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Unit 3: Energy Through Our Lives Unit-Part I

Section D. Energy Flow in Ecosystems 

(optional reading for online module participants)


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. 

Human Controlled Food Chain ​ ​ ​ ​ ​ ​
Sun Energy Transfer Corn


Energy Transfer
Beef Cow
Energy Transfer
Sun  Corn Beef Cow Steak

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.

Natural Food Chain 
​ ​ ​ ​ ​ ​ ​ ​
Energy Transfer



Energy Transfer

Energy Transfer
Energy Transfer
Sun Grass Rabbit Snake Hawk

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 calledcarnivores. If an animal can get its energy by ingesting eitherproducers or consumers, it is an omnivore.

Bison (Herbivore)

A bison is an herbivore.

Wolf (Carnivore)

A wolf is a carnivore.

Raccoon (Omnivore)

A raccoon 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. 

The amount of energy that is transferred from one organism to the next varies in different food chains. Generally, about ten percent of the energy from one level of a food chain makes it to the next. 
foodchain.gif (13842 bytes)


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.

What happens to the massive amount of organic material (and its potential energy) that is unconsumed or undigested?     

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 Energy Rules! unit for more information on energy transfer and the Laws of Thermodynamics).    

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.")


An ecosystem consists of species in a biological community (the living component) interacting with each other and with the physical and chemical factors that make up their environment (the nonliving component). An ecosystem can be as small and obscure as a blade of grass, a vernal pond, or rotting log. It can also be as large and magnificent as the Florida Everglades or the Amazon rain forest.  Some scientists even classify Earth as a working ecosystem. 

Species interactions include relationships like pollination, mutualism, predation, and decomposition. Plants and animals in an environment interact with each other in various ways. For example, plants may depend on insects or birds to pollinate flowers and on earthworms to aerate the soil; animals may depend on plants for food or shelter.
Examples of how plants and animals interact with the nonliving elements of their environment include life-sustaining processes such as photosynthesis, water purification, evaporation, and respiration. Physical factors such as sunlight, moisture, temperature, and wind influence the suitability of an area for particular organisms.
Snow Scene

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.

Energy is evident in all living and nonliving components of an ecosystem and in the interactions between the components. Sunlight and wind are energy resources, potential energy in plant and animal matter, temperature is related to thermal energy, and moisture content in soil and air is influenced by temperature. Therefore, energy influences which types of plants and animals that live in an ecosystem. To investigate the flow of energy in your neighborhood feel free to participate in the Ecosystem Survey (this is optional for online module participants). (Survey taken from the KEEP Energy Education Activity Guide "Energy Use in an Ecosystem.")

Food Chains and the Carbon Cycle

When we talk about the flow of energy in food chains, the transfer of energy between organisms also includes the transfer of matter, specifically carbon-based materials. Unlike energy, however, carbon and other elements of matter cycle within ecosystems, being used again and again as they travel through food chains, the atmosphere, soil, and water. 

carboncycle.gif (20295 bytes)

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

It is possible to make food (energy) chains out of other fuel usages besides food. For example, how we power our homes and run our cars are types of food chains. The fuel sources are mainly fossil fuels and these are burned to provide our society with energy.  Like food chains, using energy to power our homes or run our cars involves the flow of energy and the cycling of carbon (see above graphic). And, as with food chains,  energy  is "wasted" or "lost" with each transfer.


Plant Matter (Fern Frond)

Plant matter 

Heat Loss

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Fossil Fuel (Coal Cart)

Fossil fuel (coal)

Heat Loss

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Steam Engine

Steam engine

Heat Loss

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Electricity Lights Up a City


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.

Congratulations! You have completed Unit 3: Energy Through our Lives - Part I



Go back to Section C: Greenhouse Gas Emissions and Climate Change


Continue on to Unit 4: Energy Through Our Lives - Part II Section E: Home Energy Use