Explain how energy flows through ecosystems through food chains, food webs, and trophic levels, including the roles of producers, consumers, and decomposers and the ten percent rule (Virginia 2018 Biology SOL BIO.8.b).

What this topic is asking

Virginia Biology SOL standard BIO.8 is about how populations, communities, and ecosystems are dynamic, and substandard BIO.8.b focuses on how nutrients cycle with energy flow through ecosystems. The EOC expects you to know the roles organisms play (producer, consumer, decomposer), to read food chains and food webs, to use an energy pyramid and the ten percent rule, and to explain the central contrast: energy flows one way and is lost as heat, while matter cycles. Many items give a food web or an energy pyramid and ask you to predict an effect or do a quick calculation.

Producers, consumers, and decomposers

Decomposers are easy to forget, but they are essential: they recycle the matter, linking energy flow to the nutrient cycles.

Food chains, food webs, and trophic levels

A food chain is a single pathway showing who eats whom, with arrows pointing in the direction the energy flows (from the organism eaten to the organism that eats it). A food web is the more realistic picture: many food chains linked together, because most organisms eat, and are eaten by, more than one species.

A classic EOC skill is reading the consequence of a change in a food web: if one population falls, the organisms that eat it lose a food source (and may decline), while the organisms it ate may increase. Trace the arrows.

Energy pyramids and the ten percent rule

The ten percent rule has two important consequences. First, food chains are short (usually four or five links at most), because after a few transfers there is too little energy left to support another level. Second, there are far fewer top predators than producers, because each level can support only about a tenth of the biomass of the one below.

Energy flows, matter cycles

The single most important contrast in this topic: energy flows in one direction through an ecosystem (from the Sun, to producers, to consumers, and out as heat), so it must be constantly resupplied by sunlight and cannot be recycled. Matter (nutrients such as carbon and nitrogen) is not lost as heat; it cycles between organisms and the environment and is used again and again. Energy is a one-way flow; matter is a loop.

Try this

Q1. State the role of a decomposer in an ecosystem and give an example. [2]

• Cue. Decomposers break down dead organisms and wastes, returning nutrients to the soil and water for producers to reuse; examples are many bacteria and fungi.

Q2. A producer level holds 20,000 units of energy. Estimate the energy available to the primary consumers, and explain your reasoning. [2]

• Cue. About 2,000 units, because only about 10 percent of the energy passes to the next trophic level (the ten percent rule); the rest is lost, mostly as heat.