properties, is called a group. Depending on their properties, elements can be classified as metals, nonmetals, and metalloids. Atoms of metals tend to lose electrons to form positively charged ions, and include sodium (Na), calcium (Ca), aluminum (Al), iron (Fe), lead (Pb), silver (Ag), and mercury (Hg). For example, an atom of the metallic element Na, which contains 11 positively charged protons and 11 negatively charged electrons, can lose one of its electrons. It then becomes a sodium ion with a positive charge of 1 because it now has 11 positive charges (protons), but only 10 negative charges (electrons). Atoms of nonmetals, such as chlorine (Cl), oxygen (O), and sulfur (S), tend to gain electrons lost by metallic atoms and form negatively charged ions. For example, an atom of the nonmetallic element chlorine Cl can gain an electron and become a chlorine ion. The ion has a negative charge of 1 because it has 17 positively charged protons and 18 negatively charged electrons. The elements arranged in a diagonal staircase pattern between the metals and nonmetals are called metalloids because they have a mixture of metallic and nonmetallic properties (e.g., germanium (Ge) and arsenic (As)).

Some elements are required by living organisms as nutrients (e.g., N, P, Ca), while other elements are moderately or highly toxic to all or most forms of life (e.g., Hg, Pb, As). Note that some elements such as copper (Cu) serve as nutrients, but can also be toxic at high enough doses. As previously mentioned, six nonmetallic elements—carbon (C), oxygen (O), hydrogen (H), nitrogen (N), sulfur (S), and phosphorus (P)—make up about 99% of the atoms of all living things (organic compounds, defined as compounds containing C and H atoms, are essential to life on Earth).

The properties of elements are responsible for their physical state at room temperature: they may be gases, solids, or liquids. Elements also have specific chemical reactivity, the ability to combine and to chemically bond with each other. In addition to providing the atomic number for each element, the periodic table also displays the element’s atomic mass. Looking at carbon, for example, its symbol (C) and name appear, as well as its atomic number of six (in the upper left-hand corner) and its atomic mass of 12.11.

Figure 3.3. The Periodic Table. The periodic table shows the atomic mass and atomic number of each element. The atomic number appears above the symbol for the element and the approximate atomic mass appears below it. Source: “Periodic Table of Elements” by 2012rc is licensed under CC BY 3.0.

The periodic table groups elements according to chemical properties. The differences in chemical reactivity between the elements are based on the number and spatial distribution of an atom’s electrons. Atoms that chemically react and bond to each other form molecules. Molecules are simply two or more atoms chemically bonded together. Logically, when two atoms chemically bond to form a molecule, their electrons, which form the outermost region of each atom, come together first as the atoms form a chemical bond. Most matter consists of compounds, which are combinations of two or more different elements held together in fixed proportions.

Matter Undergoes Physical and Chemical Change

When a sample of matter undergoes a physical change, such as between a liquid and a gas, there is no change in its chemical composition. When a chemical change, or chemical reaction, takes place, there is a change in the chemical composition of the substances involved. Chemical reactions occur when two or more atoms bond together to form molecules or when bonded atoms are broken apart. The substances used in the beginning of a chemical reaction are called the reactants (usually found on the left side of a chemical equation), and the substances found at the end of the reaction are known as the products (usually found on the right side of a chemical equation). An arrow is typically drawn between the reactants and products to indicate the direction of the chemical reaction; this direction is not always a “one-way street.” For the creation of the water molecule shown above, the chemical equation would be:

2H + O → H₂O₂H + O → H₂O

An example of a simple chemical reaction is combustion (Figure 3.4). Combustion is a chemical process in which a substance reacts rapidly with oxygen and gives off heat. For example, coal is made up almost entirely of the element carbon (C). When coal is burned completely in a power plant, the solid carbon in the coal combines with oxygen gas from the atmosphere to form the gaseous compound carbon dioxide. Carbon dioxide is the principal product of combustion of fossil fuels since carbon accounts for 60–90 percent of the mass of fuels that we burn. The original substance is called the fuel, and the source of oxygen is called the oxidizer. During combustion, new chemical substances are created from the fuel and the oxidizer. These substances are called exhaust. Most of the exhaust comes from chemical combinations of the fuel and oxygen. When a hydrogen-carbon-based fuel (like gasoline) burns, the exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon + oxygen). But the exhaust can also include chemical combinations from the oxidizer alone. If the gasoline is burned in air, which contains 21% oxygen and 78% nitrogen, the exhaust can also include nitrous oxides (NO_x, nitrogen + oxygen), which are potent greenhouse gases.

Figure 3.4. Combustion. Three things must be present for combustion to occur: a fuel to be burned, a source of oxygen, and a source of heat. Exhausts (such as CO2) are created and heat is released as a result of combustion. Source: NASA.

Ions and Ionic Bonds

An ion is an atom or a group of atoms with one or more net positive or negative electrical charges (Figure 3.5). Like atoms, ions are made up of protons, neutrons, and electrons (more electrons than protons = negative charge; more protons than electrons = positive charge). Sodium chloride (NaCl) consists of a three-dimensional network of oppositely charged ions held together by the forces of attraction between opposite charges. The strong forces of attraction between such oppositely charged ions are called ionic bonds. They are formed when an electron is transferred from a metallic atom such as sodium (Na) to a nonmetallic element such as chlorine (Cl). Because ionic compounds consist of ions formed from atoms of metallic elements (positive ions) and nonmetallic elements (negative ions), they can be described as metal–nonmetal compounds. Sodium chloride and many other ionic compounds tend to dissolve in water and break apart into their individual ions. Ions are also important for measuring a substance’s acidity in a water solution, a chemical characteristic that helps determine how a substance dissolved in water will interact with and affect its environment. The acidity of a water solution is based on the comparative amounts of hydrogen ions (H⁺) and hydroxide ions (OH^–) contained in a particular volume of the solution. Scientists use pH as a measure of acidity. Pure water (not tap water or rainwater) has an equal number of H⁺ and OH^– ions. It is called a neutral solution and has a pH of 7. An acidic solution has more hydrogen ions than hydroxide ions and has a pH less than 7. A basic solution has more hydroxide ions than hydrogen ions and has a pH greater than 7. Cations are positive ions that are formed by losing electrons. Negative ions are formed by gaining electrons and are called anions. Anions are designated by their elemental name being altered to end in “-ide”: the anion of chlorine is called chloride, and the anion of sulfur is called sulfide, for example.

