Biology of Humans
23. Ecology, the Environment, and Us
In the previous chapter, we learned about human evolution. In this chapter, we see that humans are but one of a host of species that share our small planet. We have many of the same needs as other species, and we face many of the same threats. If we are ever to understand the world around us and our place in it, we must have some knowledge of its physical characteristics and understanding of our dependence on the other species we share it with.
Earth as an Ecosystem
Our focus in this chapter is ecology, a science whose name comes from the Greek words oikos, meaning "home," and logos, meaning "to study." Thus ecology is the study of our home— Earth—encompassing both its living (biotic) components and nonliving (abiotic) components. More precisely, ecology is the study of the interactions between organisms and between organisms and the environment. Ecologists are the scientists who study these interactions.
If we could see Earth (Figure 23.1) from space, like an astronaut does, we would make two important observations about our home. First, we would see that Earth is isolated from other planets. So we could conclude that, aside from an occasional meteorite and other bits of debris from space, our planet has no source of new materials. In fact, many of the materials that came together to form it some 4.5 billion years ago cycle repeatedly from organism to organism and between the living and nonliving components of our world. A carbon atom in your body may once have been part of a dinosaur or part of Aristotle. Also, from our vantage point in space, we would be reminded by the light reflected from Earth's surface that this planet receives one very important contribution from without— energy, in the form of sunlight. As we will see, this energy is captured by green plants and transferred from organism to organism to sustain nearly all life on Earth.
· The best legacy we can give our children is a world in ecological balance.
FIGURE 23.1. A view of Earth from space shows us that the planet is isolated. Because there is no regular input of materials, important elements must be cycled from organism to organism and between Earth’s living and nonliving components. The only input to the system is energy from the sun, which sustains nearly all life on Earth.
The part of the Earth where life exists is the biosphere. The biosphere includes many ecosystems, each made up of the organisms in a specific geographic area and their physical environment. All the living species in an ecosystem that can potentially interact form a community. A population is all the individuals of the same species that can potentially interact. Thus, populations of different species form a community.
When ecologists consider an individual species, they describe it according to its niche (sometimes called ecological niche). The niche is the organism's role in the ecosystem, defined by all the physical, chemical, and biological factors that keep the organism healthy and allow it to reproduce. Such factors include the nature of the organism's food and how it obtains this food; its predators; and its specific needs for shelter, as well as the temperature, light, water, and oxygen the organism requires to survive. Thus the organism's habitat—the physical place where it lives—is also a part of its niche. For example, the habitat of an African lion is semi-open plains. Its niche includes the habitat, when it is active (any time of day), how it obtains food (predation on large animals), how it relates to other animals (lives in social group called a pride; marks territory by roaring, urinating, and patrolling), and how it reproduces (gives birth to litters of 1 to 4 cubs after 110 days of pregnancy on average).
Ecosystems change over long periods (Figure 23.2). In fact, everything on Earth is constantly changing. The sequence of changes in the kinds of species making up a community is called ecological succession. There are two types of ecological succession: primary and secondary.
FIGURE 23.2. Primary succession is the sequence of changes in the species composition of a community that begins where no life previously existed.
Primary succession occurs where no community previously existed. Such places may be found on rocky outcroppings, or where lava has solidified into a new surface, or where land appears as a glacier recedes or ice caps melt. At the start of primary succession, no soil exists.
The first living things to invade such an area are called pioneer species. Among the most prominent of these are lichens, which are actually two species—a fungus and a photosynthetic organism, usually an alga—combined in a mutually beneficial relationship. The fungus provides the attachment to the barren surface, retains water, and releases minerals from the rock to be used by the alga; the alga provides the food. Lichens secrete acid, which helps break down the rock, beginning soil formation. In time, the lichens die and their remains mix with the rock particles, furthering soil formation in cracks and crevices.
Soil building is an extremely slow process, but once soil is in place, the rate of succession speeds up. Plants begin to appear. Their roots push into every crevice, and their leaves fall and decompose, accelerating the pace of soil building and new plant colonization. In some areas, trees eventually become the dominant plant form.
Each species that moves into the area changes the environmental conditions slightly, thus changing the available resources in ways that favor some species and hamper others. The community that eventually forms and remains, if no disturbances occur, is called the climax community (Figure 23.3). The nature of a climax community is determined by many factors, including temperature, rainfall, nutrient availability, and exposure to sun and wind.
FIGURE 23.3. Selected climax communities of Earth. Environmental conditions, particularly the temperature and the availability of water, affect the distribution of organisms. Each species is adapted to certain conditions.
When an existing ecological community is cleared away— either by natural means or by human activity—and is then left alone by humans, it undergoes a sequence of changes in species composition known as secondary succession. Secondary succession occurs in areas where soil is already in place. Such areas include old, deserted fields and farms (originally cleared and planted by humans but later abandoned) or areas damaged by catastrophic fire, flood, wind, or overgrazing (Figure 23.4). The initial invaders in secondary succession are likely to be grasses, weeds, and shrubs, but these are gradually outcompeted as other plants move in from the surrounding community. The area finally "heals," and the community that forms often resembles the one that existed before the disturbance.
FIGURE 23.4. Secondary succession following the 1988 fires in Yellowstone National Park
Virtually all the energy that propels life on this small planet comes from the sun. Only a small fraction of the sun's energy that reaches Earth's surface worldwide is captured and used by living organisms. Even so, the life that abounds on Earth owes its existence to that captured energy, which is shifted, shuffled, channeled, and scattered through various systems from one level to the next.
