Ecosystems Curriculum

Section 1: Background Information

A Connected Community

The activities in this section are designed to help students understand the connectedness within ecosystems. An ecosystem is a community of different types of living things (organisms) and their physical environment (including sunlight, rocks, soil, water, hills, holes, etc.) The organisms in an ecosystem interact just as people interact in a school. Each organism has a role (or "niche") in the ecosystem. Each living thing in the ecosystem depends on other living things.

The sun plays a critical role in the ecosystem. It provides the energy for all life on Earth and thus all Earth's ecosystems. Plants convert sunlight to make their own food, which they use to support their own lives. When animals eat plants, they eat this "ready-made" food, formed from energy originally provided by the sun. The sun’s energy is thus passed along to them. In this way, the sun’s energy fuels every living thing. Plants are called producers because they produce or make their own food. Animals are called consumers because they consume (eat) food but do not produce it on their own. There are different kinds of consumers: those that eat only plants are called primary consumers or herbivores; those that eat only animals are called carnivores; and those that eat both plants and animals are called omnivores.

Some organisms in the ecosystem are called decomposers. They decompose or break down dead matter by digesting dead plants and animals. (In this sense, they are also consumers.) They break down dead matter into basic materials, which are recycled into the soil and become nutrients that can be used by plants and some other living things. Therefore, they play an essential role in the ecosystems.

Food Chains

Students are usually taught about ecosystem feeding relationships in terms of food chains. For instance, they are shown a food chain beginning with the sun or green plants and extending to secondary consumers (those consumers that eat the organisms that eat green plants). Food chains address one of the difficulties that elementary students tend to have when reasoning about ecosystem relationships. Students usually focus on simple linear or direct effects (one thing makes another happen). For instance, students realize that the green plants are important to the organisms that eat green plants. However, students often don’t detect indirect connections. It is common for students to reason that the green plants are not important to the things that don't eat them even if those things eat things that eat green plants. Learning about food chains helps them to see the domino-like pattern and to move beyond simple direct effects to indirect effects.

Food Webs and Domino Causality

The activities in this section introduce domino causality to help students move beyond noticing only direct effects. Domino causal models describe how, like dominoes falling, effects can in turn cause other effects. The dominoes can fall in different types of extended patterns, for instance branching or radiating. For branching patterns, events closer to the "stem" have a greater effect on the rest of the branch than ones that are further away. In radiating patterns, one event can have many direct and indirect effects.

Domino causal models provide a way to visualize more extended patterns of cause and effect. They enable students to organize more information about the system. These patterns are some of the easiest causal patterns for students to learn. However, without direct teaching, many students will not learn to recognize them.

The activities in this section also introduce food webs. Like food chains, food webs illustrate the domino-like patterns of indirect effects. Learning only about food chains can reinforce a short sighted, linear view of how ecosystem members affect each other. Students are likely to miss more extended domino-like patterns, such as branching and radiating ones. A food web shows the broader energy transfer relationships within an ecosystem. It can help in predicting how changes in a population will affect other living things.

It is important to realize that focusing solely on domino patterns is not the answer to students’ difficulties either. An overemphasis on domino patterns can make it harder to detect other types of patterns such as cyclic or two-way patterns. The trick is to help students learn different types of causal patterns (simple linear, domino, cyclic, and two-way) and when each type applies.

There are at least three other concepts that give students difficulty in understanding feeding relationships in ecosystems: The direction of the arrows, overemphasizing connectedness, and reasoning about populations vs. individuals.

The Direction of the Arrows

The arrows in a food chain or food web go from the thing that is being eaten to the thing that eats it to show the transfer of energy. When students try to reason about food webs, they often reverse the direction of the arrows to show what eats what.¹ They reason about "active", more easily visualized aspects of what happens (this eats this) rather than "passive," less easily visualized aspects (this gains energy from this). Students can be encouraged to use the arrows to "point to the mouth of the thing that eats it"² to help them construct the arrows in the right direction.

Overemphasizing Connectedness

It is possible to take the connectedness message too far. There is enough redundancy and flexibility in food webs that not all seemingly harmful or disruptive events adversely affect food web members.³ Once students understand the basic idea of connectedness, this caveat can be introduced. Section six provides resources to do so.

Reasoning about Populations vs. Individuals

Young children find it hard to reason about populations as opposed to individuals.⁴ When creating a food chain, they think of the predator as eating the particular animal as prey, rather than each animal representing a population of animals. It is important to reinforce the understanding that the food web shows how populations of organisms interact. Even though one organism is pictured or talked about, the whole population is being represented.