BIOLOGICAL ENERGY USE, ECOSYSTEM FUNCTIONING OF

THE NEED FOR ENERGY

The bald eagle has been the national symbol of the United States since 1782, representing freedom, power, and majesty. The eagle's impressively large beak and talons, combined with its ability to detect details at great distances, make this bird an imposing predator. Interestingly, the most important factor determining what this bird looks like, how it behaves, the prey it seeks, how it interacts with the environment, and the number of bald eagles that are supported by the environment, is energy. In fact, energy is probably the most important concept in all of biology.

Energy is the ability to do work, and work is done when energy has been transferred from one body or system to another, resulting in a change in those systems. Heat, motion, light, chemical processes, and electricity are all different forms of energy. Energy can either be be transferred or converted among these forms. For example, an engine can change energy from fuel into heat energy. It then converts the heat energy into mechanical energy that can be used to do work. Likewise, the chemicals in food help the bald eagle to do the work of flying; heat allows a stove to do the work of cooking; and light does the work of illuminating a room. Biological work includes processes such as growing, moving, thinking, reproducing, digesting food, and repairing damaged tissues. These are all actions that require energy.

The first law of thermodynamics states that energy can neither be created nor destroyed. This seems to imply that there is an abundance of energy: If energy cannot be destroyed, then there must be plenty available to do biological work. But biological work requires high-quality, organized energy.

Energy can be converted from one form to another. According to the second law of thermodynamics, each time energy is converted, some useful energy is degraded into a lower-quality form—usually heat that disperses into the surroundings. This second law is very interesting in terms of biology because it states that every time energy is used, energy quality is lost: The more energy we use, the less there is to do useful work. As we shall see, this principle influences every biological event, from interactions between predators and their prey to how many species can live in a habitat.

Plants and Animals Get Energy from the Sun

Living systems are masters of energy transformation. During the course of everyday life, organisms are constantly transforming energy into the energy of motion, chemical energy, electrical energy, or even to light energy. Consequently, as dictated by the laws of thermodynamics, each living system is steadily losing energy to its surroundings, and so must regularly replenish its supply. This single fact explains why animals must eat and plants must harvest light energy through photosynthesis.

Where does this supply of energy come from? Most life on Earth depends on radiant energy from the Sun, which delivers about 13 × 10²³ calories to Earth each year. Living organisms take up less than 1 percent of this energy; Earth absorbs or reflects most of the rest. Absorbed energy is converted to heat, while energy is reflected as light (Figure 1). Solar energy helps create the different habitats where organisms live, is responsible for global weather patterns, and helps drive the biogeochemical cycles that recycle carbon, oxygen, other chemicals, and water. Clearly, solar energy profoundly influences all aspects of life.

Solar energy also is the source of the energy used by organisms; but to do biological work, this energy must first be converted. Most biological work is accomplished by chemical reactions, which are energy transformations that rely on making or breaking chemical bonds. Chemical energy is organized, high-quality energy that can do a great deal of work, and is the form of energy most useful to plants and animals. Solar energy can be used to create these chemical bonds because light energy that is absorbed by a molecule can boost that molecule's electrons to a higher energy level, making that molecule extremely reactive. The most important reactions involve the transferring of electrons, or oxidation-reduction (REDOX) reactions because, in general, removing electrons from a molecule (oxidation) corresponds to a release of energy. In other words, organisms can do work by oxidizing carbohydrates, converting the energy stored in the chemical bonds to other forms. This chemical energy is converted from solar energy during the process of photosynthesis, which occurs only in plants, some algae, and some bacteria.

How Plants Get Energy: Photosynthesis

A pigment is a material that absorbs light. Biologically important pigments absorb light in the violet, blue, and red wavelengths. Higher-energy wavelengths disrupt the structure and function of molecules, whereas longer (lower-energy) wavelengths do not contain enough energy to change electron energy levels. Photosynthesis is restricted to those organisms that contain the appropriate pigment combined with the appropriate structures such that the light energy can trigger useful chemical reactions. If an organism cannot perform photosynthesis, it cannot use solar energy. The Sun's energy enters the food chain through photosynthesis. Plants are the most important photosynthesizers in terrestrial systems, while photosynthesis in aquatic systems generally occurs in algae.

