Essential Science for Teachers: Life Science
Energy Flow in Communities Children’s Ideas
Children’s Ideas About Energy Flow in Communities
Below are common ideas children in grades K-6 have about this topic, compiled from research on children’s ideas about science (see the Session 7 Children’s Ideas Bibliography). For each idea, consider why a child would be likely to believe this and what evidence might refute it.
1. Interdependencies among life forms have a predetermined purpose.
In a community, food chains are a good example of interdependencies among producers, consumers, and decomposers. Scientists would argue that energy flow is a consequence of these interdependencies rather than a predetermined purpose. This is an example of teleological thinking, where nature or natural processes are considered to have a purpose or design. Children and adults often try to attribute cause in nature in this way, often because of religious or cultural beliefs. It’s more a habit of mind to be addressed during lessons than an idea to be countered by evidence.
2. Interdependencies among life forms are ultimately meant to serve the needs of people.
There are countless interdependencies in the living world that have no influence whatsoever on human beings. Think about communities in places that people have yet to explore, like parts of the deep sea or the tops of trees in tropical rain forests. This is an example of “anthropocentric” thinking — where nature or natural processes are thought to revolve around the human species. Young children especially tend to think of the natural world in human terms and, as a result, may reason about interdependencies among life forms in this way.
3. Food is defined only as those things that animals eat.
Food is any substance that provides energy and nutrients to an organism. All organisms require food. Animals, including human beings, are consumers and eat their food. Others technically do not “eat” their food. Producers, which include plants, make their own food and then process it internally for energy and matter. Decomposers absorb their food. The first meaning that children have for the term “food” is something that they eat. They also have experience observing pets and other animals eating food. The scientific meaning of food is something that is developed only with formal instruction.
4. Plants get their food from the soil.
Photosynthesis is the process by which plants make their own food. Using light energy from the sun, carbon dioxide from the air is combined with water from the soil to form the food sugar. Some of the first observations children make about plants are that they grow in soil and take in water through their roots. It’s a natural extension to think that plants get food from the soil. This may also be reinforced by the term used for fertilizer: plant food. Even after children are introduced to photosynthesis, this idea may persist because they can’t observe carbon dioxide being absorbed from the air.
5. Plants make food for the benefit of animals.
The food that plants make provides them with their own source of energy and matter. Plants were making food well before animal life evolved. And, in the absence of animals, plants still make food. Many children can’t see the “point” of a plant except as a source of food for an animal. One way to get children to broaden their ideas about plant life is to engage them in plant cultivation and other activities that establish plants as life forms with their own unique and interesting habits.
6. Energy is “formed” in biological processes rather than changed or transferred.
The first law of thermodynamics states that energy is conserved in a system: it is neither created nor destroyed. It can, however, change forms. When a plant makes food through photosynthesis, it changes light energy to chemical energy. When an animal eats a plant, chemical energy is transferred to the animal. In this way, energy flows. It doesn’t spontaneously appear. Energy is an abstract concept. Children can’t see energy, yet they may understand that objects, like food, contain energy. It seems reasonable that they think that energy is formed within an object as it comes into being.
7. Once eaten, the energy in food disappears.
Once food is eaten, it can be “burned” to obtain energy, or it can be stored in an organism’s body. In the first case, the energy is used, and most of it is changed to heat. This is an example of the second law of thermodynamics: in any energy reaction, energy tends to degrade from a higher-quality form to a lower-quality form. Although heat can’t be used to fuel life processes, this energy does not simply disappear. In the second case, think about fat. Fat storage represents an excess of energy from food eaten; when we gain weight we have evidence it hasn’t disappeared. Most children understand that food gives them energy, and they know that they have to eat food regularly. They may reason that as the energy in food is “used up,” it disappears.
8. Energy adds up through a food chain, giving top predators all of the energy.
At each link in a food chain, energy is used to fuel life processes. Most of this energy, which is stored in the chemical bonds of food, is changed into heat as it is used. For any amount of energy entering a food chain, very little is left for top predators. Children may have this idea because they think of predators as large animals that require a lot of energy. They may reason that a top predator’s energy needs can only be met by “saving it up” from links lower in a food chain. This may also reflect the belief that the links before a top predator are there to serve the top predator.
