Every organism in an ecosystem is directly or indirectly linked with other organisms in the same ecosystem. Our understanding of this concept allows us to predict how changes in the environment that affect one organism or population may affect others. The stability of an ecosystem is maintained by interdependence of organisms within it. As one population increases, it is kept in check by other species that eat it, by its supply of food, and by the availability of habitat. Habitat includes food, water, shelter, space, and air. Most populations in an ecosystem will, over time, adjust to meet climatic changes or the introduction of new or different species through migration, evolution, or human intervention.

Organisms within ecosystems are categorized by the roles they play. Organisms are divided into: (1) producers—organisms that make their own food through photosynthesis, i.e., plants; (2) consumers —organisms that depend on other organisms (producers or consumers) for food, i.e., herbivores, omnivores, carnivores, and scavengers; and (3) decomposers—organisms that break down dead plants and animals and return nutrients to the soil, water, and air.

Food chains show the transfer of energy from the sun through plants to a sequence of organisms in which each is food for the next organism in the sequence, i.e., grass g mouse g snake g hawk. A food web is the complex network of many interconnected food chains and feeding interactions in an ecosystem. Plants and animals do not live in isolation, but rather form interdependent relationships with both living and non-living parts of their environment. Each species of plant and animal relies on other species to provide for their habitat needs.

Elementary students should be able to identify organisms as part of a food chain and/or food web. They should understand that plants and animals do not live in isolation, but form interdependent relationships with both living and non-living parts of the environment. Different species rely on one another to provide different food and habitat needs.

Middle school students should continue to develop this understanding of interrelationships and interdependency within an ecosystem. They should be able to identify and describe different patterns within an ecosystem. Students at this level should be able to construct simple food chains and webs and describe the relationships within them. They should also recognize that most food chains and webs connect with other food chains and webs.

Additionally, middle school students should be able to describe how energy moves through a simple food chain. They should be able to describe in greater depth how plants get energy from the sun and use that energy to make their own food. They should be able to describe why animals need to use plants or other animals as food sources and how they, in turn, can become food sources for other organisms.

While not directly addressed in Lesson 2, students should also be encouraged to consider the origin of the food they eat and the impact its production may have on the environment. Many students, particularly in urban areas, have limited knowledge of where their food comes from or how it is produced. The extension lesson Sandwich Super Sleuth (on the MEECS Ecosystems & Biodiversity CD) is recommended as an extension to this lesson.

Source: Adapted from Michigan State Board of Education. (1991).

New Directions for Science Education in Michigan: State Objectives for K-12 Science (MEGOSE). Lansing, MI: Michigan Department of Education. Retrieved July 8, 2005, from http://elsci.lansingschools.net/MEGOSE/.