Inorganic ions in animals and plants are necessary for cellular activity. In body tissues, ions are also known as electrolytes, essential for the electrical activity needed to support muscle contractions and neuron activation. Many sports drinks and dietary supplements provide these ions to replace those lost from the body via sweating during exercise. They perform several other important functions in living organisms, such as biological messengers (e.g., Ca²⁺), antioxidants (e.g., Zn²⁺), osmotic balance (e.g., K⁺), protein synthesis (NO₃^–), and bone formation (PO₃^–).

Figure 3.5. Ions. A hydrogen atom (center) contains a single proton and a single electron. The removal of an electron produces a cation (left), while the addition of an electron produces an anion (right). Source: “Ions” by Jkwchui is licensed by CC BY-SA 3.0.

Ionic bonds are formed between ions with opposite charges (Figure 3.6). For instance, positively charged sodium ions and negatively charged chloride ions bond together to make crystals of sodium chloride, or table salt, creating a crystalline molecule with zero net charge.

Figure 3.6. Ionic Bond. In the formation of an ionic compound, metals lose electrons and nonmetals gain electrons to achieve an octet (a stable group of 8 electrons in an atom’s outer shell).

Covalent Bonds

Another way an octet of electrons can form is by the sharing of electrons between atoms to form covalent bonds. These bonds are stronger and much more common than ionic bonds in the molecules of living organisms. Covalent bonds are commonly found in carbon-based organic molecules, such as our DNA and proteins. Covalent bonds are also found in inorganic molecules like H₂O, CO₂, and O₂. One, two, or three pairs of electrons may be shared, making single, double, and triple bonds, respectively. The more covalent bonds between two atoms, the stronger their connection. Thus, triple bonds are the strongest.

The strength of different levels of covalent bonding is one of the main reasons living organisms have a difficult time in acquiring nitrogen for use in constructing their molecules, even though molecular nitrogen, N₂, is the most abundant gas in the atmosphere. Molecular nitrogen consists of two nitrogen atoms triple bonded to each other and, as with all molecules, the sharing of these three pairs of electrons between the two nitrogen atoms allows for the filling of their outer electron shells, making the molecule more stable than the individual nitrogen atoms. This strong triple bond makes it difficult for living systems to break apart this nitrogen in order to use it as constituents of proteins and DNA.

The formation of water molecules provides an example of covalent bonding (Figure 3.7). The hydrogen and oxygen atoms that combine to form water molecules are bound together by covalent bonds. The electron from the hydrogen splits its time between the incomplete outer shell of the hydrogen atoms and the incomplete outer shell of the oxygen atoms. To completely fill the outer shell of oxygen, which has six electrons in its outer shell but which would be more stable with eight, two electrons (one from each hydrogen atom) are needed: hence the well-known formula H₂O. The electrons are shared between the two elements to fill the outer shell of each, making both elements more stable.

Figure 3.7. Covalent Bonding. Two or more atoms may bond with each other to form a molecule. When two hydrogens and an oxygen share electrons via covalent bonds, a water molecule is formed.

Hydrogen Bonds and Van Der Waals Interactions

Ionic and covalent bonds between elements require energy to break. Ionic bonds are not as strong as covalent, which determines their behavior in biological systems. However, not all bonds are ionic or covalent bonds. Weaker bonds can also form between molecules. Two weak bonds that occur frequently are hydrogen bonds and van der Waals interactions. Without these two types of bonds, life as we know it would not exist. Hydrogen bonds provide many of the critical, life-sustaining properties of water and also stabilize the structures of proteins and DNA, the building block of cells.

When polar covalent bonds containing hydrogen form, the hydrogen in that bond has a slightly positive charge because hydrogen’s electron is pulled more strongly toward the other element and away from the hydrogen. Because the hydrogen is slightly positive, it will be attracted to neighboring negative charges. When this happens, a weak interaction occurs between the δ⁺ of the hydrogen from one molecule and the δ^– charge on the more electronegative atoms of another molecule, usually oxygen or nitrogen, or within the same molecule. This interaction is called a hydrogen bond. This type of bond is common and occurs regularly between water molecules. Individual hydrogen bonds are weak and easily broken; however, they occur in very large numbers in water and in organic polymers, creating a major force in combination. Hydrogen bonds are also responsible for zipping together the DNA double helix.

Like hydrogen bonds, van der Waals interactions are weak attractions or interactions between molecules. Van der Waals attractions can occur between any two or more molecules and are dependent on slight fluctuations of the electron densities, which are not always symmetrical around an atom. For these attractions to happen, the molecules need to be very close to one another. These bonds—along with ionic, covalent, and hydrogen bonds—contribute to the three-dimensional structure of the proteins in our cells that is necessary for their proper function.

Organic Compounds are the Foundation of Life

Carbon has the unique ability to form a variety of stable covalent bonds with many elements because it has four valence electrons. The ability to form four covalent bonds allows carbon, in combination with other elements, to form a large variety of both simple and complex molecules. While carbon can bind with itself to form linear chains, such as hexane (C₆H₁₄), carbon can also form ringed molecules, such as benzene (C₆H₆).

Nearly all of the ~10 million biological molecules described can be placed into one of four categories: carbohydrates, lipids, proteins and nucleic acids (Figure 3.8). Simple carbohydrates (i.e., sugars) provide energy to cells via cellular respiration or fermentation. More complex carbohydrates can serve other functions, such as energy storage (i.e., starch) or structural support (i.e., cellulose and lignin in plants). Carbohydrates consist of carbon, hydrogen, and oxygen, usually with a hydrogen:oxygen atom ratio of 2:1. Lipids, which are water-insoluble hydrocarbons, are a diverse group of molecules that serve many different functions in living organisms, including the foundation for cell membranes (phospholipids), energy storage found in fats and oils, and vehicles for cellular messaging (e.g., steroids). Proteins, formed from long chains of amino acids, serve as the cellular machinery and collectively provide many functions including: speeding up chemical reactions, transportation of materials into and out of the cell, and protection from infections. Nucleic acids, which are composed of nucleotides, include DNA and RNA, which serve as the informational storage and messaging required for the production of the cell’s proteins.