In plants, chlorophyll is the pigment that absorbs radiant energy from the Sun. This allows the transfer of electrons from water to carbon dioxide, creating the products glucose and oxygen. The equation for photosynthesis is: Photosynthesis takes atmospheric carbon dioxide and incorporates it into organic molecules—the carbon dioxide is "fixed" into the carbohydrate. These molecules are then either converted into chemical energy or used as structural molecules. The first powers living systems; the second is what living systems are composed of.

To release energy, the electrons can be removed from glucose and used to create ATP, a molecule that supplies a cell's short-term energy needs. This latter occurs in a series of reactions known as respiration. (Body heat is a by-product of these reactions.) The most efficient respiration reactions are those that use oxygen to accept the electrons removed from glucose. Thus respiration is the reverse of photosynthesis: Organic compounds are oxidized in the presence of oxygen to produce water and carbon dioxide. Photosynthesis captures energy, and both products of photosynthesis are required in the energy-releasing reactions of respiration.

By converting radiant energy from the Sun into stored chemical energy, plants essentially make their own food: All that a plant needs to survive is water, carbon dioxide, sunlight, and nutrients. Plants need not rely on any other organism for their energy needs. In contrast, the only forms of life in terrestrial systems that do not depend on plants are a few kinds of bacteria and protists. Every other living thing must eat other organisms to capture the chemical energy produced by plants and other photosynthesizers.

How Animals Get Energy: Energy Flow Through an Ecosystem

A heterotroph is an organism that relies on outside sources of organic molecules for both energy and structural building blocks. This includes all animals and fungi and most single-celled organisms. An autotroph can synthesize organic molecules from inorganic elements. Most autotrophs are photosynthetic; a few, occurring in very restricted areas such as deep ocean trenches, are chemosynthetic.

Consequently, each organism depends in some way upon other organisms; organisms must interact with each other to survive. For example, a heterotroph must eat other organisms to obtain energy and structural compounds, while many important plant nutrients originate from animal wastes or the decay of dead animals. The study of these interactions is the subject of the science of ecology. Interactions that involve energy transfers between organisms create food chains. A food chain portrays the flow of energy from one organism to another.

Three categories can describe organisms in a community based on their position in the food chain: producers, consumers, and decomposers. These categories are also known as trophic levels. Plants "produce" energy through the process of photosynthesis. Consumers get this energy either directly or indirectly. Primary consumers eat plants directly, whereas secondary consumers get their energy from plants indirectly by eating the primary consumers. Energy enters the animal kingdom through the actions of herbivores, animals that eat plants. Animals that eat other animals are carnivores, and omnivores are animals that eat both plants and animals. Decomposers get their energy from consuming nonliving things; in the process releasing inorganic nutrients that are then available for reuse by plants.

A North American prairie food chain begins with grass as the producer. The grass is eaten by a prairie dog, a primary consumer. The prairie dog falls prey to a secondary consumer, such as a black-footed ferret. Some food chains contain a fourth level, the tertiary consumer. In this example, the ferret could be eaten by a golden eagle, a top predator. Because organisms tend to eat a variety of things, food chains are generally linked to form overlapping networks called a food web. In addition to the ferret, golden eagles eat ground squirrels and rabbits, each of which are primary consumers in separate food chains. A prairie food web would depict all of the interactions among each of these food chains.

Not all of the energy produced by photosynthesis in the grass is available to the prairie dog. The grass requires energy to grow and produce structural compounds. Similarly, the prairie dog eats food for the energy it needs for the work of everyday life: It needs to find and digest the grass; to detect and avoid predators; to find a mate and reproduce. Each time one of these processes uses energy, some energy is lost to the environment as heat. By the time the predator eats the prairie dog, very little of the original energy from the grass is passed on (Figure 1).

The fact that energy is lost at each stage of the food chain has tremendous influence on the numbers of producers and consumers that can be supported by any given habitat. This can best be illustrated graphically by ecological pyramids. Figure 2 is an example of an energy pyramid. Two other types of ecological pyramids are a pyramid of numbers and a pyramid of biomass. Each section of these pyramids represents a trophic level for the represented community—the producers form the base of the pyramid, the primary consumers the second level, and the secondary and tertiary consumers the third and fourth levels, respectively. Decomposers are also often included. The size of each section represents either the number of organisms, the amount of energy, or the amount of biomass.