Food Chains and Food Webs
The flow of energy through the living world begins when light from the sun is absorbed by photosynthetic organisms, such as plants, algae, and cyanobacteria (a group of photosynthetic bacteria that usually live in water). Photosynthesis is a chemical process that essentially captures light energy and transforms it into the chemical energy of the sugar glucose, which is manufactured from carbon dioxide and water. The energy stored in the glucose molecules and oxygen released in the process of photosynthesis sustain nearly all life on Earth. The photosynthesizers themselves use some of the energy stored in these glucose molecules to fuel their own metabolic activities. Any remaining energy can be used for growth and reproduction. Once the photosynthesizer uses the energy in glucose to make new organic molecules, these molecules may become food for an animal.
The photosynthesizers, called the producers, form the lowest trophic level (trophic means "feeding"). All other organisms are consumers and use the energy that producers store (Figure 23.5).
FIGURE 23.5. Trophic levels. The width of the arrow Indicates the relative amount of energy transferred.
Consumers belong to higher trophic levels and are grouped on the basis of their food source.
• Herbivores, or primary consumers, eat plants.
• Carnivores, or secondary consumers, feed on herbivores.
Some carnivores eat other carnivores, forming still higher trophic levels—tertiary and quaternary consumers.
• Omnivores eat both plants and animals.
• Decomposers, such as bacteria, fungi, and worms, consume dead organic material for energy and release inorganic material that can then be reused by producers.
At one time, the pattern of feeding relationships responsible for the flow of energy through an ecological system was described as a food chain—a linear sequence in which A eats B, which then eats C, which then eats D, and so on. However, now we know that the chain analogy is too simplistic because many organisms eat at several trophic levels. To illustrate, consider the number of trophic levels on which humans feed. Whereas eating chickens— which are primary consumers—makes us secondary consumers, eating tuna (a predatory fish) makes us tertiary consumers (or quaternary consumers if the tuna has eaten another predatory fish). But we also can be considered primary consumers because we eat vegetables (Figure 23.6). More realistic patterns of feeding relationships, consisting of many interconnected food chains, are described as food webs. Figure 23.7 describes part of a food web in a community where land meets water.
FIGURE 23.6. Humans eat at several trophic levels. The width of the arrow indicates the relative amount of energy transferred.
FIGURE 23.7. In this simplified food web, the arrows show the direction of flow of energy in the form of food. The width of the arrow indicates the relative amount of energy transferred.
On which trophic level would the hawk be feeding if it ate the duck?
It would be a tertiary consumer.
Energy Transfer through Trophic Levels
Energy is lost as it is transferred from one trophic level to the next. Although the efficiency of transfer can vary greatly depending on the organisms in the system, on average only 10% of the energy available at one trophic level is transferred to the next higher level. As a result, ecosystems rarely have more than four or five trophic levels.
As we consider why energy transfer between trophic levels is so inefficient, keep in mind that only the energy that is converted to biomass (the dry weight of the organism) is available to the next higher trophic level. One reason for energy loss between trophic levels is that roughly two-thirds of the energy in the food that is digested is used by the animal for cellular respiration. In addition, an animal must first expend energy to obtain its food, usually by grazing or hunting (the chicken in Figure 23.8 must do a lot of pecking). Furthermore, not all the food available at a given trophic level is captured and consumed. In addition, some of the food eaten cannot be digested and is lost as feces. The energy in the indigestible material is unavailable to the next higher trophic level. However, the remaining energy can be converted to biomass and will be available to the next higher trophic level. These losses become multiplied as the energy is transferred to successively higher trophic levels, until there simply is not enough food or energy left to feed another level. To put this another way, for every 100 calories of solar energy captured by producers, 10 calories are transferred to herbivores and only 1 calorie is transferred to carnivores (secondary consumers).
FIGURE 23.8. 4s energy flows through a food web, only a small amount of it is stored as body mass and becomes available to the next higher trophic level. Some of the food energy is simply not captured, some is undigested and lost in fecal waste, and some is used for cellular respiration. Only the remaining energy that has been converted to biomass becomes available to the next higher trophic level. The width of the arrow indicates the relative amount of energy transferred.
An ecological pyramid is a diagram that compares certain properties in a series of related trophic levels. The pyramid of energy, for example, in Figure 23.9 shows the amount of energy available at each trophic level. The base of the pyramid reflects primary productivity—the amount of light energy converted to chemical energy in organic molecules, primarily by photosynthetic organisms. The tiers representing successively higher trophic levels grow smaller, forming a pyramid, because only about 10% of the energy contained in one trophic level is transferred to the next. Pyramids of biomass describe the number of individuals at each trophic level multiplied by their biomass. Because the energy available at a trophic level determines the biomass that can be supported, a pyramid of energy and a pyramid of biomass generally have the same shape.
FIGURE 23.9. Ecological pyramids. Note that each trophic level contains less energy and less biomass than the level below it.
Health and Environmental Consequences of Ecological Pyramids
Ecological pyramids convey important lessons. Let's take look at two of these lessons: (1) nondegradable substances accumulate to higher concentrations in organisms living at higher trophic levels; and (2) more humans could be nourished on a vegetarian diet than on a diet containing meat.
Biological magnification. Chemicals that are essential to life—carbon, hydrogen, oxygen, nitrogen, and phosphorus—are passed from one trophic level to the next, thus being continuously recycled from organism to organism. Organic molecules are broken down and then either metabolized for energy or put together to form the biomass of another individual.
However, certain substances—including chlorinated hydrocarbon pesticides such as DDT, heavy metals such as mercury, and radioactive isotopes—are broken down or excreted very slowly. Substances like these tend to stay in the body, often stored in fatty tissues such as liver, kidneys, and the fat around the intestines.