Figure 3.8. Organic Compounds. All living organisms are composed of the same basic building blocks of macromolecules: carbohydrates, lipids, proteins, and nucleic acids (DNA/RNA). Source: “Introduction to Chemistry” is licensed under CC BY-SA 4.0.

The Nature of Energy

Energy is a fundamental physical entity and is simply defined as the capacity of a body or system to accomplish work. In physics, work is defined as the result of a force being applied over a distance. In all of the following examples of work, energy is transformed and some measurable outcome is achieved:

• A hockey stick strikes a puck, causing it to speed toward a target
• A book is picked up from the floor, lifted, and then laid on a table
• A vehicle is driven along a road
• Heat from a stove is absorbed by water in a kettle, causing it to become hotter and eventually to boil
• The photosynthetic pigment chlorophyll absorbs sunlight, converting the electromagnetic energy into a form that plants and algae can utilize to synthesize sugars

Energy can exist in various states, each of which is fundamentally different from the others. However, under suitable conditions energy in any state can be converted into another one through physical or chemical transformations. The states of energy can be grouped into three categories: electromagnetic, kinetic, and potential.

Electromagnetic Energy

Electromagnetic energy (or electromagnetic radiation) is associated with photons. These have properties of both particles and waves and travel through space at a constant speed of 3 × 108 m/s (the speed of light). Electromagnetic energy exists as a continuous spectrum of wavelengths, which (ordered from the shortest to longest wavelengths) are known as gamma, X-ray, ultraviolet, visible light, infrared, microwave, and radio (Figure 3.9). The human eye can perceive electromagnetic energy over a range of wavelengths of about 0.4 to 0.7 μm, a part of the spectrum that is referred to as visible radiation or light (1 μm, or 1 micrometer, is 10^–6 m; see Appendix A).

Figure 3.9. The Electromagnetic Spectrum. The spectrum is divided into major regions on the basis of wavelength and is presented on a logarithmic scale (log10) in units of micrometers (1 μm = 10^–3 mm = 10^–6 m). Note the visible component and the wavelength ranges for red, orange, yellow, green, blue, and violet colors as seen above. Source: “EM Spectrum Properties” by Inductiveload, NASA is licensed under CC BY-SA 3.0.

Electromagnetic energy is given off (or radiated) by all objects that have a surface temperature greater than absolute zero (greater than -273°C or 0°K). The surface temperature of a body determines the rate and spectral quality of the radiation it emits. Compared with a cooler body, a hotter one has a much larger rate of emission, and the radiation is dominated by shorter, higher-energy wavelengths. For example, the Sun has an extremely hot surface temperature of about 6000°C, and as a direct consequence most of the radiation it emits is ultraviolet (0.2 to 0.4 μm), visible (0.4 to 0.7 μm), and near infrared (0.7 to 2 μm). (Note that the interior of the Sun is much hotter than 6000°C, but it is the surface temperature that directly influences the emitted radiation.) Because the surface temperature of Earth averages much cooler at about 15°C, it radiates much smaller amounts of energy at longer wavelengths (peaking at a wavelength of about 10 μm).

Image 3.2. The Sun. The energy of the Sun is derived from nuclear fusion reactions involving hydrogen nuclei. These reactions generate enormous quantities of thermal and electromagnetic energy. Solar electromagnetic radiation is the most crucial source of energy that sustains ecological and biological processes. Source: NASA.

Kinetic Energy

Kinetic energy is associated with bodies that are in motion. Two classes of kinetic energy can be distinguished:

Mechanical kinetic energy is associated with any object that is in motion, meaning it is travelling from one place to another. For example, a hockey puck flying through the air, a trail-bike being ridden along a path, a deer running through a forest, water flowing in a stream, or a planet moving through space are all expressions of this kind of kinetic energy. The amount of mechanical kinetic energy is determined by the mass of an object and its speed.

Thermal kinetic energy is associated with the rate that atoms or molecules are vibrating. Such vibrations are frozen at -273°C (absolute zero), but are progressively more vigorous at higher temperatures, corresponding to a larger content of thermal kinetic energy, which is also referred to as heat.

Potential Energy

Potential energy is the stored ability to perform work. To actually perform work, potential energy must be transformed into electromagnetic or kinetic energy. There are a number of kinds of potential energy:

Gravitational potential energy results from gravity, or the attractive forces that exist among all objects. For example, water stored at any height above sea level contains gravitational potential energy. This can be converted into kinetic energy if there is a pathway that allows the water to flow downhill. Gravitational potential energy can be converted into electrical energy through the technology of hydroelectric power plants.

Chemical potential energy is stored in the bonds between atoms within molecules. Chemical potential energy can be liberated by exothermic reactions (those which lead to a net release of thermal energy), as in the following examples:

• Chemical potential energy is stored in the molecular bonds of sulfide minerals, such as iron sulfide (FeS2), and some of this energy is released when the sulfides are oxidized. Specialized bacteria can metabolically tap the potential energy of sulfides to support their own productivity, through a process known as chemosynthesis (this is further examined later in this chapter).
• The ionic bonds of salts also store chemical potential energy. For example, when sodium chloride (table salt, NaCl) is dissolved in water, ionic potential energy is released as heat, which slightly increases the water temperature.
• Hydrocarbons store energy in the bonds between their hydrogen and carbon atoms (hydrocarbons contain only these atoms). The chemical potential energy of gasoline, a mixture of liquid hydrocarbons, is liberated in an internal combustion engine and becomes mechanically transformed to achieve the kinetic energy of vehicular motion.
• Organic compounds (biochemicals) produced metabolically by organisms also store large quantities of potential energy in their inter-atomic bonds (Image 3.3). The typical energy density of carbohydrates is about 16.8 kJ/g, while that of proteins is 21.0 kJ/g, and lipids (or fats) 38.5 kJ/g. Many organisms store their energy reserves as fat because these biochemicals have such a high energy density.