Biomass refers to the total weight of organisms in the ecosystem. The small amount of solar radiation incorporated into living systems translates into the production of huge amounts of biomass: On a worldwide basis, 120 billion metric tons of organic matter are produced by photosynthesis each year. However, a plant uses only a fraction of the energy from photosynthesis to create this biomass; similarly, only a fraction of the energy consumed by an animal is converted to biomass. The rest is used up for metabolic processes and daily activities. The amount of biomass, the amount of energy, and the number of species present at each level of the pyramid are therefore less than in the trophic level before it. On the Serengeti Plain in Africa, there are many more blades of grass than there are zebras, and many more zebras than there are lions.

As shown in Figure 2, when a herbivore eats a plant, only about 10 percent of the energy stored in that plant is converted to animal biomass; the rest is used up in everyday activities. The same is true for each succeeding trophic level. Note that the commonly cited "10 percent energy transfer" figure is only a rough average based on many studies of different ecosystems. Scientists have found that actual transfer rates vary from 1 to 20 percent.

In a temperate forest ecosystem on Isle Royale, Michigan, ecologists found that it takes 762 pounds (346 kg) of plant food to support every 59 pounds (27 kg) of moose, and that 59 pounds of moose are required to support every one pound (0.45 kg) of wolf. The basic point is that massive amounts of energy do not flow from one trophic level to the next: energy is lost at each stage of the food chain, so there are more plants than herbivores and more herbivores than carnivores.

As links are added to the food chain, the amount of energy becomes more and more limited; this ultimately limits the total number of links in the food chain. Most habitats can support food chains with three to four trophic levels, with five being the usual limit. One study of 102 top predators demonstrated that there are usually only three links (four levels) in a food chain.

These factors lie behind the idea that adopting a vegetarian diet is a strategy in line with a sustainable lifestyle. Take the example of the plant → moose → wolf food chain given above. More energy is available to the wolf if it eats the plant rather than the moose. If a person is stuck on a desert island with a bag of grain and ten chickens, it makes little energetic sense to first feed the grain to the chickens, then eat the chickens. More energy is conserved (because fewer energy conversions are required) if the person first eats the chickens, then the grain. Similarly, if everybody were to adopt a vegetarian diet, much more food would be available for human consumption.

Thirty to 40 percent of the calories in a typical American diet come from animal products. If every person in the world consumed this much meat, all the agricultural systems in the world would be able to support only 2.5 billion people. In 1999 these same systems supported 6 billion people, primarily because the majority of the people living in less developed countries consume fewer animal products.

The energy flow through an ecosystem is the most important factor determining the numbers, the types, and the interactions of the plants and animals in that ecosystem.

Where an Animal's Energy Goes

During an animal's lifetime, about 50 percent of the energy it consumes will go to general maintenance (everyday metabolic processes), 25 percent to growth; and 25 percent to reproduction.

Animal nutritionists have developed formulas to guide them in recommending the amount of food to feed animals in captive situations such as in zoos. First, the number of calories needed to maintain the animal while at rest is determined—this is called the basal metabolic rate (BMR). In general, a reptile's BMR is only 15 percent that of a placental mammal, while a bird's is quite a bit higher than both a reptile's and a mammal's. For all animals, the number of calories they should receive on a maintenance diet is twice that used at the basal metabolic rate. A growing animal should receive three times the number of calories at the BMR, while an animal in the reproductive phase should receive four to six times the BMR.

Recall that during respiration, animals gain energy from glucose by oxidizing it—that is, by transferring the electrons from the glucose to oxygen. Because molecules of fat contain more hydrogen atoms (and therefore electrons) than either glucose or proteins, the oxidation of fat yields almost twice the calories as that of carbohydrates or proteins. Each gram of carbohydrate yields four calories, a gram of protein yields five calories, while a gram of fat yields nine calories.