The concentrations of such substances become magnified at each higher trophic level. Remember that, on average, only 10% of the energy available at one trophic level transfers to the next. Consequently, consumers at one trophic level must eat many organisms from the previous trophic level to obtain enough energy to support life. If the organisms that are eaten are contaminated, the pollutant will accumulate in the consumer. Consider a simplified example. Mercury, a nondegradable, potentially harmful substance, enters water through volcanic activity as well as through coal combustion and improper disposal of medical waste. Bacteria convert the mercury to a particularly toxic form, methyl mercury, which then works its way through the food web. If mercury pollutes the water in a certain aquatic ecosystem, phytoplankton (minute photosynthetic organisms that are abundant in aquatic environments) will absorb the mercury, and mercury levels in the phytoplankton will be in some dilute concentration that we will represent as 1. An herbivore such as a zooplankton (a minute non-photosynthetic organism) feeds on large numbers of phytoplankton. The mercury ingested from the phytoplankton thus becomes more concentrated in the zooplankton's tissues than it was in the phytoplankton—let's say about 10 times more. Large numbers of zooplankton may in turn be eaten by a small fish. Once in the fish's body, the mercury stays there and accumulates, let's say another 10 times, to reach a relative concentration of 100. When these mercury-containing fish are eaten by a tuna, the mercury passes to the tuna's body and accumulates. Because many small fish must be consumed to keep a tuna alive, the mercury concentration in the tuna's body might be 1000 times greater than that in water. This tendency of a nondegradable chemical to become more concentrated in organisms as it passes along the food chain is known as biological magnification (Figure 23.10).
FIGURE 23.10. Substances such as mercury that are broken down slowly or excreted slowly tend to accumulate in the body of an organism. The concentration becomes magnified at each successive trophic level, because a consumer must eat many individuals from a lower trophic level to stay alive. Because of this biomagnification, the mercury concentration in the tuna eaten by a human at the end of this food chain is about 1000 times greater than the mercury concentration in the phytoplankton at the beginning.
Biological magnification is of more than theoretical concern for humans. We are often top carnivores, and we continue to pollute our environment with nondegradable, potentially harmful substances. If a human eats a long-lived predator—such as a tuna, swordfish, or shark—that has accumulated methyl mercury through biological magnification as described, that person could begin to accumulate dangerously high levels of mercury, too. High levels of mercury in humans affect the nervous system, causing muscle tremors, personality disorders, and birth defects. For this reason, the Food and Drug Administration now recommends that pregnant women, breast-feeding women, women of child-bearing age, and children stop eating shark, marlin, and swordfish altogether, and limit their intake of albacore (white) tuna to one 6-oz serving a week.
Polychlorinated biphenyls (PCBs) are industrial chemicals suspected of causing cancer and nervous system damage. Like mercury, PCBs are nondegradable and accumulate in the food web. PCBs can be found in chicken, beef, and dairy products. Some farm-raised Atlantic salmon also have PCB levels high enough to trigger health warnings from the Environmental Protection Agency. (Wild salmon have lower levels of PCBs.) In addition, PCBs have been called environmental estrogens because they mimic the effects of the sex hormone estrogen or enhance estrogen's effects. Scientists, recognizing that PCBs have had feminizing effects on certain male animals, are investigating the possibility that PCBs may be reducing human fertility (for example, by reducing human sperm count) and affecting the rates of cancer in certain reproductive structures (breasts, ovaries, prostate gland, and testes).
World hunger. The world's human population has grown at an alarming rate (a problem discussed in Chapter 24). The pyramid of energy suggests a way to feed the growing population more efficiently: persuade people to eat at lower trophic levels. Because only about 10% of the energy available on one trophic level is transferred to the next one, we see that
10,000 calories of corn energy → 1000 calories of beef energy → 100 calories of human energy
However, if humans began to eat one level lower on the food chain, about 10 times more energy would be available to them:
10,000 calories of grain (corn, wheat, rice and so on) → 1000 calories of human energy
In short, more people could be fed and less land would have to be cultivated if we adopted a largely, or exclusively, vegetarian diet (Figure 23.11). This is one reason that people in densely populated regions of the world, including China and India, are primarily vegetarians. As the human population continues to expand, meat is likely to become even more of a luxury throughout the world than it is today.
FIGURE 23.11. Energy pyramids may hold an important lesson for humans. Because only about 10% of the energy available at one trophic level is available at the next higher level, approximately 10 times more people could be fed if they ate a vegetarian diet rather than a diet containing only animal protein.
Stop and think
The traditional diet of the Inuit, one of the native peoples of the North American coast, is part of a relatively long food chain:
Diatoms (producers) → Tiny marine animals → Fish → Seals → Inuits
How might the length of this food chain be one factor contributing to the small population size of Inuit groups?
Earth's resources are limited. Life on Earth is demanding. Many of Earth's reserves would be depleted quickly if not for nature's cycling. Materials move through a series of transfers, from living to nonliving systems and back again (Figure 23.12). Let's look at some of the more important of these biogeochemical cycles, the recurring pathways through which certain materials travel between living and nonliving systems.
FIGURE 23.12. Through biogeochemical cycles, matter cycles between living organisms and the physical environment. This cycling of matter (gray arrows) is in contrast to the path of energy (yellow arrows), which flows through the ecosystem in one direction.
