food chain and food web article

Solar energy sustains almost all of 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 photosynthetic 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 different 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 recycled back into the plants. In contrast, deserts are very unproductive ecosystems because plants receive limited water and nutrients and are growth-restricted.

In addition to producers, ecosystems also house species called herbivores (also called primary consumer) 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 flows along a linear feeding relationship (producers, herbivores and a variety of carnivores) that we call a food chain. Thus, energy moves 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 4 - 5 feeding levels.

	Why the arrows keep shrinking between different feeding (trophic) levels

	1. The mouse receives energy from the food it eats. 2. The mouse's cells extract the food's energy so it can be used for the mouse's survival needs (e.g., growth, acquiring food, escaping enemies). The process is not 100% efficient and a lot of energy is lost as heat. The mouse can use this heat to help keep its body temperature at a normal level during cold weather. 3. Some of the energy that is in the food is lost in the mouse's waste (feces). 4. The remaining energy is stored in the mouse's body and is available to the organism that preys on it. About 90% of the energy acquired in the mouse's food is used or lost by the mouse and only 10% is available to predators.

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 especially) that accumulate 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 nutrients and energy from this resource as opposed to the sun. 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.

In addition to food chains and food webs, a third way that the flow of energy can be depicted in an ecosystem is to create one of the three types of ecological pyramids. A Pyramid of Numbers can be generated by counting all of the organisms at the different feeding levels. On occasion this approach will not work. For example, one tree (a producer) can represent an ecosystem and harbor numerous populations of herbivores and carnivores.

A second type of pyramid is a Pyramid of Biomass where organisms are collected from each feeding level, dried and then weighed. This dry weight (biomass) represents the amount of organic matter (available energy) of the organisms. While this approach will generally create a pyramid that illustrates energy flow, its use can occasionally also produce an inverted pyramid. In aquatic ecosystems, phytoplankton reproduce and are then eaten rapidly by zooplankton. Therefore, it would be possible to have a few phytoplankton and a lot of zooplankton when a collection is taken.

A third type of pyramid called a Pyramid of Energy Flow tends to resolve these problems. This approach necessitates measuring the caloric value of the different organisms that make up the community. It nicely shows how energy is continually decreasing along the food chain from producers to top level carnivores.

A Few Recommended Websites on Food Chains/Food Webs:

1. http://www.vtaide.com/png/foodchains.htm - Includes descriptive information and several interactive lessons (Early Elementary)

2. http://www.bbc.co.uk/schools/revisewise/science/living/index.shtml - Interactive with a test included (Early Elementary)

3. http://curriculum.calstatela.edu/courses/builders/lessons/less/biomes/introbiomes.html - Good text and graphics (Elementary - Middle)

[click any one of the images to view three simplified illustrations]