THE EFFECTS OF ENERGY

Adaptations

Adaptations are features an organism has or actions it does that help it survive and reproduce in its habitat. Each organism has basic needs, including energy, water, and shelter; adaptations allow an organism to obtain its basic needs from its habitat. Body structures, behavioral characteristics, reproductive strategies, and physiological features are all examples of adaptations. Different habitats pose different challenges to organisms, and adaptations can be thought of as solutions to these challenges. For example, important adaptations in a desert habitat would include those that allow an animal to conserve water and store energy.

Energy affects adaptations in two fundamental ways. First, plants and animals need energy to survive, and adaptations may allow them to obtain, use, and, in many cases, conserve energy. Not only do plants and animals need a great deal of energy to fuel the chemical reactions necessary for survival, but also this energy can be in limited supply, particularly to consumers. Accordingly, many adaptations seen in different species revolve around the conservation of energy.

Second, solar radiation can result in striking differences among habitats. A rain forest is fundamentally different from a tundra first and foremost because there is a much greater amount of energy available to a rain forest. The average daylight hours in Barrow, Alaska, vary from about one hour in January to nearly twenty-four hours in June. The reduced growing season limits the amount of energy that plants can produce. Contrast this with Uaupés, Brazil, which receives twelve hours of sunlight each month of the year. The year-long growing season contributes to the fact that rain forests are the most productive habitats on Earth.

Regulating Body Temperature. Metabolism refers to all the chemical reactions that take place within an organism. To occur at rates that can sustain life, metabolic reactions have strict temperature requirements, which vary from species to species. There are two ways by which an organism can achieve the appropriate temperature for metabolism to occur. The first is to capture and utilize the heat generated by various energy conversions; the second is to rely on external energy sources such as direct sunlight. In other words, body temperature can be regulated either internally or externally.

An endothermic animal generates its own body temperature, while an ectothermic animal does not. In general, endothermic animals have constant body temperatures that are typically greater than that of the surrounding environment, while ectothermic animals have variable temperatures. Ectotherms rely on behavioral temperature regulation—a snake will move from sun to shade until it finds a suitable microclimate that is close to its optimal body temperature. When exposed to direct sunlight, an ectotherm can increase its body temperature as much as 1°C (32.8°F) per minute.

Endothermic animals can achieve and sustain levels of activity even when temperatures plummet or vary widely. This can be a huge advantage over ectothermy, especially in northern latitudes, at night, or during the winter. In colder climates, an ectothermic predator such as a snake will tend to be more sluggish and less successful than an endothermic predator. There are no reptiles or insects in the polar regions.

However, endothermy is a costly adaptation. An actively foraging mouse uses up to thirty times more energy than a foraging lizard of the same weight; at rest, an endothermic animal's metabolism is five to ten times greater than that of a comparably sized ectotherm. In certain habitats, this translates to a substantial advantage to ectothermy. Because of their greater energy economy and lower food requirements, tropical ectotherms outnumber endotherms in both number of species and number of individuals.

Birds and mammals are endothermic vertebrates. Not coincidentally, they are the only vertebrates with unique external body coverings—feathers and hair, respectively. For both groups, these body coverings evolved as an adaptation to reduce heat loss. A bird's feathers were originally adaptive because they helped keep the animal warm, not because they helped it to fly.

The Energetics of Body Size. Larger animals have lower energy requirements than smaller ones. Gram for gram, a harvest mouse has twenty times the energy requirements of an elephant. Part of the advantage of size probably stems from the fact that a larger animal has proportionately less surface area than a smaller one. When heat leaves the body, it does so through the body surface. It is more difficult for a smaller object to maintain a constant body temperature because it has a greater amount of surface area relative to its volume. One consequence of this relationship is that a whale, because of its lower metabolic rate, can hold its breath and thus remain underwater for longer periods of time than a water shrew.

The Costs of Locomotion. Because oxygen is required for energy-producing metabolic reactions (respiration), there is a direct correlation between the amount of oxygen consumed and the metabolic rate. Not surprisingly, metabolic rates increase with activity. During exercise, a person will consume fifteen to twenty times more oxygen than when at rest.

A comparison of different species reveals that a larger animal uses less energy than a smaller one traveling at the same velocity. This seems to be related to the amount of drag encountered while moving. Small animals have relatively large surface-area-to-volume ratios and therefore encounter relatively greater amounts of drag. Adaptations that reduce drag include streamlined body shapes. However, because a larger body must first overcome a greater amount of inertia, there is a greater cost of acceleration. Therefore, small animals tend to be able to start and stop abruptly, whereas larger animals have longer start-up and slow-down periods.

