Students review the concepts of food chains and the roles of organisms in a food chain through a simple card sorting activity. Cards representing different individuals in a California ecosystem are first sorted by herbivore, carnivore, dentrivore, and omnivore, then are reordered to create several food chains. In addition, students begin to understand the idea of a food pyramid – since all living things use energy to move, reproduce, respond to the environment and grow, less energy is available to pass on at each link of the food chain.
Can define and construct a food chain.
Can identify the role of organisms within a food chain.
Can trace the path of energy through a food chain.
Can build a food pyramid and explain how it functions in terms of energy transfer at each level.
Dentritivore (or Decomposer)
This activity teaches students 2 things, 1) it reviews the concept of food chains and the major roles of organisms with a food chain and 2) it illustrates the concept of a food pyramid and the transfer of energy from one organism to the next through the food chain. Most students should be familiar with the concept of a food chain and food pyramid from elementary school science, but it is worth at least one class period to thoroughly review these concepts in preparation for more in depth investigations of food webs, ecosystems, and population change.
Food chains are the most simple arrangement of who eats whom assuming that each organism only eats one thing. Of course, in real life this is not the case. Still, it is useful to consider food webs as tangled food chains; therefore, understanding food chains is an essential prerequisite.
The roles that organisms play within a food chain are very well defined. Producers make their own food through photosynthesis. Consumers eat producers or other consumers and may be divided into 4 major categories: herbivores which eat producers, carnivores which eat herbivores or other carnivores, dentritivores (also called decomposers) which recycle the energy from dead organisms to make nutrients available for producers, and omnivores which eat producers and consumers.
Although it is tempting to emphasize that every food chain begins with the sun as the source of energy on which photosynthesis depends, in fact, not all food chains begin with the sun. Organisms near hydrothermal vents at the bottom of the ocean depend on sulfur as the initial energy source. These bacteria use a process called chemosynthesis, taking hydrogen sulfide and oxidizing it, thereby releasing energy. This is typically far more advanced than middle school classrooms although you may wish to allude to the existence of food chains that do not rely on the sun as the initial energy source. For further information, see RESA has a superb website about life at hydrothermal vents.
The concept of a food pyramid adds a level of complexity to the concept of food chains. A producer uses energy from sunlight to grow, reproduce, and survive. Only a small fraction of that energy can be used by a herbivore that eats that producer. Similarly, that herbivore needs to use energy to grow, reproduce, and survive. A carnivore that eats that herbivore does get some energy from that herbivore but only a small percentage. Another way to think of it is to consider how many seeds a plant produces in its lifetime, how many seeds a chicken consumes in its lifetime, and how many chickens a human will consume in its lifetime. Clearly, energy is used and lost at each level of the food chain. Using a food pyramid to illustrate this concept helps students see this visually.
At this point, introduce the idea of producers as plants, or more scientifically, as organisms that make their own food through photosynthesis. Introduce the idea of consumers as animals, or more scientifically, as organisms that eat producers or other consumers.
Introduce the vocabulary words herbivore, carnivore, omnivore, and dentritivore at this point and give the formal definitions.
The idea for the food chain card game came from Project WILD's book Alaska Ecology available for purchase online for $22. Debbie Breeding's activity Food Chains and Webs provided the information and pictures for the food chain cards. The illustrations on the activity pages are by Paula McKenzie, copied from Mountains to the Sea- A Visitor's Guide to the Santa Monica Mountains and Seashore. Another web resource that provides a similar activity is available from the British Ecological Society. Some of the illustrations on the activity pages are copied from the food chain cards available on their site.
Grade 6 Ecology (Life Sciences)
5. Organisms in ecosystems exchange energy and nutrients among themselves and with the environment. As a basis for understanding this concept:
a. Students know energy entering ecosystems as sunlight is transferred by producers into chemical energy through photosynthesis and then from organism to organism through food webs.
b. Students know matter is transferred over time from one organism to others in the food web and between organisms and the physical environment.
c. Students know populations of organisms can be categorized by the functions they serve in an ecosystem.
e. Students know the number and types of organisms an ecosystem can support depends on the resources available and on abiotic factors, such as quantities of light and water, a range of temperatures, and soil composition.