Earth Systems / Earth's Interconnected Cycles
Energy Balance Model

Learning Objective Define energy, and state the first and second laws of thermodynamics. Distinguish among producers, consumers, and decomposers. Summarize how energy flows through a food web.

Energy is the capacity or ability to do work. Organisms require energy to grow, move, reproduce, and maintain and repair damaged tissues. Energy exists as stored energy—called potential energy—or as kinetic energy, the energy of motion. Think of potential energy as an arrow on a drawn bow (Figure 5-3). When the string is released, this potential energy is converted to kinetic energy as the motion of the bow propels the arrow. Similarly, the grass a bison eats has chemical potential energy, some of which is converted to kinetic energy and heat as the bison runs across the prairie. Thus, energy changes from one form to another.

Figure 5-3    Potential and kinetic energy Potential energy is stored in the drawn bow (A) and is converted to kinetic energy (B) as the arrow speeds toward its target. Photographed in Athens, Greece, during the 2004 Summer Olympics.

Thermodynamics is the study of energy and its transformations. Two laws about energy apply to all things in the universe: the first and second laws of thermodynamics. According to the first law of thermodynamics, an organism may absorb energy from its surroundings, or it may give up some energy into its surroundings, but the total energy content of the organism and its surroundings is always the same. An organism can't create the energy it requires to live. Instead, it must capture energy from the environment to use for biological work, a process involving the transformation of energy from one form to another. In photosynthesis, plants absorb the radiant energy of the sun and convert it into the chemical energy contained in the bonds of sugar molecules (Figure 5-4). Later, an animal that eats the plant may transform some of the chemical energy into the mechanical energy of muscle contraction, enabling the animal to walk, run, slither, fly, or swim.

Figure 5-4    Energy flow in the environment The sun powers photosynthesis, producing chemical energy stored in the leaves and seeds of this umbrella tree. Photographed in Hanging Rock State Park, North Carolina.

As each energy transformation occurs, some of the energy is changed to heat energy that is released into the cooler surroundings. No organism can ever use this energy again for biological work; it is “lost” from the biological point of view. However, it isn't gone from a thermodynamic point of view because it still exists in the surrounding physical environment. The use of food to enable us to walk or run doesn't destroy the chemical energy once present in the food molecules. After you have performed the task of walking or running, the energy still exists in the surroundings as heat energy.

According to the second law of thermodynamics, the amount of usable energy available to do work in the universe decreases over time. The second law of thermodynamics is consistent with the first law—that is, the total amount of energy in the universe isn't decreasing with time. However, the total amount of energy in the universe available to do biological work is decreasing over time.

Less usable energy is more diffuse, or disorganized. Entropy is a measure of this disorder or randomness. Organized, usable energy has low entropy, whereas disorganized energy such as heat has high entropy. Another way to explain the second law of thermodynamics is that entropy, or disorder, in a system tends to increase over time. As a result of the second law of thermodynamics, no process requiring an energy conversion is ever 100 percent efficient because much of the energy is dispersed as heat, resulting in an increase in entropy. For example, an automobile engine, which converts the chemical energy of gasoline to mechanical energy, is between 20 and 30 percent efficient: Only 20 to 30 percent of the original energy stored in the chemical bonds of the gasoline molecules is actually transformed into mechanical energy, or work.

The organisms of an ecosystem are divided into three categories based on how they obtain nourishment: producers, consumers, and decomposers. Virtually all ecosystems contain representatives of all three groups, which interact extensively with one another, both directly and indirectly.

Plants and other photosynthetic organisms are producers and manufacture large organic molecules from simple inorganic substances, generally carbon dioxide and water, usually using the energy of sunlight. Producers are potential food resources for other organisms because they incorporate the chemicals they manufacture into their own bodies. Plants are the most significant producers on land, and algae and certain types of bacteria are important producers in aquatic environments.

Animals are consumers—they consume other organisms as a source of food energy and bodybuilding materials. Consumers that eat producers are primary consumers or, herbivores. Grasshoppers, deer, and rabbits are examples of primary consumers (Figure 5-5A). Secondary consumers eat primary consumers, whereas tertiary consumers eat secondary consumers. Both secondary and tertiary consumers are carnivores that eat other animals. Lions, spiders, and lizards are examples of carnivores (Figure 5-5B). Other consumers, called omnivores eat a variety of organisms. Bears, pigs, and humans are examples of omnivores.

Figure 5-5    Consumers and decomposers A.   The arctic hare is a herbivore, or primary consumer. The chemical energy stored in flowers and leaves transfers to the juvenile hare as it eats. B.   A Madagascar day gecko (a tertiary consumer) feeds on a spider (a secondary consumer). Both the gecko and the spider are carnivores. C.   Ghost crabs forage in the sand for detritus. Photographed along the West African coast. D.   These mushrooms are growing on a dead beech tree in Ostmuritz/Serrahn National Park, Germany. The mushrooms you see are reproductive structures; the invisible branching, threadlike body of the mushroom grows underground, decomposing dead organic material.

Some consumers, called detritus feeders or detritivores, consume detritus, organic matter that includes animal carcasses, leaf litter, and feces (Figure 5-5C). Detritus feeders, such as snails, crabs, clams, and worms, are especially abundant in aquatic environments, where they consume the organic matter in the bottom muck. Earthworms are terrestrial (land-dwelling) detritus feeders, as are termites, beetles, snails, and millipedes. Detritus feeders work together with microbial decomposers to destroy dead organisms and waste products.

Bacteria and fungi are important examples of decomposers, organisms that break down dead organisms and waste products (Figure 5-5D). Decomposers release simple inorganic molecules, such as carbon dioxide and mineral salts, which producers can then reuse.

In an ecosystem, energy flow occurs in food chains, in which energy from food passes from one organism to the next in a sequence (Figure 5-6). Each level, or “link,” in a food chain is a trophic level (the Greek tropho means nourishment). Producers form the first trophic level, primary consumers form the second trophic level, secondary consumers the third trophic level, and so on. At every step in a food chain are decomposers, which respire organic molecules in the carcasses and body wastes of all members of the food chain.

Figure 5-6    Energy flow through a food chain Energy enters ecosystems from the sun, flows linearly—in a one-way direction—through ecosystems, and exits as heat loss. Much of the energy acquired by a given level of the food chain is used and escapes into the surrounding environment as heat. This energy, as the second law of thermodynamics stipulates, is unavailable to the next level of the food chain.

Simple food chains rarely occur in nature because few organisms eat just one kind of organism. The flow of energy through an ecosystem typically takes place in accordance with a range of food choices for each organism involved. In an ecosystem of average complexity, numerous alternative pathways are possible. An owl eating a rabbit is a different energy pathway than an owl eating a snake. A food web, a complex of interconnected food chains in an ecosystem, is a more realistic model of the flow of energy and materials through ecosystems (Figure 5-7 on previous page).

Figure 5-7    A meadow food web This food web is greatly simplified compared to what actually happens in nature. Many species aren't included, and numerous links in the web aren't shown. (The energy-flow diagram along the left side of the figure does not correspond exactly to the food web diagram. For example, deer mice and cottontails are primary consumers, and the spider is a secondary consumer.)