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Earth: A Living Planet
During the last few years, people around the world have finally begun to understand that our remarkable planet is home not only to people but also to other forms of life. And the health of this life is essential to the health of human society.
In order to properly care for our planet, we must understand how the living world operates. To do so, we must study ecology. Ecology is the study of the interactions of organisms with one another and with their physical surroundings. The word ecology comes from the Greek word oikos, which means house. The "house" includes the environment in which organisms live, the interactions of organisms with one another, and the interactions of organisms with the nonliving environment. Scientists who study ecology are called ecologists.
From space, it is easy to see that Earth is a single living system that has been named the biosphere, or living globe. The biosphere is that part of the Earth in which life exists. It includes all the areas-of land, air, and water on the entire planet, as well as all the life that populates these areas. The biosphere extends from about 8 kilometers above the Earth's surface to as far as 8 kilometers below the surface of the ocean. Living organisms are not distributed uniformly throughout the biosphere. For example, few organisms live in polar regions, whereas tropical rain forests swarm with life.
Ecologists divide the biosphere into a variety of smaller units called ecosystems. Ecosystems consist of all the biotic (living) and abiotic (nonliving) factors that surround organisms and affect their way of life. Biotic factors are all the living organisms. Abiotic factors include soil type, rainfall, elevation, temperature, and location on the Earth. A pond ecosystem is made up of the pond itself and all the plants, animals, protists, and bacteria that live within it. A forest ecosystem is composed of everything from the soil to the fungi living in rotting leaves to the birds on the tips of the trees' tallest branches.
It is important for you to remember that although it may appear as if ecosystems are isolated from one another, they really are 'not. One ecosystem blends into another. For example, as water flows through a forest into a pond, frogs in the pond eat insects that fly through the woods and feed on plants and animals living near the pond. You should also remember that every organism in an ecosystem constantly changes the ecosystem. For example, leaves from trees near a pond fall onto the ground as well as into the pond. With the passage of time, the leaves decay, returning important nutrients to the forest floor and introducing new nutrients into the pond.
Altitude and Latitude Zonation Stages of Sucession SucessionBecause so many organisms alter their surroundings, environments are constantly changing in the process of ecological succession. In ecological succession, an existing community (organisms living in a common environment and interacting with one another) is gradually replaced by another community.
Ecological succession sometimes occurs in areas where there has been no previous living community. For example, when a new island rises out of the sea, the newly cooled lava rock is devoid of life. Soon, however, weathering by the wind, rain, and other agents creates a surface for the growth of lichens. The lichens produce organic acids that further help to dissolve the rock.
Eventually the lichens die and add their organic matter to the forming soil. Now the soil is fit for the growth of mosses, which break up the rock more and also add their remains to the soil. As the soil becomes more suitable for other organisms, new species move in and change the environment in their own ways. Ecological succession may take thousands of years and continues until a stable community becomes established.
Over long periods of time, ecological succession can produce dramatic changes in an ecosystem. For example, as a lake or pond ages, it may slowly fill up with silt and fallen leaves. Gradually the pond or lake turns into a marsh and then into dry land. Eventually plants and animals from the surrounding communities colonize the area.
Ecological succession also occurs in areas where farmland is abandoned and forests are cut down or are destroyed by fire or other natural disasters and diseases. Often, ecological succession in these areas re-establishes the original community. For example, on an abandoned farm in New England, grasses are usually the first plants to grow, followed by weeds and small shrubs. Later, fast-growing trees, such as poplars and sumac, start to grow. Soon larger trees develop. Ecological succession continues until a forest develops.
Ecological succession often leads to a relatively stable collection of plants and animals called a climax community. In reality, a climax community is described by the major forms of plant life. However, the types of animals that live in a climax .community depend directly or indirectly on the types of plants. Ecologists often describe different parts of the world in terms of their most common climax community. If, for example, you tell an ecologist that an area is a "temperate zone beech-maple forest" he will have a good idea of what the area is like. Tell a marine biologist that you visited a "South Pacific coral atoll" and she too will know what you are talking about.
Although we describe a climax community as relatively stable, it does not mean that it never changes. Fires, floods, or droughts can disturb a climax community and cause the area to undergo ecological succession again. Similarly, tsunamis (incorrectly called tidal waves) and hurricanes can upset climax communities in many marine environments. In some cases, disasters can completely wipe out an entire climax community, causing ecological succession to begin.