Interestingly, two different species of similar body size have similar energetic costs when performing the same type of locomotion. Differences in energy expenditure are seen when comparing the type of locomotion being performed rather than the species of animal. For a given body size moving at a given velocity, swimming is the most energetically efficient mode of locomotion, flying is of intermediate cost, and running is the most energetically expensive. An animal swimming at neutral buoyancy expends less effort than an animal trying to stay aloft while flying; running costs the most because of the way the muscles are used.

Nonetheless, birds have higher metabolic rates than mammals of similar size. Most small mammals reduce energy costs by seeking protected environments; birds spend much of their time exposed. Also, because fat is heavy, the need to fly restricts a bird's ability to store energy. Even with a high-protein diet, a bird must eat as much as 30 percent of its body weight each day. This factor alone probably accounts for some birds' summer migratory journey from the tropics to northern latitudes, which, because of their longer days, allow a bird more daylight hours in which to feed itself and its young.

The Energetics of Mating. Sexual reproduction requires a considerable amount of energy, including the energy invested while competing for mates; mating itself (including the production of gametes); and caring for the offspring. Three main mating systems are found in the animal kingdom: polygyny (one male, more than one female), monogamy (one male, one female), and polyandry (one female, more than one male. Each system can be defined in terms of the relative energetic investments of each sex. In a monogamous pair bond, both sexes invest approximately equal amounts of energy; consequently, courtship behaviors tend to be rather involved, and competition is equal between the sexes. Males in a polygynous system spend a great deal of energy competing for mates, while females invest more heavily in parental care, and therefore tend to be very particular with whom they mate. A polyandrous system is the opposite: Females invest in competition and mating, while males invest in parental care.

Adaptations to Habitats. Because of Earth's geometry and the position of its axis, the equator receives more solar energy per unit area than the polar regions. Because Earth's axis is tilted relative to the plane of Earth's orbit around the Sun, this angle of incident radiation varies seasonally. These factors, combined with Earth's rotation, establish the major patterns of temperature, air circulation, and precipitation.

A habitat, or biome, is made up of interacting living and nonliving elements. The major terrestrial habitats include deserts, temperate forests, grasslands, rain forests, tundra, and various types of wetlands. The boundaries of these different habitats are determined mainly by climatological factors such as temperature, precipitation, and the length of the growing season. These conditions are created by the influence of solar radiation, often in conjunction with local factors such as topography.

The amount of biomass produced in a habitat—the productivity of the habitat—is determined by the types of plants (some species are more efficient photosynthesizers than others), the intensity and duration of solar radiation, the amount of nutrients available, and climatic factors such as temperature and precipitation. Aquatic habitats tend to be less productive than terrestrial ones, largely because there is less sunlight available at depth and there is a scarcity of mineral nutrients. Tropical rain forests have conditions that favor high productivity; one result is that they also have the highest biodiversity of any habitat.

Cold Habitats. Because of considerations of surface area relative to body mass, animals that live in cold habitats tend to have larger body sizes and smaller extremities (especially ears and legs) compared to their counterparts in warmer habitats. Animals that live in cold habitats also have a greater amount of insulation, such as fat, fur, or feathers. Behavioral adaptations include gathering in groups, which effectively decreases the exposed surface area of each individual.

If energy resources are seasonally low, some animals adopt the strategy of migrating to areas with greater resources. A bird that is insectivorous is more likely to be migratory than one that is a seed-eater. Hibernation is another adaptation in response to seasonal energy shortages. The body temperature of a true hibernator closely matches that of its surroundings. The heart slows (a ground squirrel's heartbeat drops from 200 to 400 beats per minute to 7 to 10 beats per minute), and metabolism is reduced to 20 to 100 times below normal. Hibernators tend to have much longer lifespans than non-hibernators.

Differences also can be seen in human populations living in cold habitats. Among the Inuit, the body maintains a high temperature by burning large amounts of fat and protein. Increased blood flow to the arms, legs, fingers, and toes helps prevent frostbite. An Australian Aborigine may sleep in below-freezing temperatures with little clothing or shelter, yet conserves energy by allowing the temperature in the legs and feet to drop. Heat is maintained in the trunk, where it is needed most.

