Use a model to illustrate how energy flows through an ecosystem from producers to consumers and decomposers, and why it decreases at each trophic level (Tennessee Academic Standards for Science, Biology I, BIO1.LS2).

What this topic is asking

The Tennessee LS2 standards (Ecosystems) ask you to model how energy flows through an ecosystem and why it decreases at each level. For the Biology I EOC that means knowing the roles of producers, consumers, and decomposers, reading food chains and food webs, identifying trophic levels, and using the 10 percent rule to calculate how much energy passes up an energy pyramid. Items frequently give a pyramid or web and ask a calculation or to predict the effect of removing an organism.

Producers, consumers, and decomposers

Consumers are ranked by what they eat: primary consumers (herbivores) eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. All of the energy in the system originally enters through the producers, which is why they form the base of every food chain.

Food chains and food webs

A food chain shows one path of energy, for example grass to grasshopper to frog to snake. The arrows point in the direction the energy flows (from the eaten to the eater). But real ecosystems are more complex: most organisms eat and are eaten by several others. A food web shows all the interconnected food chains in an ecosystem.

The web matters because it shows interdependence: removing one species (say, a predator) can affect many others. EOC items often ask you to predict what happens to one population if another increases or is removed, by tracing the arrows.

Trophic levels and the energy pyramid

Each step in a food chain is a trophic level: producers are the first level, primary consumers the second, and so on. An energy pyramid stacks these levels, with the most energy at the producer base and less at each higher level, so it narrows toward the top.

The 10 percent rule

This is the most calculated idea in LS2. To move up a level, multiply by $0.1$; to move down, divide by $0.1$ (multiply by 10).

Energy flows one way

A crucial contrast: energy flows in one direction through an ecosystem and is not recycled. It enters as sunlight, passes up the trophic levels, and is ultimately lost as heat. This is unlike matter (carbon, nitrogen), which is recycled (see the cycling of matter). The EOC tests this distinction: energy flows through and is lost; matter cycles and is reused.

Try this

Q1. Explain why a food chain rarely has more than four or five trophic levels. [2]

• Cue. Only about 10 percent of energy passes to each higher level, so by the fifth level there is too little energy left to support another level of consumers.

Q2. State the difference between how energy and matter move through an ecosystem. [2]

• Cue. Energy flows in one direction and is lost (mainly as heat), so it is not recycled; matter (such as carbon and nitrogen) is recycled and reused through the ecosystem.