Areas that are similar in climate and other physical factors develop similar types of climax communities. These areas are called biomes. A biome is an environment that has a characteristic climax community. The Earth is made up of two main types of biomes: land biomes and aquatic biomes. Aquatic biomes will be discussed in the next section.
Most land biomes are named for their climax community, or the dominant type of plant life. The major land biomes are the tundra, taiga, temperate deciduous forest, grassland, tropical rain forest, and desert.
Northern North America, Asia, and Europe are covered by biomes called tundra. The tundra is the northernmost land biome. This nearly treeless biome is covered by mosses, lichens, and grasses. A few stunted trees survive here and there. Many animals migrate into the tundra during the summer to feed on the plants that grow there. Caribou and reindeer are two examples. In turn, wolves, foxes, and hordes of mosquitoes feed on these animals. A great many birds summer here, then fly south in early autumn.
The most characteristic feature of the tundra is permafrost, which is a layer of permanently frozen subsoil. During the summer the ground thaws to a depth of a few centimeters and becomes soggy and wet; in the winter it freezes again. This cycle of thawing and freezing, which rips and crushes plant roots, is what keeps the plants small and stunted.
South of the tundra are biomes dominated by great coniferous, or cone-bearing, forests of fir, pine, and spruce. These biomes are called taiga (TICH-guh). The term taiga comes from the Russian word that means primeval forest.
The taiga stretches across much of North America and Asia, with a narrow band reaching into Norway and Sweden (in Europe). The taiga rises to the higher elevations of many mountain ranges in the United States, including the Rocky and Appalachian mountains. The taiga also extends along the coasts of Washington, Oregon, and northern California, where it is home to the giant redwoods--some of the tallest trees in the world. Redwoods can reach heights of more than 60 meters.
Although winters in the taiga are cold, summers are mild enough and long enough to allow many animals and plants to reproduce. The ground thaws during the warmer months, although in some places the thawing does not last for long. Many rivers, ponds, lakes, and bogs provide homes for a variety of living things. Many small birds and mammals live in the taiga and either hibernate or move to warmer regions during the long, cold winters. Typical inhabitants of the taiga include black bears, grizzlies, wolves, moose, elk, and dozens of smaller animals such as voles, wolverines, and grouse.
Covering the eastern coast of the United States, the southern coast of Canada, most of Europe, and parts of Japan, China, and Australia are biomes that are characterized by changing seasons and leaf fall. These biomes are called temperate deciduous forests. A temperate deciduous forest gets its name from its forests of oak, maple, beech, and birch--trees that are deciduous, or shed their leaves in autumn. Although rainfall is sufficient year round, cold winters halt plant growth for several months. Because deciduous trees shed their leaves every autumn, this biome goes through striking seasonal changes. In the spring, many small plants in the forest practically burst out of the ground and grow quickly so that they can flower and bear fruit before they are shaded by the trees.
Although a great number of animals once inhabited these forests, many have been hunted to near extinction. With careful protection and hunting regulations, deer, moose, gray foxes, and several other species are beginning to reappear. An enormous variety of birds spend their summers in this biome, and chipmunks, raccoons, opossums, and squirrels make it their permanent home.
In the temperate deciduous forest biome, an abundance of organic matter and nutrients are stored in a layer of decaying leaves and twigs called humus (HYOO-muhs). Because humus enriches the soil, these forests make good farmland. And this has encouraged human activities that have greatly altered the biome. For example, forest land in New England was cleared of trees and used for farming. Fortunately, much of the original deciduous forest has since recovered.
Usually found in the interior portions of many continents, grasslands are vast areas covered with grasses and small leafy plants. Although this biome may receive significant rainfall (25 to 75 centimeters per year), most of it falls in one season. The grasslands of the world include the plains and prairies of North America, the steppes of the Soviet Union, the veld of South Africa, and the pampas of Argentina.
In the midwestern United States, grasslands are characterized by hot summers and cold winters. In some tropical grasslands, however, there is little seasonal change in temperature. Instead, the seasons change from wet to very dry. These tropical grasslands, which are dotted with groves of trees, are called savannas.