The Water Cycle
Each drop of rain reminds us that water recycles continuously, precipitating from the atmosphere and falling to Earth; collecting in ponds, lakes, or oceans; and then evaporating back into the atmosphere. Most of the rain or snow that falls over land returns to the sea at some point. This cycle provides us with a renewable source of drinking water. Because water is so critical to life, large amounts of it pause for a time in the bodies of living things. In living cells, water helps regulate temperature and acts as a solvent for biological reactions. The very oxygen we breathe is produced from water through the reactions of photosynthesis. Water also cycles back to the environment from living things; plants return 99% of the water they absorb to the atmosphere in the process of transpiration (the evaporation of water from the leaves and stems of plants). All living things carry out cell respiration, which generates water that is exhaled as water vapor. The water cycle is shown in Figure 23.13.
FIGURE 23.13. The water cycle. Water, which is essential to life, cycles from the atmosphere to the land as precipitation, collects in oceans and other bodies of water, and evaporates back to the atmosphere. Water also returns to the atmosphere in the form of vapor lost from the leaves of plants (transpiration). This cycling gives us a renewable source of drinking water. The width of the arrow indicates the relative contribution of the process to the cycle.
Human activities and the water cycle. Humans have disrupted the water cycle in several ways. We have cut down all the trees in vast areas of forest. This deforestation has reduced the amount of transpiration and, therefore, the amount of water vapor returned to the atmosphere. Our urban areas have altered runoff patterns. When rain falls on land, some water seeps down to replenish groundwater. In cities, most rainwater flows into sewers and becomes part of waste water. The most significant way that humans disrupt the water cycle is by using more freshwater than is replenished (discussed in Chapter 24).
On a brighter side, we have developed some ways of recovering freshwater from the oceans and from wastewater. Some coastal cities are building desalination plants to remove salts from ocean water and make it suitable for human consumption. However, desalination is an expensive process that uses a lot of energy. The Tampa Bay Seawater Desalination Plant in Florida is the largest of its kind in North America. The plant produces about 95,000 cubic meters (25 million gallons) of freshwater a day, which accounts for 10% of the drinking water for the Tampa area. Texas is planning to reduce the energy cost of desalination by using a wave-powered plant placed in the Gulf of Mexico. We have also developed technology to recover freshwater from waste water. This technology, often called "toilet-to-tap," is the subject of an Environmental Issue essay in Chapter 2.
The Carbon Cycle
In the carbon cycle, carbon moves from the environment, into the bodies of living things, and back to the environment (Figure 23.14). Living organisms need carbon to build the molecules that give them life: proteins, carbohydrates, fats, and nucleic acids. Conversely, certain processes of living organisms—photosynthesis and respiration—are an integral part of the carbon cycle.
FIGURE 23.14. The carbon cycle. Carbon cycles between the environment and living organisms. Carbon dioxide (CO2) is removed from the environment as producers use it to synthesize organic molecules by photosynthesis. The carbon in those organic molecules then moves through the food web, serving as a carbon source for herbivores, carnivores, and decomposers. Carbon is returned to the atmosphere as CO2 when organisms use the organic compounds in cellular respiration. The width of the arrow indicates the relative contribution of the process to the cycle.
The primary movement of carbon from the environment into living organisms occurs during photosynthesis, as plants, algae, and cyanobacteria use carbon dioxide (CO2) to produce sugars and other organic molecules. When photosynthesizers are eaten by herbivores, these organic molecules serve as a carbon source for the herbivores. The herbivores use that carbon to produce their own organic molecules, which then serve as a carbon source for carnivores. When an organism dies, the organic molecules in its body will serve as a carbon source for decomposers. However, while alive, all organisms cycle carbon back to the environment through cellular respiration, which breaks down organic molecules to CO2.
Some carbon is significantly delayed before being cycled back into the environment. For example, carbon may remain tied up in the wood of some trees for hundreds of years. Most of the carbon that is currently not cycling is thought to be stored in limestone, a type of sedimentary rock formed from the shells of marine organisms that sank to the bottom of the ocean floor and were covered and compressed by newer sediments. Other vast carbon stores are the fossil fuels (oil, coal, and natural gas), so named because they formed from the remains of organisms that lived millions of years ago.
Three processes release carbon from long-term storage and return it to the environment: decomposition, erosion, and combustion. When trees die, for example, the natural process of decomposition will make the carbon available for new organisms, which will respire and release CO2 to the atmosphere. The carbon in limestone is recycled through erosion. Millions of years after it forms, sedimentary rock containing limestone can be lifted to Earth's surface by movement of tectonic plates, where it is eroded by chemical and physical weathering. These changes make the carbon available to cycle through the food web once again. Combustion, or burning, returns the carbon in fossil fuels to the environment. Today, fossil fuels such as coal, oil, and natural gas are being burned in huge amounts, and the carbon they contain is being returned to the atmosphere as CO2.
Increasing carbon dioxide levels. Two human activities— burning fossil fuels and deforestation—are altering the carbon cycle and increasing the atmospheric carbon dioxide level. Recall that fossil fuels were formed from deposits of dead plants and animals that were buried in sediments and escaped decomposition between 345 million and 280 million years ago. High temperatures and pressure over millions of years converted the deposits to coal, oil, and natural gas. The burning of these fuels returns carbon to the environment in the form of CO2. Deforestation—the removal of a forest without adequate replanting, as is occurring in areas of the Amazon rain forest and the U.S. Pacific Northwest— increases atmospheric CO2 in two ways. First, it reduces the removal of CO2 from the atmosphere by eliminating trees' photosynthesis. Second, the trees are often burned after cutting to clear the area, and burning adds CO2 to the atmosphere.
The rise in atmospheric CO2 raises concerns because CO2 is one of the greenhouse gases that play a role in warming our atmosphere. Indeed, the scientific consensus is that the increase in atmospheric CO2 is causing a rise in temperatures throughout the world—known popularly as global warming. Global climate change and its many consequences are discussed in the next chapter.
