Use mathematical representations to support explanations of the flow of energy through food chains and food webs in an ecosystem (Louisiana Student Standards for Science, High School Biology, HS-LS2-4).

What this topic is asking

Louisiana's LS2 standards (HS-LS2-4) ask you to use mathematical representations to explain how energy flows through an ecosystem. For LEAP 2025 Biology you should know the roles of producers and consumers, how to read food chains and webs, the idea of trophic levels, and the ten percent rule that explains why energy pyramids narrow toward the top. Because this is a "use mathematics" standard, the test often asks you to calculate the energy passed between levels.

Producers, consumers, and decomposers

Consumers are grouped by what they eat: a primary consumer (herbivore) eats producers; a secondary consumer eats primary consumers; a tertiary consumer eats secondary consumers. Because producers capture the energy that everything else depends on, an ecosystem cannot function without them.

Food chains and food webs

A food chain is a simple sequence showing the flow of energy from one organism to the next (for example, grass to grasshopper to frog to snake). A food web is a more realistic diagram showing the many interconnected food chains in a community, because most organisms eat, and are eaten by, more than one kind of organism. The arrows in both always point in the direction the energy flows (from the food to the eater).

Trophic levels and the ten percent rule

To use the rule mathematically: multiply the energy at one level by 0.10 to estimate the energy available to the next. So if producers hold 10,000 units, a primary consumer gets about 1,000, a secondary consumer about 100, and so on.

Why energy flows one way

A crucial contrast for the test: energy flows one way through an ecosystem (sunlight to producers to consumers, with heat lost at every step), and it is not recycled, which is why ecosystems need a constant input of sunlight. Matter (such as carbon), by contrast, is recycled through the ecosystem, the subject of the next topic.

Try this

Q1. State the role of producers and explain why they are essential to a food web. [2]

• Cue. Producers make their own food by photosynthesis and form the base of the food web; they capture the energy that all consumers ultimately depend on.

Q2. A trophic level contains 8,000 units of energy. Estimate the energy available to the next level and explain your reasoning. [2]

• Cue. About $8{,}000 \times 0.10 = 800$ units, because only about ten percent of the energy passes to the next trophic level (the rest is lost as heat and in life processes).