In grasslands, as in other biomes, interactions among animals and plants shape the environment. In fact, many grasslands do not undergo ecological succession and thus do not become forests primarily because of the grazing of large animals and periodic fires. On the Serengeti grasslands of Africa, impala, gazelles, wildebeests, and elephants graze.
Wheat, corn, and other grains are heavily farmed in the grasslands of the midwestern United States and in the Ukraine in the Soviet Union. In the past, these grasslands were almost destroyed by overfarming. Overfarming strips the grasslands of their protective layer of vegetation, thus allowing windstorms to blow away kilometers of topsoil. This is exactly what happened in the 1930s when windstorms carried away hundreds of millions of metric tons of rich topsoil, creating the great dust bowls.
Rainforest Canopy LayersIn parts of the world where the temperature stays warm and rain falls year round, tropical rain forest biomes are found. Typically, tropical rain forests receive 200 to 400 centimeters of rainfall each year. Temperatures remain constant at about 25°C throughout the year. This biome covers large areas of South America, Southeast Asia, Africa, and Central America.
The tropical rain forests are home to more species of plants and animals than can be found in all the rest of the land biomes combined! Here, many trees grow to a height of 70 meters, and their tops form a dense covering called a canopy. The tall trees also provide surfaces on which many other plants grow. Lianas (lee-AH-nuhz) are large woody vines that use the trees to support their rapid growth.
Animal life in the rain forests is rich and varied. Colorful insects and birds are particularly abundant. Reptiles, small mammals, and amphibians are common inhabitants of this biome. It is not surprising that many of the animals living here are tree dwellers, as the floor of tropical rain forests bristles with danger.Many of the animals and plants that inhabit the rain forests · produce chemicals that may be useful in fighting some types of diseases. Unfortunately, the world's tropical rain forests are being destroyed by the rapid growth of the human population. If the destruction continues at its present rate, almost all of the tropical rain forests will disappear by the end of this century! Along with the rain forests will go thousands of plant and animal species found only in this fascinating biome.
Deserts are biomes that usually occur in areas where there is less than 25 centimeters of rainfall a year. There are many different kinds of deserts around the world. In deserts such as the Sahara in Africa--the world's largest desert--rain almost never falls, and the wind is hot and dry. Because almost nothing grows in this type of desert, the landscape looks as barren as the surface of the moon. Other deserts are home to many species of lizards, insects, scorpions, snakes, and birds.
In seasonal deserts there is some rainfall during the year. Rapidly growing plants soak up the water as quickly as possible, then grow, flower, fruit, and become dormant until the next rainfall. In the deserts of the southwestern United States and Mexico, rainfall is more even, but it is sparse. Here, sagebrush, Cacti, and only a few types of trees survive. Another type of desert is found on mountains and plateaus where the high altitudes cause a decrease in temperature. These deserts are called cold deserts. Cold deserts have a brief rainy season that permits the growth of grasses and shrubs.
If modern science can find a way to bring water to the deserts, they can be made suitable for farming. Desert soil is often very fertile and, of course, receives plenty of sunlight. In parts of the Middle East, archaeologists have uncovered the ruins of waterworks that were used thousands of years ago by desert inhabitants to collect rainwater. In several areas these waterworks have been rebuilt, making farming possible again.
Aquatic Biomes Voyage to the Deep
The aquatic biomes are the water ecosystems. They include the freshwater biome, marine biome, and estuaries. These biomes support more organisms than do the land biomes.
Some of the abiotic factors that affect the kinds of organisms found in the aquatic biomes are light intensity, amounts of oxygen and carbon dioxide dissolved in the water, and the availability of organic and inorganic nutrients. The aquatic biomes do not vary in temperature as much as land biomes do.
Rivers, streams, and lakes are the lifeblood of our continents and are considered the freshwater biomes of the Earth. Not only do they provide much of our drinking water, but they are also an important source of food. Tiny floating plants and animals drift and swim through the water. These organisms are eaten by fishes and amphibians, which also eat the vegetation and insects that fall into the water from overhanging trees. Trout are typical of the fast swimming fishes that live in mountain streams. Large rivers such as the Amazon in South America and the Nile in Africa are home to many species of insects, fishes, amphibians, reptiles, and mammals.
Unfortunately, people all over the world are using rivers and lakes as dumping grounds for wastes. The results of this carelessness are beginning to catch up with us. We will discuss the problems of water pollution in more detail in Chapter 49.