Adaptations to Warm Habitats. When water evaporates into the surroundings, the vaporized molecules carry a great deal of heat away with them. One of the best ways to cool an animal's body is to evaporate water from its surface. Adaptations that take advantage of this property include sweating, panting, and licking the body. But water often is a limited resource in warm habitats such as deserts, so many desert animals have adaptations that reduce the amount of water that evaporates from the body. Most small desert animals avoid the heat and reduce water loss by being nocturnal and living in burrows. Large extremities, particularly ears, help to bring heat away from the body and dissipate it to the surroundings.

Other adaptations are perhaps best exemplified by examining the camel, which is able to conserve water by excreting a very concentrated urine. Also, the upper lip is split, so moisture dripping from the nose reenters the body through the mouth. More importantly, the camel can tolerate dehydration: It can lose an impressive 25 percent of its body weight in water with no ill effects. Its internal body temperature can fluctuate by as much as 6°C (10.8°F). By increasing its temperature during the day and dropping it at night, it can more closely track changes in external temperatures. This also helps to reduce water loss by as much as five liters of water per day. The fat stored in the camel's hump represents an important energy supply in the sparsely vegetated habitat in which it lives.

ENERGY AND HUMANS

The Costs of Technology

Paradoxically, organisms must use energy to get energy: A lion must hunt to eat, while a zebra must sometimes move long distances to find food. Most organisms can gain 2 to 20 calories in food energy for each calorie they use to obtain that energy. This holds for both a hummingbird, whose metabolic rate is 330 calories per minute, and a damselfly, which uses less than 1 calorie per day.

This is also true for human hunter-gatherer societies. Without technology, people use about 1 calorie to gain 5 to 10 calories. The energy return increases to 20 calories through the use of shifting agricultural practices.

Ironically, the cost of getting our food has increased with technological advances. In 1900 we gained a calorie for each calorie we used, while in 1995, for each calorie we invested we got only 0.1 calorie in return. Some of the energy costs associated with food include human labor, the cost of fertilizer, the cost of fuel for the farm machinery, and the cost of transportation of the food.

More developed countries rely heavily on burning fossil fuels to meet energy needs. Fossil fuels are the remains of plants that, over millions of years, have been transformed into coal, petroleum, and natural gas. Just like the natural systems examined in this article, our energy ultimately comes from the Sun and photosynthesis. Although more developed countries have less than 20 percent of the world's population, they use more than 80 percent of the world's energy.

People in less developed countries burn wood, plant residues, or animal dung to fuel stoves and lanterns. About 2 billion people rely on wood to cook their daily meals. Typically, four to five hours per day are spent gathering wood fuels from the surrounding habitat. Because these countries tend to have high population growth rates, there has been an everincreasing demand for more wood, resulting in a significant amount of habitat degradation in many areas.

The Effects of Human Energy Use

The two major ways by which humans get energy is to either burn fossil fuels or to burn wood for fuel. Both contribute substantially to air pollution, and both can have serious effects on (1) the health of plants and animals and (2) the workings of Earth's atmosphere.

Burning fossil fuels can release air pollutants such as carbon dioxide, sulfur oxides, nitrogen oxides, ozone, and particulate matter. Sulfur and nitrogen oxides contribute to acid rain; ozone is a component of urban smog, and particulate matter affects respiratory health. In fact, several studies have documented a disturbing correlation between suspended particulate levels and human mortality. It is estimated that air pollution may help cause 500,000 premature deaths and millions of new respiratory illnesses each year.

Physiological effects of air pollution are dependent on dosage, the ability of the exposed organism to metabolize and excrete the pollution, and the type of pollutant. Many pollutants affect the functioning of the respiratory tract; some change the structure and function of molecules; others can enter the nucleus and turn genes on or off; and some cause chromosomal aberrations or mutations that result in cancer. For example, exposure to the air toxin benzene can increase the risk of getting myelogenous leukemia or aplastic anemia, while exposure to ground-level ozone can cause a 15 to 20 percent decrease in lung capacity in some healthy adults.