Electrical potential energy results from differences in the quantity of electrons, which are subatomic, negatively charged particles that flow from areas of high density to areas where it is lower. When an electrical switch is used to complete a circuit connecting two areas with different electrical potentials, electrons flow along the electron gradient. The electric energy may then be transformed into uses as light, heat, or work performed by a machine. A difference in electrical potential is known as voltage, and the current of electrons must flow through a conducting material, such as a metal.

Elasticity is a kind of potential energy that is inherent in the physical qualities of certain flexible materials and that can perform work when released, as occurs when a drawn bow is used to shoot an arrow.

Compressed gases also store potential energy, which can do work if expansion is allowed to occur. This type of potential energy is present in a cylinder containing compressed or liquefied gas.

Nuclear potential energy results from the extremely strong binding forces that exist within atoms. This is by far the densest form of energy. Huge quantities of electromagnetic and kinetic energy are liberated when nuclear reactions convert matter into energy. A fission reaction involved the splitting of isotopes of certain heavy atoms, such as uranium-235 and plutonium-239 (²³⁵U and ²³⁹P), to generate smaller atoms plus enormous amounts of energy. Fission reactions occur in nuclear explosions and, under controlled conditions, in nuclear reactors used to generate electricity. A fusion reaction involves the combining of certain light elements, such as hydrogen, to form heavier atoms under conditions of extremely high temperature and pressure, while liberating huge quantities of energy. Fusion reactions involving hydrogen occur in stars and are responsible for the unimaginably large amounts of energy that these celestial bodies generate and radiate into space. It is thought that all heavy atoms in the universe were produced by fusion reactions occurring in stars (see Chapter 4). Fusion reactions also occur in a type of nuclear explosive device known as a hydrogen bomb. Technology has not yet been developed to exploit controlled fusion reactions to generate electricity; if and when available, controlled fusion could be used to generate virtually unlimited amounts of commercial energy (see Chapter 18).

Image 3.3. Energy Transformation. Organic matter and fossil fuels contain potential chemical energy, which is released during combustion to generate heat and electromagnetic radiation. This forest fire was ignited by lightning and burned the organic matter of a pine forest. Source: NASA.

Units of Energy

Although energy can exist in various forms, all of them can be measured in the same or equivalent units. The internationally accepted system for scientific units is the SI system (Système International d’Unités), and its recommended unit for energy is the joule (J). One joule is defined as the energy required to accelerate 1 kg of mass at 1 m/s² (1 meter per second per second) over a distance of 1 m.

A calorie (or gram-calorie, abbreviation cal) is another unit of energy. One calorie is equivalent to 4.184 J, and it is defined as the amount of energy required to raise the temperature of 1 g of pure water by 1°C (specifically, from 15°C to 16°C). Note, however, that the dietician’s “Calorie” is equivalent to 1000 calories (1 Calorie = 1 kcal). However, the energy content of many food products is now listed in kJ in countries using the SI system of units (only three countries worldwide- Myanmar, Liberia, and the United States- have not adopted the SI system).

Energy Transformations

As was previously noted, energy can be transformed among its various states. For example, when solar electromagnetic radiation is absorbed by a dark object, it is transformed into thermal energy and the absorbing body increases in temperature. The gravitational potential energy of water stored at a height is converted into the kinetic energy of flowing water at a waterfall, which may be harnessed using hydroelectric technology to spin a turbine and generate electrical energy. As well, visible wavelengths of solar radiation are absorbed by chlorophyll, a green pigment in plant foliage, and some of the captured energy is converted into chemical potential energy of sugars via the biochemistry of photosynthesis.

All transformations of energy must behave according to certain physical principles, which are known as the laws of thermodynamics. These are universal principles, meaning they are always true, regardless of the circumstances.

The First Law of Thermodynamics

The first law of thermodynamics, also known as the law of conservation of energy, can be stated as follows: Energy can undergo transformations among its various states but it is never created or destroyed; therefore, the energy content of the universe remains constant. A consequence of this law is that there is always a zero balance among the energy inputs to a system, any net storage within it, and the energy output from the system.

Consider the case of an automobile driving along a highway. The vehicle consumes gasoline, an energy input that can be measured. The potential energy of the fuel is converted into various other kinds of energy, including kinetic energy embodied in forward motion of the vehicle, electrical energy powering the lights and windshield wipers, heat from friction between the vehicle and the atmosphere and road surface, and hot exhaust gases (thermal energy) and unburned fuel (chemical potential energy) that are vented through the tailpipe. Overall, in accordance with the first law of thermodynamics, an accurate measuring of all of these transformations would find that, while the energy of the gasoline was converted into various other forms, the total amount of energy was conserved (it remained constant).

The Second Law of Thermodynamics

The second law of thermodynamics can be expressed as follows: Transformations of energy can occur spontaneously only under conditions in which there is an increase in the entropy of the universe. Entropy is a physical attribute related to disorder, and is associated with the degree of randomness in the distributions of matter and energy. As the randomness (disorder) increases, so does entropy. A decrease in disorder is referred to as negative entropy. Consider, for example, an inflated balloon. Because of the potential energy of its compressed gases, that balloon may slowly leak its contents to the surrounding atmosphere, and it may even burst. Either of these outcomes can occur in a spontaneous fashion, because both processes would represent an increase in the entropy of the universe. This is because compressed gases are more highly ordered than those widely dispersed in the atmosphere. In contrast, dispersed gases in the atmosphere would never spontaneously relocate to inflate a balloon. A balloon will only inflate only if energy is expended through a local application of work, such as by a person blowing into the balloon. In other words, energy must be expended to cause a local decrease of entropy in a system. However, note that this energy cost itself gives rise to an increase in the entropy of the universe. For instance, the effort of a person inflating a balloon involves additional respiration, which uses biochemical energy and results in heat being released into the environment.

