Ecology Education Consulting, Inc.

Credits: Vole, Owl, Fox, Rodents (Dorling Kindersley) / Owl on Book (Voronin76|Shutterstock)
In addition to producers, ecosystems also house herbivores (also called primary consumers) that eat photosynthetic organisms in order to obtain the energy and nutrients that they will need to stay alive.  In turn, carnivores eat herbivores.  Carnivores can also eat one another so an ecosystem may have different levels of carnivores - 1st level, 2nd level, 3rd level or secondary, tertiary or quaternary consumers.  In the process, energy that is stored in the bodies of each organism flow along a linear feeding relationship (producers, herbivores and a variety of carnivores) that we call a food chain.  Thus, energy is traveling in one direction.  It is NOT recycled.  At each level, much of this energy is lost from the organism’s body as heat and with waste matter.  As a general rule of thumb, about 10% (5 - 20% range) of the energy taken in is available for the next feeding (trophic) level.  Since higher trophic levels receive progressively less energy, there are fewer species at these levels.  As a result, food chains rarely go beyond four - five feeding levels.

Definition of a food chain: A linear series of feeding relationships in an ecosystem?

Food Chains and Food Webs

What is a food chain? What is a food web? After reading this short ecology article, hopefully you will better understand each concept. The sun's energy sustains almost all the 1.8 million species of living things on our planet; without the sun’s energy, life on Earth would cease.  Solar energy is converted into chemical energy (in the form of sugar) through the process of photosynthesis, which is performed by plants and other organisms (e.g., Cyanobacteria).  This is why we call plants and other photosynthetic organisms producers.  [A few organisms acquire their energy from geothermal processes on the ocean floor instead of from the sun.]  Different ecosystems produce varied amounts of chemical energy (sugar) because they vary in the amount of sunlight, water and nutrients available to the plants, as well as temperature conditions.  Tropical rain forests, for example, are very productive (energy-rich) because these ecosystems get a lot of sunlight, have fairly uniform warm temperatures and receive a great deal of rainfall, thus promoting plant growth all year long.  In addition, dead matter is rapidly decomposed, and nutrients are recycled back into the plants.  In contrast, deserts are very unproductive ecosystems because plants receive limited water and nutrients and are growth-restricted.

Owl food chain

Owl Food Chain

Notice the arrows decrease in size to signify the loss of energy from one feeding (trophic) level to the next.  At each level, the organisms use up some of the energy they acquire in order to meet their survival challenges.  The process is not 100% efficient and a lot of energy is lost from the organism’s body as heat.  

There are two kinds of food chains -
grazing food chains and detritus food chains.  Grazing food chains derive their energy directly from the sun.  These are the chains that we are familiar with because we can see them, and because they are more frequently described in articles about energy flow through food chains.  However, think about the enormous amount of dead matter (leaves, for example) that accumulates on a forest’s floor or the large amount of dead plants that wind up on the bottom of lakes and ponds each year.  This dead matter (detritus) is rich in energy and nutrients.  Decomposers and detritivores obtain their energy and nutrients from this resource in contrast to directly using the sun's energy.  All the organisms feeding on the detritus are part of the detritus food chain (rarely mentioned or illustrated in children’s books on food chains).  Those feeding directly on the detritus are primary detritus feeders and those preying on these organisms are secondary detritus feeders.  In nature, the two types of food chains mix as organisms from one chain feed on those from the other food chain.  

Rarely do organisms just eat one type of food.  Carnivores eat other carnivores, as well as herbivores.  Some may even eat both animals and plants and are called omnivores.  If we listed every species that occurred in an ecosystem and then drew arrows connecting them to each of their food sources, we would see so many crisscrossing arrows that it would give the appearance of a spider web.  Therefore, we call the entire complex array of feeding relationships in an ecosystem a
food web.  Food webs more accurately describe the feeding relationships that exist in an ecosystem than do simple food chains.  However, when depicting food webs, the arrows do not show the energy that is being lost at each trophic level.   

Vole - a herbivore or
primary consumer

Owl - a carnivore or
secondary consumer

Producers

Click Owl to
learn more about food chains and food webs

Definition of a food web: the complex array of feeding relationships in an ecosystem. A food web consists of many interconnected food chains.
A food web that shows three trophic levels. The illustration above has 4 trophic levels. Within the food web are numerous food chains. For example the grass, rats and owl represent a food chain. The grass, rabbit and fox is another food chain.
Simplified Food Web
Second Level Carnivore or Tertiary Consumers
Simplified Food Web
Producers
Herbivores or Primary Consumers
First Level Carnivores or Secondary Consumers
Simplified Food Web
Definition of a food chain: A linear sequence of feeding relationships.
Producers
Herbivores or Primary Consumers
First Level Carnivores or Secondary Consumers
1st Trophic
Level
2nd Trophic
Level
3rd Trophic
Level