Air pollution affects plant health as well. Acid rain and ozone can directly damage a plant's leaves and bark, interfering with photosynthesis and plant growth. More serious effects can occur if soil nutrients are leached away and heavy metals are mobilized in the soils upon which plants depend. Without proper nutrients, plants become susceptible to a variety of diseases. The overall result is a decrease in the amount of energy produced by plants. Acid rain affects over 345,960 square miles (900,000 sq. km) of Eastern Europe, where it has taken a significant toll on cities, forest, lakes, and streams (Kaufman and Franz, 1993). Moreover, air pollution is reducing U.S. food production by 5 to 10 percent, costing an estimated $2 billion to 5 billion per year (Smith, 1992).

Burning fossil fuel releases carbon into the atmosphere—more than 6.3 billion tons in 1998 alone. Significant amounts of carbon also come from burning of live wood and deadwood. Such fires are often deliberately set to clear land for crops and pastures. In 1988 the smoke from fires set in the Amazon Basin covered 1,044,000 square miles. By far the most serious implication of this is the significant threat to Earth's ecosystems by global climate change.

Like all matter, carbon can neither be created nor destroyed; it can just be moved from one place to another. The carbon cycle depicts the various places where carbon can be found. Carbon occurs in the atmosphere, in the ocean, in plants and animals, and in fossil fuels. Carbon can be moved from the atmosphere into either producers (through the process of photosynthesis) or the ocean (through the process of diffusion). Some producers will become fossil fuels, and some will be eaten by either consumers or decomposers. The carbon is returned to the atmosphere when consumers respire, when fossil fuels are burned, and when plants are burned in a fire. The amount of carbon in the atmosphere can be changed by increasing or decreasing rates of photosynthesis, use of fossil fuels, and number of fires.

Scientists have been able to compare the seasonal changes in atmospheric carbon dioxide to the seasonal changes in photosynthesis in the Northern Hemisphere. Plants take up more carbon dioxide in the summer, and animals continue to respire carbon dioxide in the winter, when many plants are dormant. Correspondingly, atmospheric carbon dioxide increases in the winter and decreases in the summer.

Atmospheric carbon dioxide, water vapor, methane, and ozone are all "greenhouse gases." When solar energy is reflected from Earth's surface, the longer wavelengths are trapped in the troposphere by these greenhouse gases. This trapped radiation warms Earth. In fact, without greenhouse gases, which have been present for several billion years, Earth would be too cold to support life.

Habitat destruction is another important contributing factor to increased atmospheric carbon dioxide levels. The world's forests are being cut, burned, or degraded at an astounding rate: More than half of the world's tropical rain forests have been lost in the past one hundred years. The forests supply fuel wood for energy; land for crops or pastures; and the wood demands of the global economy. Burning the forests releases carbon dioxide into the atmosphere; cutting or degrading the forests results in fewer plants available to take carbon dioxide out of the atmosphere.

Although scientists have been able to measure increasing levels of carbon dioxide, it is difficult to predict what the effects will be. For example, some models predict that warmer temperatures and the greater availability of atmospheric carbon dioxide will stimulate productivity, which in turn will remove carbon dioxide from the atmosphere, thus neutralizing the problem. However, the availability of carbon dioxide is not what generally limits plant growth. Rather, plant productivity tends to be restricted by the availability of resources such as nitrogen, water, or sunlight. Therefore, increasing the amount of carbon dioxide available to plants probably will have little effect on productivity, especially because it likely will result in greater evaporation rates and changed weather patterns. Although increased levels of evaporation may actually increase rain in some parts of the world, it may not be in those places currently containing rain forests.

A 1999 study by the Institute of Terrestrial Ecology predicts that tropical rain forests will be able to continue to absorb carbon dioxide at the current rate of 2 billion tons per year until global temperatures rise by 8°F (4.5°C). At this point, evaporation rates will be high enough to decrease rainfall for the forests, leading to the collapse of tropical ecosystems. This collapse will decrease the amount of carbon dioxide leaving the atmosphere and have dire consequences for all life.

Allison Brody

See also: Acid Rain; Agriculture; Air Pollution; Atmosphere; Biological Energy Use, Cellular Processes of; Climatic Effects; Environmental Problems and Energy Use; Green Energy; Thermodynamics.

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