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How does energy flow through an ecosystem, and why is it lost at each level?

Explain how energy flows through an ecosystem in food chains and food webs, and why energy decreases at each trophic level (North Carolina Standard Course of Study, Biology, LS.Bio.4).

A standard-level answer on energy flow for the North Carolina Biology EOC: producers and consumers, trophic levels, food chains and webs, energy pyramids, and why only about 10 percent of energy passes up each level.

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  1. What this topic is asking
  2. Producers, consumers, and decomposers
  3. Food chains and food webs
  4. Why energy decreases: the ten percent rule
  5. Try this

What this topic is asking

North Carolina LS.Bio.4 asks how energy flows through an ecosystem and why it decreases at each level. For the Biology EOC you need to know producers and consumers, trophic levels, how food chains and food webs show energy flow, the energy pyramid, and the ten percent rule (only about 10 percent of energy passes to the next level). Items often give a food web or pyramid and ask you to trace energy or apply the rule.

Producers, consumers, and decomposers

Producers are the entry point of energy into the ecosystem: they capture sunlight and store it as chemical energy in glucose. Consumers are grouped by what they eat: primary consumers (herbivores) eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. Decomposers (bacteria and fungi) feed on dead matter, returning nutrients to the soil. Each of these feeding positions is a trophic level.

Food chains and food webs

A food chain is a single line showing the flow of energy from one organism to the next: for example, grass to grasshopper to frog to snake. The arrows point in the direction the energy flows (from the eaten to the eater). A food web is more realistic: it shows the many interconnected food chains in a community, because most organisms eat, and are eaten by, more than one species. A food web makes it clear how removing one species can affect many others, which links to ecosystem stability.

Why energy decreases: the ten percent rule

Because energy is lost at every step, the producers must capture far more energy than is available to the top consumers. To apply the rule, multiply by about 0.1 for each level up: 10,000 units in producers gives about 1,000 to primary consumers and about 100 to secondary consumers.

Try this

Q1. State the original source of energy for almost all ecosystems and how it enters the food web. [2]

  • Cue. The Sun; producers capture it by photosynthesis and store it as chemical energy.

Q2. Explain why only about 10 percent of energy passes to the next trophic level. [2]

  • Cue. Most energy is lost at each level as heat from cellular respiration (and in movement and waste), so only about 10 percent is stored and passed on.

Exam-style practice questions

Practice questions written in the style of NCDPI exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

NC Biology EOC (style)1 marksIn a food chain, the original source of energy for almost all ecosystems is: (A) the producers. (B) the Sun. (C) the consumers. (D) the decomposers.
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A 1-point item on the energy source.

The correct answer is B. The Sun is the original source of energy; producers capture sunlight by photosynthesis and store it as chemical energy, which then flows to consumers. Producers are the entry point, but the Sun is the original source.

The Sun is the source; producers capture it.

NC Biology EOC (style)2 marksAn energy pyramid shows 10,000 units of energy in the producers. (a) Estimate the energy available to the second-level consumers (two levels up) and explain. (b) State why energy decreases at each level.
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A 2-point item applying the ten percent rule.

(a) 1 point: about 100 units. Roughly 10 percent passes to each next level, so 10,000→1,00010{,}000 \to 1{,}000 (first-level consumers) →100\to 100 (second-level consumers).
(b) 1 point: most energy is lost at each level as heat from cellular respiration (and in movement and undigested waste), so only about 10 percent is stored and passed on.

Markers reward the calculation and a heat-loss explanation for the decrease.

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