Marine Biomes Marine Ecosystems
The vast habitats of the ocean, or the marine biomes, cover most of the surface of the Earth. Because sunlight penetrates only a short distance before it is absorbed by the water, photosynthesis can take place only in the uppermost region of a marine biome. This region is called the photic (FOHT-ihk) zone. The photic zone may be as shallow as 30 meters in the North Atlantic Ocean or as deep as 200 meters in the South Pacific Ocean. It is in this thin ocean layer that phytoplankton (tiny free-floating photosynthetic organisms) and algae grow.
Oceanographers have divided marine biomes into ecologically distinct zones depending on depth and distance from shore. Each of these zones contains organisms that are adapted to the conditions there.
INTERTIDAL ZONE The intertidal zone is the most difficult zone for organisms to live in. Those that live here must tolerate radical changes in their surroundings: Once or twice a day they are submerged in ocean water; the remainder of the time they are exposed to air and sunlight. Organisms in the intertidal zone have adapted in some way to the pounding and surging of waves. Some organisms, such as clams, burrow into the sand to keep from being washed out to sea. Others, such as barnacles and seaweed, attach themselves to rocks. Still others, such as snails, sea urchins, and starfish, cling to rocks by their feet or suckers.
NERITIC ZONE The neritic zone is the part of a marine biome that extends from the low-tide line to the edge of the open sea. Large algae (seaweed) are abundant here because this part of the ocean is in the photic zone. For example, off the coast of California grow huge forests of giant kelp (brown algae). In shallow areas of tropical waters, meadows of turtle grass provide food for fishes, invertebrates, and turtles. And along the ocean floor, lobsters and crabs crawl while flounder and rays swim above them.
OPEN-SEA ZONE In the open-sea zone, phytoplankton are responsible for 80 to 90 percent of the Earth's photosynthetic activity. Phytoplankton are in turn eaten by larger animals. Thus the chain of life in the sea begins with these tiny organisms. Swimming rapidly through the open-sea zone are fishes of all shapes and sizes and mammals such as dolphins and whales. The open ocean is also home to sea birds such as albatrosses, which live most of their life at sea.
Because nutrients are scarce in most of the open sea, the growth of phytoplankton is relatively slow. This limits the number of animals that can live there. Closer to the shore, however, nutrients are more abundant, and countless fishes swim there to feed and reproduce. Unfortunately, these rich fishing areas are much more susceptible to pollution than is the open sea.
DEEP-SEA ZONE The deep-sea zone is an area of high pressure, cold temperature, and total darkness. Until recently, biologists thought the deep-sea zone was completely devoid of life. But it is now known that this area is home to some of the Earth's strangest creatures. Gulper eels with mouths that make up almost half of their body and giant squid with glowing spots along their sides inhabit the ocean depths.
Here, too, zooplankton (free-floating microscopic animals) wait for night in order to migrate to the ocean's surface and feed on phytoplankton. Herds of bottom-dwellers, such as sea cucumbers, crawl along the ocean floor. Hardly a day goes by that an interesting life form is not found in the deep-sea zone.
Estuaries (EHS-tyoo-er-eez) are found at the boundary between fresh water and salt water. Salt marshes, mangrove swamps, lagoons, and the mouths of rivers that empty into the ocean are examples of estuaries. These areas contain a mixture of fresh water and salt water.
Estuaries support a variety of life forms. Because estuaries are usually shallow, sunlight is able to penetrate the water completely. Photosynthesis occurs at all levels, making estuaries a suitable environment for aquatic plants. The abundance of such plants, in turn, supports many types of fishes, shrimps, and crabs. In fact, many fishes and invertebrates spawn, hatch, and nurse their young in estuaries. As the young mature, they head for the open sea, then return to the estuaries to reproduce. Several species of birds use estuaries for nesting, feeding, and resting.
Energy and Nutrients: Building the Web of Life
One of the most important factors in any ecosystem is the flow of energy through the ecosystem. Of all the sun's energy that reaches the Earth's surface, only a small amount--approximately 0.1 percent on a worldwide basis--is used by living things. Yet this amount, as small as it is, is responsible for the production of several thousand grams of organic matter per square meter of forest per year.