9. There are more individuals at the ends of food chains than at the beginning.
Think about a natural community: a forest, for example. Are there more trees, shrubs, and grasses than hawks or bears? Scientists have actually conducted studies where they’ve censused populations, and there is no doubt that in most communities, there are fewer individuals at the ends of food chains than at the beginning. This is a consequence of energy flow; so much energy is used at each link that few individuals can be supported at the end. Note that this idea applies only to producers and consumers: there can be billions of decomposers at the end of a food chain. Children may believe this because they are more familiar with animals that are top predators. Often, these are the animals that are featured in books and other media, leading children to equate this with numerical dominance. Even when challenged, children may think that these animals are hiding, otherwise more would be observed.
10. The decomposers recycle energy.
If the decomposers recycled energy, plants would be expected to be able to survive without energy input from the sun. If you isolate a plant without light, even in soil rich with decaying matter, it will eventually die. When teaching about food chains, energy and matter are often equated with one another. Children are taught that nutrients (i.e., matter) are recycled. They are also taught that the nutrients in food provide energy. It seems reasonable that they think that the nutrients recycled by decomposers provide energy.
Children's Ideas Bibliography
The Children’s Ideas listed in this section of the Web site were compiled from the following research:
- Barker, M., & Carr, M. (1989). “Teaching and learning about photosynthesis.” International Journal of Science Education, 11(1), 48-56.
- Driver, et al. (1992). “Life and living processes.” Leeds National Curriculum Support Project, Part 2. Leeds City Council and the University of Leeds, UK.
- Eisen, Y., & Stavy, R. (1988) “Students’ understanding of photosynthesis.” American Biology Teacher, 50, 208-212.
- Leach, J. et al. (1996) “Children’s ideas about ecology: Ideas found in children aged 5-16 about the interdependency of organisms.” International Journal of Science Education, 18, 19-34
- Webb, P., & Boltt., G. (1990). “From food chain to food web: A natural progression?” Journal of Biological Education, 24(3): 187-190.
Session 1 What Is Life?
What distinguishes living things from dead and nonliving things? No single characteristic is enough to define what is meant by "life." In this session, five characteristics are introduced as unifying themes in the living world.
Session 2 Classifying Living Things
How can we make sense of the living world? During this session, a systematic approach to biological classification is introduced as a starting point for understanding the nature of the remarkable diversity of life on Earth.
Session 3 Animal Life Cycles
One characteristic of all life forms is a life cycle — from reproduction in one generation to reproduction in the next. This session introduces life cycles by focusing on continuity of life in the Animal Kingdom. In addition to considering what aspects of life cycles can be observed directly, the underlying role of DNA as the hereditary material is explored.
Session 4 Plant Life Cycles
What is a plant? One distinguishing feature of members of the Plant Kingdom is their life cycle. In this session, flowering plants serve as examples for studying the plant life cycle by considering the roles of seeds, flowers, and fruits. A comparison to animal life cycles reveals some surprising similarities and intriguing differences.
Session 5 Variation, Adaptation, and Natural Selection
What causes variation among a population of living things? How can variation in one generation influence the next generation? In this session, variation in a population will be examined as the "raw material" upon which natural selection acts.
Sessions 6 Evolution and the Tree of Life
Why are there so many different kinds of living things? Comparing species that exist today reveals a lot about their relationships to one another and provides evidence of common origins. This session explores the theory of evolution: change in species over time.
Session 7 Energy Flow in Communities
Communities are populations of organisms that live and interact together. The structure of a community is defined by food web interactions. The process of energy flow is the focus of this session as the interactions between producers, consumers, and decomposers are examined.
Session 8 Material Cycles in Ecosystems
Studying an ecosystem involves looking at interactions between living things as well as the nonliving environment that surrounds them. Life depends upon the nonliving world for habitat, as well as energy and materials. In this session, material cycles will be explored as critical processes that sustain life in an ecosystem.