The rising level of atmospheric CO2 is also making the oceans more acidic. When CO2 dissolves in water, it forms carbonic acid. The acidity decreases the availability of calcium carbonate, which is needed by certain forms of marine life to form skeletons and shells. Some of these animals are at the bottom of the food chain, so their loss could have consequences for other animals that depend on them for food. Corals also build their skeletons from calcium carbonate, and coral reefs form the foundation for many fisheries. The arctic oceans are particularly vulnerable, because carbon dioxide is more soluble in cold water. Studies are underway to determine the possible effects of the acidification of arctic waters.
The Nitrogen Cycle
Nitrogen is a principal constituent of several molecules needed for life, including proteins and nucleic acids. Nitrogen is often in short supply to living systems, so its cycling is particularly important (Figure 23.15).
FIGURE 23.15. The nitrogen cycle. Atmospheric nitrogen can be converted to ammonium by nitrogen-fixing bacteria, which then convert the ammonium to nitrate, the main form of nitrogen absorbed by plants. Plants use nitrate to produce proteins and nucleic acids. Animals eat the plants and use the plant’s nitrogen-containing chemicals to produce their own proteins and nucleic acids. When plants and animals die, their nitrogen-containing molecules are converted to ammonium by bacteria. Denitrifying bacteria return nitrogen to the atmosphere. The width of the arrow indicates the relative contribution of the process to the cycle.
The largest reservoir of nitrogen is the atmosphere, which is about 79% nitrogen gas (N2). However, nitrogen gas cannot interact with life directly. (As you sit reading, you are bathed in nitrogen gas, but you do not interact with it.) Before it can be used, nitrogen gas must be converted to a form that living organisms can use—ammonium (NH4+). The process of converting nitrogen gas to ammonium is called nitrogen fixation. This is performed by nitrogen-fixing bacteria, many of which live in nodules on the roots of leguminous plants such as peas and alfalfa. Ammonium is then converted to nitrite (NO2-) and then to nitrate (NO3-) by nitrifying bacteria living in the soil, in a process called nitrification. The ammonium or nitrates are first absorbed by plants, which use the nitrogen to form plant proteins and nucleic acids. The nitrogen then passes through the food web and is incorporated into the nitrogen-containing compounds of animals.
When plants and animals die, decomposers such as bacteria break down the waste products and dead bodies of plants and animals, producing ammonium (NH4+). Much of the ammonium is converted to nitrate by nitrifying bacteria. Other bacteria balance the nitrogen cycle by performing a process called denitrification, in which the nitrates that are not assimilated into living organisms are converted to nitrogen gas. Denitrifying bacteria are found in wet soil, swamps, and estuaries.
Human activities and the nitrogen cycle. Human burning of fossil fuels has affected the nitrogen cycle by adding nitrogen dioxide (NO2) gas (as well as sulfur dioxide—SO2—gas) to the atmosphere, where they react with water vapor to form acids. The acids eventually fall to Earth as acid rain, which is killing trees in northern forests and fish and amphibians in aquatic environments. (Acid rain is discussed in Chapter 2.) Sunlight can cause hydrocarbons and NO2 to form photochemical smog that contains ozone, which is especially harmful to the respiratory system.
The Phosphorus Cycle
Phosphorus is an important component of many biological molecules, including the genetic material DNA, energy-transfer molecules such as adenosine triphosphate (ATP), and phospholipids found in membranes. Phosphorus is also essential in vertebrate bones and teeth.
Unlike the water, carbon, and nitrogen cycles, the phosphorus cycle does not have an atmospheric component (Figure 23.16). Instead, the reservoir for phosphorus is sedimentary rock, where it is found as phosphate ions. The cycle begins when erosion caused by rainfall or runoff from streams dissolves the phosphates in the rocks. The dissolved phosphate is readily absorbed by producers and incorporated into their biological molecules. When animals eat the plants, the phosphates are passed through food webs. Decomposers return the phosphates to the soil or water, where they become available to plants and animals once again. Much of the phosphate is lost to the sea. Although some of this phosphate may cycle through marine food webs, most of it becomes bound in sediment. The phosphates in sediment become unavailable to the biosphere unless geological forces bring the sediment to the surface.
FIGURE 23.16. The phosphorus cycle. Phosphates dissolve from rocks in rainwater. They are then absorbed by producers and passed to other organisms in food webs. Decomposers return phosphates to the soil. Some phosphates are carried to the oceans and eventually deposited in marine sediments. The width of the arrow indicates the relative contribution of the process to the cycle.
Eutrophication. Human activity is also disturbing the phosphorus and nitrogen cycles. Inadequate nitrogen and phosphorus in the soil can slow plant growth. For this reason, many crops are fertilized with synthetic preparations containing both nitrogen (as ammonium) and phosphates. Phosphates from fertilizer can wash into nearby streams, rivers, ponds, or lakes in runoff. Fertilizer runoff is one cause of eutrophication, the enrichment of water in a lake or pond by nutrients. The nutrient boost can lead to an explosive growth of photosynthetic organisms, such as algae in deeper water and weeds in shallow areas. When these organisms die, their remains accumulate at the bottom of the lake. Decomposers then thrive, depleting the water of dissolved oxygen. Gradually, fish, such as pike, sturgeon, and whitefish, which require higher oxygen concentrations in the water, are replaced by species such as catfish and carp that can tolerate lower levels of dissolved oxygen. Eventually, the oxygen depletion can kill all fish and bottom-dwelling life.