Another example concerns planet Earth. The planet continuously receives solar radiation, almost all of which is visible and near-infrared wavelengths in the range of about 0.4 to 2.0 μm. Some of this electromagnetic energy is absorbed and converted to thermal energy, which heats the atmosphere and surface. The planet cools itself of the absorbed solar radiation in various ways, but ultimately that energy is dissipated by an emission of electromagnetic energy to outer space as longer-wave infrared radiation (of a spectral quality that peaks at a wavelength of 10 μm). In this case, relatively short-wavelength solar radiation is ultimately transformed into the longer-wavelength radiation emitted by Earth, a process that represents a degradation in quality of the energy and an increase in the entropy of the universe.

An important corollary (or secondary proposition) of the second law of thermodynamics is that energy transformations can never be completely efficient—some of the initial content of energy must always be converted to heat so that entropy increases. This helps to explain why, even when using the best available technology, only about 30% of the potential energy of gasoline can be converted into the kinetic energy of a moving automobile, and no more than about 40% of the energy of coal or natural gas can be transformed into electricity in a generating station. There are also thermodynamic limits to the efficiency of photosynthesis, the process by which plants convert visible radiation into biochemical, even when it is occurring under ideal conditions with optimal supplies of nutrients, water, and light.

A superficial assessment might suggest that life in general appears to contradict the second law of thermodynamics. Plants, for example, absorb visible wavelengths of electromagnetic radiation and use this highly dispersed form of energy to fix simple inorganic molecules (carbon dioxide and water) into extremely complex and energy-dense biochemicals. The plant biomass may then be consumed by animals and microbes, which synthesize their own complex biochemicals. These various bio-syntheses represent energy transformations that greatly decrease local entropy because relatively dispersed electromagnetic energy and simple inorganic compounds are being converted into the complex, highly ordered biochemicals of organisms. Do these biological transformations contravene the second law of thermodynamics?

This seeming paradox of life can be resolved using the following logic: the localized bio-concentration of negative entropy can only occur because the system (ultimately referring to the biosphere, or all life on Earth) receives a constant input of energy in the form of solar radiation. If this external source of energy were somehow terminated, all of the organisms and organic materials would spontaneously degrade, releasing simple inorganic molecules and heat and thereby increasing the entropy of the universe. Therefore, life and ecosystems cannot survive without continual inputs of solar energy, which are required to organize and maintain their negative entropy. In this sense, the biosphere can be viewed as representing an “island” of negative entropy, highly localized in space and time, and continuously fueled by the Sun as an external source of energy.

Earth: An Energy Flow-Through System

Electromagnetic radiation emitted by the Sun is by far the major input of energy that drives ecosystems. Solar energy heats the planet, circulates its atmosphere and oceans, evaporates its water, and sustains almost all its ecological productivity. Eventually, all of the solar energy absorbed by Earth is re-radiated back to space in the form of electromagnetic radiation of a longer wavelength than what was originally captured. In other words, Earth is a flow-through system, with a perfect balance between the input of solar energy and output of re-radiated energy, and no net storage over the longer term.

In addition, almost all ecosystems absolutely depend on solar radiation as the source of energy that photosynthetic organisms (such as plants and algae) utilize to synthesize simple organic compounds (such as sugars) from inorganic molecules (carbon dioxide and water). Plants and algae then use the chemical potential energy in these sugars, plus inorganic nutrients (such as nitrate and phosphate), to synthesize a huge diversity of biochemicals through various metabolic reactions. Plants grow and reproduce by using these biochemicals and their potential energy. Moreover, plant biomass is used as food by the enormous numbers of organisms that are incapable of photosynthesis. These organisms include herbivores that eat plants directly, carnivores that eat other animals, and detritivores that feed on dead biomass. (The energy relationships within ecosystems are described later.)

Less than 0.02% of the solar energy received at Earth’s surface is absorbed and fixed by photosynthetic plants and algae. Although this represents a quantitatively trivial component of the planet’s energy budget, it is extremely important qualitatively because this biologically absorbed and fixed energy is the foundation of ecological productivity. Ultimately and eventually, however, the solar energy fixed by plants and algae is released to the environment again as heat and is eventually radiated back to outer space. This reinforces the idea of Earth being a flow-through system for energy, with a perfect balance between the input and output.

Earth’s Energy Budget

An energy budget of a system describes the rates of energy input and output as well as any internal transformations among its various states, including changes in stored quantities. Figure 3.10 illustrates key aspects of the physical energy budget of Earth.

The rate of input of solar radiation to Earth averages about 8.36 J/cm²-minute (2.00 cal/cm²-min), measured at the outer limit of the atmosphere. About half of this energy input is visible radiation and half is near infrared. The output of energy from Earth is also about 8.36 J/cm²-min, occurring as longer-wave infrared. Because the rates of energy input and output are equal, there is no net storage of energy, and the average surface temperature of the planet remains stable. Therefore, the energy budget of Earth can be characterized as a zero-sum, flow-through system.

However, the above is not exactly true. Over extremely long scales of geological time, a small amount of storage of solar energy has occurred through an accumulation of undecomposed biomass that eventually transformed into fossil fuels. In addition, relatively minor long-term fluctuations in Earth’s surface temperature occur, representing an important element of climate change. Nevertheless, these are quantitatively trivial exceptions to the statement that Earth is a zero-sum, flow-through system for solar energy.

Figure 3.10. Important Components of Earth’s Physical Energy Budget. About 30% of the incoming solar radiation is reflected by atmospheric clouds and particulates and by the surface of the planet. The remaining 70% is absorbed and then dissipated in various ways. Much of the absorbed energy heats the atmosphere and terrestrial surfaces, and most is then re-radiated as long-wave infrared radiation. Atmospheric moisture and greenhouse gases interfere with this process of re-radiation, keeping the surface considerably warmer than it would otherwise be (see also Chapter 21). The numbers refer to the percentage of incoming solar radiation. See the text for a more detailed description of important factors in this energy budget. Source: Modified from Schneider (1989).