Approximately one half of the energy plants absorb from the sun is used immediately. The rest is stored in plant tissues in the form of energy-containing compounds (carbohydrates). Animals that eat the plants obtain this energy. But because the animals must use much of this energy to carry on their life activities, they store an even smaller amount. Energy cannot be recycled, or used again. Thus energy in an ecosystem is referred to as a flow rather than a cycle.
Nutrients, on the other hand, are generally recycled through an ecosystem. When an animal dies, its matter does not disappear. Rather, it decomposes and eventually gets used by another organism.
You may recall from Chapter 6 that the sun is the ultimate source of energy for all living things. During photosynthesis, green plants and certain bacteria trap sunlight and use it to assemble carbon dioxide and water into carbohydrates. Because photosynthetic organisms are able to make their own food from inorganic substances, they are called producers.
Animals, on the other hand, are consumers. Consumers get their energy either directly or indirectly from producers. Consumers that feed directly on producers are called primary consumers. Primary consumers are also called herbivores (plant-eating animals). Consumers that feed on primary consumers are called secondary consumers. There may be tertiary (third-level) or quaternary (fourth-level) consumers that feed on secondary and tertiary consumers, respectively. Secondary and higher level consumers are usually carnivores (flesh-eating animals). For example, an insect that eats plants is a primary consumer, a frog that eats the insect is a secondary consumer, a snake that eats the frog is a tertiary consumer, and so on. Energy flows through an ecosystem from the sun to producers and then to consumers.
When plants and animals in an ecosystem die, their remains do not build up because of the presence of decomposers. Decomposers are organisms that obtain their energy from nonliving organic matter. Some examples of decomposers are bacteria and fungi.
Each step in this series of organisms eating other organisms is called atrophic, or feeding, level. The term trophic comes from the Greek word trophe which means food. There is no limit to the number of trophic levels in a particular ecosystem. However, at each higher trophic level, less and less of the energy originally captured by the producers is available. This is because the energy obtained from digested food is used to maintain the metabolism of the organism and to power its daily activities. A small amount of the energy taken in by herbivores (primary consumers) is changed into new animal biomass. Biomass is the total mass of all the organisms in atrophic level.
As a rule, approximately 10 percent of the energy at one trophic level can be used by animals at the next trophic level. Thus 10 percent of the energy in plants becomes stored in the tissues of herbivores, and 10 percent of the energy in herbivores becomes stored in the tissues of carnivores. At each successive trophic level, less energy is available to an organism.
Ecologists use ecological pyramids to represent the energy relationships among trophic levels. There are three types of ecological pyramids. A pyramid of energy shows the total amount of incoming energy at each successive level. Notice in Figure 47-15 that energy (in the form of heat) is lost going from one trophic level to another.
The trophic levels of an ecosystem can also be represented by a pyramid of biomass, which shows the total mass-of living tissue at each level. See Figure 47-15. This pyramid of biomass shows, for example, that a large amount of grass is needed to feed a single rabbit, and a large number of rabbits is needed to nourish a single hawk.
Relationships among trophic levels may also be represented by a pyramid of numbers. A pyramid of numbers illustrates the total number of organisms at each level. In a grassland, for example, a large amount of grass (producers) is needed to support the herbivores (primary consumers). Usually the number of organisms decreases at each successive level. Sometimes, however, this is not the case. In a temperate deciduous forest, one tree (producer) can support a large number of insects (primary consumers).
Like pyramids of biomass, pyramids of numbers show only the amount of organic material present at one time. They do not give the total amount of material produced or the rate at which it is produced, as do pyramids of energy.
Geochemical CycleAlthough energy moves in a one-way direction through an ecosystem, nutrients are recycled. All organisms require certain essential nutrients in order to grow. Plants need water, carbon dioxide, phosphorus, potassium, and many other elements. Animals require complex compounds (such as proteins and amino acids), several types of vitamins, and many of the same elements plants do.
As members of each trophic level eat members of the level beneath them, they acquire the complex organic molecules and elements they need in addition to energy. Although energy and nutrients move together from one trophic level to the next, they move through the biosphere differently. Nutrients move through the biosphere in a series of physical and biological processes called biogeochemical, or nutrient, cycles. They are called cycles because nutrients, unlike energy, may be used over and over again by living systems.