Eutrophication is not just a problem in lakes and ponds; there are now more than 400 coastal regions of oceanic dead zones, regions where marine life can't survive. The dead zones are caused by fertilizer runoff, which increases available nutrients for algae, and by global climate change, which increases the temperature of the top, well-oxygenated layer of the water, and prevents mixing with the lower, less-oxygenated layers. One of the largest dead zones is in the Gulf of Mexico. This dead zone, which is sometimes the size of New Jersey, is caused by the fertilizer runoff from the farms in the Midwest that is carried to the Gulf by the Mississippi River.
What would you do?
The nitrates that cause the dead zone in the Gulf of Mexico come from farms along the upper Mississippi River, but the results are experienced by the people living along the coast of the Gulf. Who do you think should be financially responsible for addressing this problem? Why?
Stop and think
A dead zone is caused by the expansion of a population of organisms because of an increase in nutrient supply and then the depletion of oxygen caused by the decomposition of those organisms. Do you predict that the Deepwater Horizon oil spill will increase or decrease the size of the dead zone in the Gulf of Mexico. Why?
Maintaining Our Remaining Biodiversity
Faced with powerful evidence of a marked reduction in Earth's biodiversity, some experts express guarded optimism that we may be able to slow or even stop this trend. Various measures are being suggested.
More developed (richer) and less developed (poorer) countries must take a careful census of their life-forms to produce inventories of the species they harbor. This need is especially acute in tropical countries, which, unfortunately, are often least able to afford such programs. Advocates thus argue that the effort must be worldwide, with developed countries subsidizing the research in less affluent areas.
Nations must cooperate in linking the economic development of impoverished regions, particularly the tropics, to conservation. Lending agencies must stipulate that certain areas be set aside and left undeveloped. Decisions about which locations are to remain undeveloped should be made with the advice of ecologists, so the effects on the world's biodiversity are minimized. (What effect do you think such rules would have had on the development of the United States if such restrictions were in place during the industrial revolution?)
Educating the people whose decisions have the most immediate effect on biodiversity should be a priority. For example, recent studies have shown that native people can make more money through sustainable use of a forest (as in gathering nuts and fruits) than they would if the forest were cleared for agriculture or ranching. Many people in developing countries are also learning that scientists worldwide are interested in the medicines and healing knowledge they have, so in a sense, the educational opportunities are reciprocal.
Tropical rain forests are the most productive and biologically diverse ecosystems on land.
Biologists worldwide must work more closely with zoning and land-use personnel to maximize the use of areas being cleared or cultivated. Such multiple land-use planning includes, for example, rotating crops to maintain healthy soil and increase biodiversity or replacing single-species forests planted by paper companies with more diverse forests that could harbor more species of animals. Of course, the paper companies may ask why they should be required to reduce their profits to increase biodiversity. This is the type of issue that will have to be negotiated in the process of bringing people to an understanding that what is good for the environment is ultimately good for everyone.
Questions to Consider
• Many of the issues regarding preserving biodiversity involve trade-offs between activities that are valuable to society and activities that reduce the biodiversity that allows ecosystems to provide a stable food supply. How can these trade-offs be evaluated? Who should be responsible for making these decisions?
• Should agencies that work to maintain biodiversity receive governmental funding or rely on public funding?
In this chapter we see that organisms are not randomly scattered over Earth. Instead, each is adapted to a particular niche, which allows it to remain healthy and to reproduce. Recall that an organism's niche includes physical, chemical, and biological factors. We also see in this chapter that the only input to nearly every ecosystem is solar energy, which producers use to manufacture organic molecules. Energy then flows through food webs. Resources, such as water, carbon, nitrogen, and phosphorus, cycle through living and nonliving systems. It's no surprise, then, that even small changes to an organism's niche can have ripple effects that disturb the entire ecosystem.
A current example of this ripple effect is damage being done to coral reefs. The damage begins with humans upsetting the carbon cycle by burning fossil fuels and cutting down forests. These human activities have raised the level of carbon dioxide in the atmosphere. Elevated atmospheric carbon dioxide causes more carbon dioxide to diffuse into the oceans, where it dissolves, forming carbonic acid. As a result, the oceans are becoming more acidic, causing damage to coral reef ecosystems. This damage, in turn, damages many productive fisheries.
As the human population grows, it consumes and pollutes more resources (discussed in the next chapter), which can have widespread effects on ecosystem Earth. Due to the interconnectedness of living and nonliving systems in an ecosystem, one effect of human disturbance may be changes in biodiversity—species richness. Biodiversity includes the number of species living in a given area and the abundance of each. Recently, scientists have raised an alarm worldwide in response to indications that globally, and especially in certain critical areas, biodiversity is decreasing. We are losing species, perhaps as many as 100 each day, largely because of human activity. Mass extinctions, the loss of many species from Earth, have occurred in previous eras, but never before because of humans. Today, many species are being forced to compete with us as we change the environment to suit ourselves, rendering it unsuitable for them.
How many species are there? No one knows. Scientific estimates range from 8 million to 30 million species now in existence, with about 90% of them living on land. However, only about 1.7 million of these species have been identified, and only 3% of those have been studied.
Most terrestrial species live in the tropics. Although tropical rain forests cover only 7% of Earth's surface, they are home to between 50% and 80%—estimated as 7 million to 8 million—of Earth's species (see the Ethical Issue essay, Maintaining Our Remaining Biodiversity).