Even though the amount of energy emitted by Earth eventually equals the quantity of solar radiation that is absorbed, many ecologically important transformations occur between the initial absorption and eventual re-radiation. These are the internal elements of the physical energy budget of the planet (see Figure 3.10). The most important components are described below:

Reflection – On average, Earth’s atmosphere and surface reflect about 30% of incoming solar energy back to outer space. Earth’s reflectivity (albedo) is influenced by such factors as the angle of the incoming solar radiation (which varies during the day and over the year), the amounts of reflective cloud cover and atmospheric particulates (also highly variable), and the character of the surface, especially the types and amounts of water (including snow and ice) and darker vegetation.

Absorption by the Atmosphere – About 25% of incident solar radiation is absorbed by gases, vapors, and particulates in the atmosphere, including clouds. The rate of absorption is wavelength-specific, with portions of the infrared range being intensively absorbed by the so-called “greenhouse” gases (especially water vapor and carbon dioxide; see Chapter 21). The absorbed energy is converted to heat and re-radiated as infrared radiation of a longer wavelength than what has been initially absorbed.

Absorption by the Surface – On average, about 45% of incoming solar radiation passes through the atmosphere and is absorbed at Earth’s by living and non-living materials at the surface, a transformation that increases their temperature. However, this figure of 45% is highly variable, depending partly on atmospheric conditions, especially cloud cover, and also on whether the incident light has passed through a plant canopy. Although over the longer term (years) and even the medium term (days) the global net storage of heat is essentially zero, in some places there may be substantial changes in the net storage of thermal energy within the year. This occurs everywhere in the United States because of the seasonality of its climate, in that environments are much warmer during the summer than in the winter. Nevertheless, almost all of the absorbed energy is eventually dissipated by re-radiation from the surface as long-wave infrared.

Evaporation of Water – Some of the thermal energy of living and non-living surfaces causes water to evaporate in a process known as evapotranspiration. This process has two components: the evaporation of water from lakes, rivers, streams, moist rocks, soil, and other non-living substrates, and transpiration of water from any living surface, particularly from plant foliage, but also from moist body surfaces and lungs of animals.

Melting of Snow and Ice – Absorbed thermal energy can also cause ice and snow to melt, representing an energy transformation associated with a change of state of water from a solid to a liquid form.

Wind and Water Currents – There is a highly uneven distribution of the content of thermal energy at and near the surface of Earth, with some regions being quite cold (such as the Arctic) and others much warmer warm (the tropics). Because of this irregular allocation of heat, the surface develops processes to diminish the energy gradients by transporting mass around the globe, such as by winds and oceanic currents (see also Chapter 4).

Biological Fixation – A very small but ecologically critical portion of incoming solar radiation (globally averaging less than 0.02%) is absorbed by chlorophyll in plants and algae and used to drive photosynthesis. This biological fixation allows some of the solar energy to be temporarily stored as potential energy in biochemicals, thereby serving as the energetic basis for ecological productivity and life on Earth.

Energy in Ecosystems

An ecological energy budget focuses on the absorption of energy by photosynthetic organisms and the transfer of that fixed energy through the trophic levels of ecosystems (“trophic” refers to the means of organic nutrition). Ecologists classify organisms in terms of the sources of energy they utilize.

Autotrophs are capable of synthesizing their complex biochemicals using simple inorganic compounds and an external source of energy to drive the process. The great majority are photoautotrophs, which use sunlight as their external source of energy. Photoautotrophs capture solar radiation using photosynthetic pigments, the most important of which is chlorophyll. Green plants are the most abundant examples of photoautotrophs, but algae and some bacteria are also photoautotrophic.

A much smaller number of autotrophs are chemoautotrophs, which harness some of the energy content of certain inorganic chemicals to drive a process called chemosynthesis. The bacterium Thiobacillus thiooxidans, for example, oxidizes sulfide minerals to sulphate and uses some of the energy liberated during this reaction to chemosynthesize organic molecules.

Because autotrophs are the biological foundation of ecological productivity, ecologists refer to them as primary producers. The total fixation of solar energy by all of the primary producers within an ecosystem is known as gross primary production (GPP). Primary producers use some of this production for their own respiration (R) – that is, for the physiological functions needed to maintain their health and to grow. Respiration is the metabolic oxidation of biochemicals, and it requires a supply of oxygen and releases carbon dioxide and water as waste products. Net primary production (NPP) refers to the fraction of GPP that remains after primary producers have used some for their own respiration. In other words: NPP = GPP – R.

The energy fixed by primary producers is the basis for the productivity of all other organisms, known as heterotrophs, which heterotrophs rely on other organisms, living or dead, to supply the energy they need. Animal heterotrophs that feed on plants are known as herbivores (or primary consumers); three familiar examples are deer, geese, and grasshoppers. Heterotrophs that consume other animals are known as carnivores (or secondary consumers), such as timber wolf, peregrine falcon, sharks, and spiders. Some species feed on both plant and animal biomass and are known as omnivores –the grizzly bear is a good example, as is our own species. Many other heterotrophs feed primarily on dead organic matter and are called decomposers or detritivores, such as vultures, earthworms, and most fungi and bacteria.

Image 3.4. Measuring Plant Productivity. Plant productivity is sustained by solar energy, which is fixed by chlorophyll in the plant and used to combine carbon dioxide, water, and other simple inorganic compounds into the complex molecular structures of organic matter. These ecologists are using gas analyzers to measure photosynthesis of tulip poplar leaves and collecting leaf samples to later analyze for chlorophyll concentrations. Source: NIST.

Productivity is production expressed as a rate function, that is, per unit of time and area. Productivity in terrestrial ecosystems is often expressed in units such as kilograms of dry biomass (or its energy equivalent) per hectare per year (kg/ha-y or kJ/ha-y), while aquatic productivity is often given as grams per cubic meter per year (g/m³-y).