THE WATER CYCLE The movement of water from the atmosphere to the Earth and back to the atmosphere is called the water cycle. The water cycle consists of an alternation of evaporation and condensation. Water molecules enter the air by evaporation from the ocean and other bodies of water. In the air, the water molecules condense (in clouds) and then return to the Earth in the form of precipitation (rain). On land, most of the rainwater runs along the surface of the ground until it enters a river or stream that carries it to a larger body of water. Some water sinks into the ground and is called ground water. The upper surface of ground water is known as the water table.
THE NITROGEN CYCLE All organisms require nitrogen to build proteins. Nitrogen is available to organisms in several ways. Free nitrogen gas makes up 78 percent of the atmosphere. Nitrogen is also found in the wastes produced by many organisms and in dead and decaying organisms. The movement of nitrogen through the biosphere is called the nitrogen cycle. The Nitrogen Cycle However, most of this nitrogen cannot be directly used by living things. It must be converted into other forms.
Certain bacteria that live on roots of plants such as legumes (beans, peas, and peanuts) change free nitrogen in the atmosphere into nitrogen compounds (nitrates and nitrites) that can be used by living things. This process is known as nitrogen fixation, and the bacteria are called nitrifying bacteria. Symbiotic Nitrogen Fixation Once the nitrogen compounds are available, plants use them to make plant proteins. Animals then eat the plants and use the proteins to make animal proteins. When the plants and animals die, the nitrogen compounds return to the soil.
Eventually other bacteria in the soil break down these nitrogen compounds into free nitrogen in a process called denitrification (dee-nigh-trih-fih-CAY-shuhn). These bacteria are called denitrifying bacteria. Through the process of denitrification, free nitrogen is returned to the atmosphere.
THE CARBON AND OXYGEN CYCLES The process by which carbon is moved through the environment is called the carbon cycle. The Carbon Cycle During photosynthesis, green plants and algae use carbon dioxide from the atmosphere to form glucose. Consumers and decomposers use glucose in respiration, during which they produce carbon dioxide. Carbon dioxide is then released into the atmosphere, completing the carbon cycle.
The movement of oxygen through the environment is called the oxygen cycle. During photosynthesis, water molecules are split, releasing oxygen into the atmosphere. The oxygen is used by most organisms for respiration. During respiration, water is released. The water is absorbed by plants, and the cycle begins again.
Nutrient Limitation Ecosystem Productivity
The rate at which producers can capture energy and use it to produce living tissue is controlled by several factors, one of which is the amount of available nutrients. If a nutrient is in short supply--thus limiting an organism's growth--it is called a limiting factor.
For example, coastal ocean water often contains sufficient supplies of several nutrients to support much more plant growth than is normally present. The producers in these ecosystems, however, are slowed by the lack of sufficient nitrogen. If nitrogen is added to this system in large amounts, there is a tremendous growth, or bloom, of algae.
Sometimes adding nutrients does not hurt an ecosystem. A little extra fertilizer may even help the system produce more plants and animals for human food. But we must be very careful about tampering with natural ecosystems in this way. If an algal bloom in a lake or river gets too big, it may cover the surface of the water. If that happens, plants below die because they receive no sunlight. Bacteria grow and use up much of the available oxygen. Animals may then suffocate.
A Food Web Food WebAnimals and plants in the biosphere are tied together in complicated networks of feeding relationships. The simplest feeding relationship is a food chain. Food Chains In one food chain, a big fish eats little fishes that eat tiny fishes that eat plankton. But nature is almost never that simple.
In nature, plants absorb nutrients and grow. Herbivores eat plants. Carnivores eat herbivores and each other. Scavengers eat dead animals. Bacteria and fungi decompose dead tissue, returning essential elements to the environment. Filter feeders strain floating organisms from the water. Detritus feeders eat bacteria and the wastes of other organisms. Parasites live at the expense of host organisms. All are connected to one another in many complex food webs. As Figure 47-20 shows, food webs have many crisscrossing strands.
In the next chapter you will learn that feeding relationships in food webs often contain a built-in system of checks and balances. These checks and balances regulate the number of individuals of each species in the food web. Because of this, many natural ecosystems are remarkably stable.
SUMMARIZING THE CONCEPTS
The key concepts in each section of this chapter are listed below to help you review the chapter content. Make sure you understand each concept and its relationship to other concepts and to the theme of this chapter.