Habitat destruction is largely responsible for the loss of biodiversity. The primary reasons habitats are destroyed are to create living space for humans and to provide room or resources for economic development. Calculations indicate that most of the original tropical rain forest will be destroyed by 2040. This means that in the next 30 years, we could lose a quarter of all the species on Earth. Evolutionary biologists tell us that, currently, we are losing species at the rate of 1000 to 10,000 times the average rate for the last 65 million years, since the extinction of the dinosaurs.
Rain forest is not the only type of habitat being destroyed. Lumbering and acid rain are destroying northern temperate forests. Also, marine ecosystems are being destroyed by pollution, overfishing, and coastal development. For example, shark populations have declined by more than 70% in the last 15 years. Commercial fishing is largely to blame for the reduction of shark populations. People eat shark meat and use the fins to make soup. Moreover, some sharks, hammerheads for example, are accidentally caught when they try to prey on the herring and squid being used as bait by people fishing for tuna or swordfish. The United States has federal regulations that restrict shark fishing; however, shark fishing is not regulated in Europe. If shark numbers continue to fall, we may not be able to save them from extinction. Sharks grow slowly, mature late in life, and have few young at a time; and females do not reproduce every year.
Perhaps you are wondering why we should care about the loss of species, especially those we never knew of. Several materialistic reasons immediately come to mind. The first direct benefit to humans of maintaining biodiversity is that biodiversity preserves the genetic diversity encoded in the chromosomes of species. This genetic diversity is useful for crossbreeding. The crossbreeding of two strains of a species, each possessing different desirable traits, can combine those desirable traits in a single new strain. Interestingly, the usefulness of genetic diversity as a reservoir of genetic stores has increased dramatically with modern technology. Owing to recent advances in genetic engineering, specific genes can be isolated from their source organisms and moved into other kinds of organisms. Thus the maintenance of genetic stores becomes even more important as we use genetic engineering to maximize crop yields or otherwise create new forms of organisms that might be useful to society (see Chapter 21).
The second direct benefit to humans of maintaining biodiversity lies in developing new kinds of medicines. About 25% of all known drugs come from plants. The rosy periwinkle from Madagascar, as discussed in Chapter 1, is a source of two anticancer drugs, vincristine and vinblastine; and the Mexican yam was once a source of oral contraceptives. Most of the plants with medicinal value have been found in the tropics, which as noted are being rapidly destroyed.
A third benefit is the many services that biodiversity provides in functioning ecosystems—cleansing water and air, enriching soil, cycling minerals, and pollinating crops, for example. Recall the important role of organisms in the biogeochemical cycles discussed earlier in this chapter. For example, as pollinators, bees contribute $15 billion to the productivity of agriculture in the United States each year. This is one reason for great concern over the decline in bee populations in recent years. Most of the organisms that could become crop pests are kept under control by their own predators.
We see, then, that there are many practical reasons to preserve biodiversity. Earth and the organisms inhabiting it are interconnected through energy flow through food webs and biogeochemical cycles. The health of ecosystem Earth depends on biodiversity.
In this chapter, we learned about Earth, our home planet— the energy flow through ecosystems, chemical cycles, and biodiversity. In the next chapter, we will consider the growth of the human population and how that growth affects Earth's resources.
Highlighting the Concepts
Earth as an Ecosystem (p. 491)
• Ecology is the study of the interactions between organisms and between organisms and their environment.
• Materials on Earth are recycled among living organisms and the environment. The only outside contribution to Earth is energy from the sun.
Biosphere (p. 492)
• The biosphere consists of all the ecosystems on Earth. An ecosystem consists of a community of organisms and their physical environment. An organism's niche is the specific role it plays in the community; its habitat is the place where it lives.
Ecological Succession (pp. 492-494)
• Ecological succession is the sequence of changes in communities over long periods. Primary succession occurs where no community previously existed. The first settlers of a community are called pioneer species. A climax community eventually forms and remains as long as no disturbances occur. Secondary succession describes the sequences of changes that occur when an existing community is disturbed by human or natural means.
Energy Flow (pp. 494-498)
• Most of the energy in living systems comes from solar energy absorbed and stored in the molecules of photosynthetic organisms, called producers. The energy stored in producers' molecules enters the animal world through plant-eating herbivores (primary consumers), which may be eaten by carnivores (secondary consumers) or by animals that eat other carnivores (tertiary consumers). Decomposers consume dead organic material, releasing inorganic compounds that can be used by producers. The position of an organism in these feeding relationships is referred to as a trophic level.
• The feeding relationships that result in the one-way flow of energy through the ecosystem and in the cycling of materials among organisms are called food chains or food webs.
• Ecological pyramids depict the amount of energy or biomass at each trophic level. Pyramids of energy show the loss of energy from one trophic level to another. On average, only 10% of the energy available at one trophic level is available at the next higher level. Ecosystems generally have only four or five trophic levels. Pyramids of biomass have the same shape as pyramids of energy, because the available energy determines the biomass that can be formed.
• Biological magnification—the tendency for certain nondegradable substances to become more concentrated in organisms as it passes through a food web—is a consequence of the energy loss between trophic levels. Thus, top carnivores are most likely to be poisoned by nondegradable toxic substances in the environment.
• Because energy is lost with each transfer in the trophic scale, one way to feed more people would be for humans to adopt a largely vegetarian diet.
Chemical Cycles (pp. 498-503)
• In biogeochemical cycles, materials cycle between organisms and the environment.
• The water cycle is the pathway of water as it falls as precipitation; collects in ponds, lakes, and seas; and returns to the atmosphere through evaporation.
• The water cycle can be disturbed because of humans causing shortages through overuse of water supplies. Most water use is for agriculture, primarily irrigation. Irrigation can cause salts to accumulate in the soil (salinization), which can make the land unusable for agriculture.