Many studies have been made of the productivity of the various trophic levels in ecosystems. For example, studies of a natural oak–pine forest found that the total fixation of solar energy by the vegetation (the annual gross primary production) was equivalent to 4.81 × 104 kJ/m²-y (48 100 kJ/m²-y) (Odum, 1993). This fixation rate was equivalent to less than 0.1% of the annual input of solar radiation. Because the plants used 2.72 × 104 kJ/m²-y during their respiration, the net primary productivity was 2.09 × 104 kJ/m²-y, represented mainly by the growing biomass of the trees. The various heterotrophic organisms in the forest used 1.26 × 104 kJ/m²-y to support their respiration. Ultimately, the net accumulation of biomass by all organisms in the ecosystem (referred to as the net ecosystem productivity) was equivalent to 0.83 × 104 kJ/m²-y, or 8.3 × 103 kJ/m²-y.

The primary productivities of the world’s ecosystems are shown in Figures 3.11 and 3.12. Across types of ecosystems, the rate of production is greatest in tropical forests, wetlands, coral reefs, and estuaries. The production for each ecosystem type is calculated as its productivity multiplied by its area. However, the largest amounts of production occur in tropical forests and the open ocean. Note that the open ocean has a relatively small productivity per unit area, but its total global production is large because of its enormous area.

Figure 3.11. Net Primary Production of the Earth in February 2016. The chlorophyll map (top) shows milligrams of chlorophyll per cubic meter of seawater each month. Places where chlorophyll amounts were very low, indicating very low numbers of phytoplankton, are blue. Places where chlorophyll concentrations were high, meaning many phytoplankton were growing, are dark green. For terrestrial NPP (bottom), values range from near 0 grams of carbon per square meter per day (tan) to 6.5 grams per square meter per day (dark green). A negative value means decomposition or respiration overpowered carbon absorption; more carbon was released to the atmosphere than the plants took in. Source: NASA.

Figure 3.12. Net Primary Production of the Earth in July 2016. The chlorophyll map (top) shows milligrams of chlorophyll per cubic meter of seawater each month. Places where chlorophyll amounts were very low, indicating very low numbers of phytoplankton, are blue. Places where chlorophyll concentrations were high, meaning many phytoplankton were growing, are dark green. For terrestrial NPP (bottom), values range from near 0 grams of carbon per square meter per day (tan) to 6.5 grams per square meter per day (dark green). A negative value means decomposition or respiration overpowered carbon absorption; more carbon was released to the atmosphere than the plants took in. Source: NASA.

An ecological food chain is a linear model of feeding relationships among species. An example of a simple food chain in Alaska is lichens and sedges, which are eaten by caribou, which are eaten by wolves. A food web is a more complex model of feeding relationships, because it describes the connections among all food chains within an ecosystem. Wolves, for instance, are opportunistic predators that may feed on snowshoe hare, voles, lemming, beaver, birds, and other prey in addition to their usual prey of deer, moose, and caribou. Therefore, wolves participate in various food chains within their ecosystem. However, no natural predators feed on wolves, which are therefore referred to as top carnivores or top predators.

Figure 3.13 illustrates important elements of the food web of Lake Erie, one of the Great Lakes. In this large lake, shallow-water environments support aquatic plants, while phytoplankton occur throughout the upper water column. The shallow-water plants are consumed by ducks, muskrat, and other herbivores, while phytoplankton are consumed by tiny crustaceans (zooplankton) and bottom-living filter-feeders such as clams. Zooplankton are eaten by small fish such as smelt, which are eaten by larger fish, which may eventually be eaten by cormorants, bald eagles, or humans. Dead biomass from any level of the food web may settle to the bottom, where it enters a detrital food web and is eaten by small animals and ultimately decomposed by bacteria and fungi.

Figure 3.13. Major Elements of the Food Web in Lake Erie. Food webs are complex systems, involving many species and various food chains.

In accordance with the second law of thermodynamics, the transfer of energy in food webs is always inefficient because some of the fixed energy must be converted into heat. For example, when a herbivore consumes plant biomass, only some of the energy content can be assimilated and transformed into its biomass. The rest is excreted in feces or utilized in respiration (Figure 3.14). Consequently, in all ecosystems the amount of productivity by autotrophs is always much greater than that of herbivores, which in turn is always much greater than that of their predators. As a broad generalization, there is about a 90% loss of energy at each transfer stage. In other words, the productivity of herbivores is only about 10% of that of their plant food, and the productivity of the first carnivore level is only 10% of that of the herbivores they feed upon.

Figure 3.14. Model of Energy Transfer in an Ecosystem. Lower levels of a food web always have a greater production than higher levels. For this reason, the trophic structure is roughly pyramidal. According to the second law of thermodynamics, some of the energy content in food webs is converted into heat or respiration (R). There is about a 90% loss of energy at each transfer stage.

These productivity relationships can be displayed graphically using a so-called ecological pyramid to represent the trophic structure of an ecosystem. Ecological pyramids are organized with plant productivity on the bottom, that of herbivores above the plants, and carnivores above the herbivores. If the ecosystem sustains top carnivores, they are represented at the apex of the pyramid. The sizes of the trophic boxes in Figure 3.14 suggest the pyramid-shaped structure of ecosystem productivity.

The second law of thermodynamics applies to ecological productivity, a function that is directly related to energy flow. The second law does not, however, directly explain the accumulated biomass of an ecosystem. Consequently, it is only the trophic structure of productivity that is always pyramid-shaped. In some ecosystems, other variables may have a pyramid-shaped trophic structure, such as the amounts of biomass (standing crop) present at specific times, or the sizes or densities of populations. However, these particular variables are not pyramid-shaped in all ecosystems.

For example, in the open ocean, phytoplankton are the primary producers, but they often maintain a biomass similar to that of the small zooplankton that feed upon them. The phytoplankton cells are relatively short-lived, and their biomass turns over quickly because of their high rates of productivity and mortality. In contrast, the individual zooplankton animals are longer-lived and much less productive than the phytoplankton. Consequently, the productivity of the phytoplankton is much larger than that of the zooplankton, even though at any particular time these trophic levels may have a similar biomass.

