Explain how energy flows through an ecosystem from producers to consumers along food chains and webs, and use the idea that only about 10 percent of energy passes between trophic levels to interpret energy pyramids (MA STE HS-LS2-3, HS-LS2-4, energy and matter).

What this topic is asking

The Massachusetts STE framework (HS-LS2-3 and HS-LS2-4) asks you to use models and mathematics to explain the flow of energy through an ecosystem. On the High School Biology MCAS, this is one of the more quantitative ecology topics: you read food chains and energy pyramids, and you apply the rule that only about 10 percent of energy passes between levels. The crosscutting concept is energy and matter, with the key idea that energy flows one way and is lost as heat (the same point you met comparing photosynthesis and respiration).

Producers, consumers, and trophic levels

Energy enters almost every ecosystem through producers, which capture light energy by photosynthesis and store it as chemical energy in glucose. Consumers then obtain energy by feeding:

• Primary consumers (herbivores) eat producers.
• Secondary consumers eat primary consumers.
• Tertiary consumers eat secondary consumers, and so on.

A food chain shows one pathway of energy (grass to grasshopper to frog to snake). A food web combines many interlinked chains, which is more realistic because most organisms eat more than one thing. Decomposers (bacteria and fungi) break down dead organisms and waste, releasing nutrients back to the ecosystem.

The 10 percent rule

The single most important quantitative idea is that only about 10 percent of the energy at one trophic level passes to the next. The other roughly 90 percent is lost:

• as heat released during cellular respiration,
• used for movement and life processes,
• and lost in undigested waste (not everything eaten is absorbed).

Because so much is lost at each step, the energy available shrinks rapidly as you move up the chain. If producers hold $10000$ units of energy, primary consumers receive about $1000$, secondary consumers about $100$, and tertiary consumers about $10$. This is why an energy pyramid is widest at the producers and narrows toward the top.

Why this shapes ecosystems

The 10 percent rule explains several patterns the MCAS asks about:

• Top predators are rare. There is so little energy left at the top that an ecosystem can support only a few large predators.
• Food chains are short. After four or five levels, there is too little energy left to support another level, so chains rarely go further.
• Pyramids narrow upward. Each level holds about a tenth of the energy of the level below, so a pyramid of energy always narrows.

Above all, energy flows one way: it enters as light, passes along the chain, and leaves as heat. Unlike matter (which cycles, see cycling of matter in ecosystems), energy cannot be recycled, so the Sun must constantly resupply it.

Try this

Q1. State roughly what percentage of energy passes from one trophic level to the next, and where the rest goes. [2]

• Cue. About 10 percent passes on; the rest (about 90 percent) is lost mainly as heat through respiration, plus movement and undigested waste.

Q2. Explain why food chains rarely have more than four or five trophic levels. [2]

• Cue. Because about 90 percent of the energy is lost at each transfer, after a few levels there is too little energy left to support another level.