• The carbon cycle is the worldwide circulation of carbon from the carbon dioxide in air to the carbon in organic molecules of living organisms and back to the air. Carbon enters living systems when photosynthetic organisms incorporate carbon dioxide into organic materials. Carbon dioxide is formed again when the organic molecules are used by the living organism for cellular respiration.
• Humans have disturbed the carbon cycle through activities that increase carbon dioxide levels in the atmosphere: burning fossil fuels, which directly adds carbon dioxide, and deforestation, which decreases the removal of carbon dioxide from the atmosphere. Carbon dioxide is a greenhouse gas that traps heat in Earth's atmosphere and causes global warming.
• The nitrogen cycle is the worldwide circulation of nitrogen from nonliving to living systems and back again. Atmospheric nitrogen (N2) cannot enter living systems. Nitrogen-fixing bacteria living in nodules on the roots of leguminous plants convert N2 to ammonium (NH4+), which is converted to nitrites (NO2-) and then to nitrates (NO3-) by nitrifying bacteria. The ammonium and nitrates are then available to plants to use in their proteins and nucleic acids. Next, the nitrogen is transferred to organisms that consume the plants. Nitrates that are not assimilated into living organisms can be converted to nitrogen gas by denitrifying bacteria.
• Humans affect the nitrogen cycle by burning fossil fuels, which adds nitrogen dioxide to the atmosphere. Nitrogen dioxide reacts with water vapor in the atmosphere to form nitric oxide that falls to Earth as acid rain. Sunlight converts nitrogen dioxide and hydrocarbons to photochemical smog that damages the respiratory system.
• Phosphorus in the form of phosphates is washed from sedimentary rock by rainfall. The dissolved phosphates are used by producers to produce important biological molecules, including DNA and ATP. When animals eat producers or other animals, phosphates are passed through the food webs. Decomposers release phosphates into the soil or water from dead organisms.
• Humans have disrupted the nitrogen and phosphorus cycles through the industrial fixation of nitrogen to produce fertilizer and the addition of phosphates to fertilizer. Some of the phosphates wash into nearby waterways. Fertilizer runoff is one cause of eutrophication.
Biodiversity (pp. 503-505)
• Biodiversity, the number and variety of living things, is being reduced dramatically, largely because of human activity. Most of the loss is occurring in the tropics and is due to habitat destruction. Two practical reasons for concern over the loss of biodiversity are that the disappearing species could have genes that would someday prove useful or that they could be found to produce chemicals with medicinal qualities.
Reviewing the Concepts
1. What is ecological succession? How does primary succession differ from secondary succession? pp. 492-494
2. Explain how energy flows through an ecosystem. pp. 494-496
3. Define the following terms: producer, primary consumer, secondary consumer, and decomposer. Explain the role each plays in cycling nutrients through an ecosystem. p. 494
4. Explain why the feeding relationships in a community are more realistically portrayed as a food web than as a food chain. p. 494
5. Explain the reasons for the inefficiency of energy transfer from one trophic level to the next higher one. Why does this loss of energy limit the number of possible trophic levels? p. 496
6. Define an energy pyramid. What causes the pyramidal shape? p. 496
7. What is meant by a biomass pyramid? How does it compare to an energy pyramid? p. 496
8. Define biological magnification. Explain how biological magnification is a consequence of the energy loss between trophic levels. Why should humans be concerned about biological magnification? p. 497
9. Explain why more people could be fed on a vegetarian diet than on a diet that contains meat. pp. 497-498
10. Describe the water cycle, the carbon cycle, the nitrogen cycle, and the phosphorus cycle. pp. 498-503
11. Explain some of the ways that humans are disturbing the water cycle. p. 499
12. Which human activities are primarily responsible for the rising level of atmospheric carbon dioxide? pp. 500-502
13. Define biodiversity. Where is it greatest? Why should we be concerned about the loss of biodiversity? pp. 503-505
14. Which of the following is not constantly recycled in the biosphere?
15. Secondary succession is most likely to be found
a. on rocky outcroppings.
b. in areas that were clear-cut for timber.
c. where an island has appeared.
d. in areas where estuaries have filled in to form land.
16. An organism's role in a community is its _____.
17. An animal that eats an herbivore is called a _____.
Applying the Concepts
1. In a particular grassland ecosystem, energy flows in the following path:
Grass → Crickets → Frogs → Herons
Assuming that the efficiency of energy transfer is typical, how many calories will the herons receive if there were 100,000 calories of grass?
2. Explain why the mercury levels in a lake may be low enough for the water to be safe to drink, yet fish from the same lake may be poisonous.
3. You are a crime scene investigator at a murder scene. Stuck to the bottom of the victim's shoe is a leaf unlike any others in the vicinity of the crime. What clues could you get from the leaf?
Becoming Information Literate
Malaria is a disease transmitted by mosquitoes that infects 300 million people a year and kills about 200 million of them. DDT is a pesticide that is an affordable and effective way to kill mosquitoes. After being widely used in the 1950s, DDT was blamed for the alarming decline in certain species of birds during the 1960s and 1970s. The United States banned the use of DDT during the 1970s, and many other countries followed suit. Because of these bans, neither the United States nor the World Health Organization will fund the use of DDT to control malaria-causing mosquitoes. Some of the areas hit hardest by malaria—Africa, Asia, and Latin America—are too poor to afford other pesticides, which cost four to six times more than DDT.
Use at least three reliable sources (journals, newspapers, websites) to consider the social, health, economic, and political controversies regarding outside funding for the use of DDT to control mosquito populations in poor, malaria-stricken countries. Cite your sources, and explain why you chose the ones you used.