Some ecosystems may even have an inverted pyramid of biomass, characterized by a smaller biomass of plants than of herbivores. This sometimes occurs in grasslands, in which the dominant plants are relatively small herbaceous species that can be quite productive but do not maintain a large biomass. In comparison, some of the herbivores that feed on the plants are large, long-lived animals, which may maintain a greater total biomass than the vegetation. Some temperate and tropical grasslands have an inverted biomass pyramid, especially during the dry season when there may be large populations (and biomass) of long-lived herbivores such as antelope, bison, deer, elephant, gazelle, hippopotamus, or rhino. However, in accordance with the second law of thermodynamics, the annual (or long-term) productivity of the plants in these grasslands is always much larger than that of the herbivores.

In addition, the population densities of animals are not necessarily smaller than those of the plants that they eat. For instance, insects are the most important herbivores in many forests and they commonly maintain large populations. In contrast, the numbers of trees are much smaller, because each individual plant is large and occupies a great deal of space. Forests typically maintain many more herbivores than trees and other plants, so the pyramid of numbers is inverted in shape. As in all ecosystems, however, the pyramid of forest productivity is much wider at the bottom than at the top.

Because of the inefficiency of the energy transfer between trophic levels, there are energetic limits to the numbers of top carnivores (such as eagles, killer whales, sharks, and wolves) that can be sustained by an ecosystem. To sustain a viable population of top predators, there must be a suitably large production of prey that these animals can exploit. This prey must in turn be sustained by an appropriately high plant productivity. Because of these ecological constraints, only extremely productive or very extensive ecosystems can support top predators. Of all Earth’s terrestrial ecosystems, none supports more species of higher-order carnivores than the savannas and grasslands of Africa. The most prominent of these top predators are the cheetah, hyena, leopard, lion, and wild dog. This unusually high richness of top predators can be sustained because these African ecosystems are immense and quite productive of vegetation, except during years of drought. In contrast, the tundra of Alaska can support only one natural species of top predator, the wolf. Although the tundra is an extensive biome, it is a relatively unproductive ecosystem.

Some pre-industrial human populations functioned as top predators. This included certain Aboriginal peoples of the United States, such as the Inuit of the Arctic and indigenous groups of the great plains, such as the Blackfoot and Plains Apache. As an ecological consequence of their higher-order feeding strategy within their food web, these cultures were not able to maintain large populations. In most modern economies, however, humans interact with ecosystems in an omnivorous manner—we harvest an extremely wide range of foods and other biomass products of microbes, fungi, algae, plants, and invertebrate and vertebrate animals. One of the consequences of this kind of feeding is that a large human population can be sustained.

Conclusions

Energy can exist in various states, but transformations from one to another must obey the laws of thermodynamics. Organisms and ecosystems would spontaneously degrade if they did not have continuous access to external sources of energy. Ultimately, sunlight is the key source of energy that supports almost all life and ecosystems. Sunlight is used by photoautotrophs to combine carbon dioxide and water into simple organic molecules through the metabolic process of photosynthesis. The fixed energy of plant biomass supports ecological food webs. Plants may be eaten by herbivores and the energy obtained is used to support their own growth. Herbivores may then be eaten by carnivores. Dead biomass supports a decomposer food web. Sunlight also drives important planetary functions, such as the hydrologic and climatic systems. Human activities can have a large and degrading influence on food webs, and even on Earth’s climatic system by influencing the intensity of the planet’s greenhouse effect.

Questions for Review

1. What forms of matter are described in this chapter?
2. What forms of energy are described in this chapter? How can each be changed into other forms?
3. What are the first and second laws of thermodynamics? How do they govern transformations of energy?
4. What are the major elements of Earth’s physical energy budget?
5. Why is the trophic structure of ecological productivity pyramid-shaped?

Questions for Discussion

1. According to the second law of thermodynamics, systems always spontaneously move toward a condition of greater entropy. Yet life and ecosystems on Earth represent local systems where negative entropy is continuously being generated. What conditions allow this apparent paradox to exist?
2. Why are there no natural higher-order predators that kill and eat lions, wolves, and sharks?
3. Why would it be more efficient for people to be vegetarian? Discuss your answer in view of the pyramid-shaped structure of ecological productivity.
4. Make a list of the key sources and transformations of energy that support you and your activities on a typical day. What is the ultimate source of each of the energy resources you use (such as sunlight and fossil fuels)?

Exploring Issues

1. As part of a study of the cycling of pollutants, you have been asked to describe the food web of two local ecosystems. One of the ecosystems is a natural forest (or prairie) and the other is an area used to grow wheat (or another crop). How would you determine the major components of the food webs of these ecosystems, the species occurring in their trophic levels, and the interactions among the various species that are present?

References Cited and Further Reading

Botkin, D.B. and E.A. Keller. 2014. Environmental Science: Earth as a Living Planet. 9th ed. Wiley & Sons, New York, NY.

Freedman, B. 1995. Environmental Ecology. 2nd ed. Academic Press, San Diego, CA. Gates, D.M. 1985. Energy and Ecology. Sinauer, New York, NY.

Hinrichs, R.A. and M. Kleinbach. 2012. Energy: Its Use and the Environment. 5th ed. Brook Cole, Florence, KY.

Houghton, J.T. 2009. Global Warming. The Complete Briefing, 4th ed. Cambridge University Press, Cambridge, UK.

Odum, E.P. 1993. Basic Ecology. Saunders College Publishing, New York, NY

Liu, P.I. 2009. Introduction to Energy, Technology, and the Environment. 2nd ed. ASME Press, New York, NY.

Priest, J. 2012. Energy: Principles, Problems, Alternatives. 8th ed. Kendall Hunt Publishing Co., Dubuque, IO.

Schneider, S.H. 1989. The Changing Climate. Scientific American, 261(3): 70-9

Whittaker, R.H. and G.E. Likens. 1975. The Biosphere and Man. pp. 305-28. In: Primary Productivity of the Biosphere. (H. Lieth and R.H. Whittaker, eds.). Springer-Verlag, New York, NY.