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Units:
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Unit:Energy Through Our Lives Unit-Part I
Food Chains
"Hey,
what's for dinner?"
"Yes, sunshine." Almost everything you eat can be traced back through food chains to the sun. A food chain consists of a series of organisms in which the first organism is eaten by a second and the second is eaten by a third. During this process, nutrients and energy in the eaten organism are transferred to the organism that eats it. Most of the food we eat comes from simple food chains derived from human-controlled agricultural ecosystems. For example, the beef we eat comes from a cow that ate corn. The corn received its energy from the sun.
However, in natural ecosystems, a hawk may eat a snake that may have received its energy from a mouse, a frog, or a rabbit. If it ate a mouse, that mouse may have consumed seeds from any number of plants. None of these food chains is exactly alike, which makes studying energy transfer complex.
The food chain begins with producers, organisms such as green plants, that can make their own food. Through photosynthesis, producers convert solar energy to chemical energy, energy in the chemical bonds of the food. Of all the energy a plant receives from the sun, only about three percent is converted into chemical energy. The amount of chemical energy varies depending on the plant species and the location of the plant. (Refer to the What is Energy? unit for more information on chemical energy.) Plants are eaten by consumers, which are organisms that cannot make their own food. Herbivores are consumers that eat only producers. Consumers that prey on other consumers are called carnivores. If an animal can get its energy by ingesting either producers or consumers, it is an omnivore.
A food chain does not consist of a set amount of organic matter and energy being passed along like a baton from one organism to another. In reality, the baton gets smaller and smaller with each transfer. When an herbivore eats a plant, it does not get all the energy the plant received from the sun. This decrease is because the herbivore may not eat all parts of the plant, and it may not be able to digest what it does eat. These undigested plant parts are excreted as waste. The same holds true for other organisms along the food chain (i.e., when one organism eats a second, the consumer does not receive all the energy obtained by and contained within the second organism). Another reason energy obtained by one organism isn't passed on in the food chain is because it is no longer available (Second Law of Thermodynamics). Some energy has already been used by the first organism. A plant uses some of the energy it receives to grow and function. An herbivore uses its energy to grow, but also to look for food and run away from predators. A predator uses large amounts of energy to chase after its food in addition to its regular life processes (e.g., breathing, digesting food, moving). The energy these organisms use eventually leaves their bodies in the form of heat.
Because energy is "lost" with each successive link, there must be enough energy in the organisms to allow for this loss and still have enough energy remaining for the consumers in the next level. In other words, the total biomass (organic matter) of the producers must be greater than the total biomass of the herbivores they support, and the total biomass of the herbivores must be greater than that of the carnivores. Because of this energy loss there are usually more producers than herbivores, and more herbivores than carnivores in an ecosystem.
Food chains cycle nutrients within an ecosystem and provide the mechanism for energy to flow through the ecosystem. In natural ecosystems, these food chains have many alternate routes through which energy can flow, creating integrated, complex food webs. Through agriculture, humans have simplified food chains so the energy flow is more direct. It is very easy to trace almost anything you eat back to its original source of energy: the sun. Can you trace what you ate today back to the sun? (Taken from the KEEP Energy Education Activity Guide "Food Chain Game.") Ecosystems
The
interaction of living and nonliving components affects the qualities and
characteristics of an ecosystem. These interactions can influence the
climate within the area (often called a micro-climate). For example, in a
forest, tall trees block the sunlight resulting in a shady moist under
story where only certain plants can live. Food Chains and the Carbon Cycle
Energy enables carbon to move through these different components of an ecosystem, however, it is important to note that carbon cycles within a system and energy flows through an ecosystem. Carbon Transfers and Human Societies
Other waste products of energy-use associated with burning our fuels (mainly fossil fuels) have contributed to important environmental issues in our society such as acid rain and global climate change. Next reading: Energy Through Our Lives-Part II - Section E. Home Energy Use.
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Decomposers, such as, bacteria, fungi,
and small animals such as ants and worms, eat nonliving organic matter.
Decomposers cycle nutrients back into food chains and the remaining
potential energy in unconsumed matter is used and eventually dissipated as
heat. Therefore, decomposers are an integral component of all ecosystems (First
and Second Laws of Thermodynamics). (Refer to the 
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