Grade 10-11 (age 15-17)

high school Ecosystems & Energy Flow

Ecosystem & Energy Flow

In an ecosystem, energy enters from the Sun through producers and flows one way toward the consumers, and at every step most of it leaves as heat through respiration. Matter such as carbon and nitrogen goes round and round between living things and their surroundings, but energy scattered as heat cannot be used again. Roughly 10% of the energy at one trophic level reaches the next, so stacking the levels gives an energy pyramid that narrows toward the top. In the diagram you can change the transfer efficiency from 5% to 20% and see the energy at each level, drawn as bar width, along with the amount lost as heat at each step.

Energy Flows One Way

☀️ From Sunlight to Heat

①Producers bring energy into the ecosystem by turning the Sun's light energy into the chemical energy of organic matter through photosynthesis

②That energy passes along the food chain to primary and then secondary consumers, and at each step most of it is used in respiration and leaves as heat

③Matter returns to producers by way of decomposers, but energy scattered as heat cannot be used by producers again, so it leaves the ecosystem

Energy Pyramid

Energy transfer efficiency (%)10%

💡 Changing the Efficiency

①The default of 10% is the average known as the "10% rule"; in real ecosystems the transfer efficiency lies roughly between 5% and 20%

②At 10%, the primary consumers get 1,000 kcal, the secondary 100 kcal and the tertiary 10 kcal, only 0.1% of the producers' energy

③Even at 20% the energy shrinks to a fifth at each step, leaving the tertiary consumers just 0.8% of the producers' energy

④Because energy runs short so quickly toward the top, food chains rarely go beyond four or five levels

What Makes Up an Ecosystem

Ecosystem components

The Parts of an Ecosystem

Interactions Between Species

Interactions in a community

What Each Species Gains or Loses

Type	Species A	Species B	Example
Interspecific competition	harm (−)	harm (−)	two species fighting over the same food
Predation	benefit (+)	harm (−)	lion (A) and antelope (B)
Parasitism	benefit (+)	harm (−)	parasite (A) and host (B)
Mutualism	benefit (+)	benefit (+)	bee and flower; termite and its gut protists
Commensalism	benefit (+)	no effect (0)	remora (A) and shark (B)

Worked Examples

Example 1

Producers hold 10,000 kcal of energy and the transfer efficiency is 10%. How many kilocalories reach the secondary consumers?

Each step up, only 10% of the energy passes to the next level.

producers 10,000 kcal → primary consumers 1,000 kcal → secondary consumers 100 kcal

▸ 100 kcal

Two steps up leaves 10% of 10%, that is, 1%. With the efficiency at 10% in the pyramid, the secondary-consumer bar reads 100 kcal, and the two levels below it lose 9,000 kcal and 900 kcal as heat.

Example 2

A bee takes nectar from a flower, and the flower gets pollinated thanks to the bee. What kind of interspecific interaction is this, and does each species gain or lose?

When both species gain (+, +), the interaction is mutualism.

The bee gets food in the form of nectar and the flower gets the pollination it needs to reproduce, so both gain.

▸ mutualism (bee +, flower +)

Interspecific interactions are sorted by pairing how each species is affected: gain (+), harm (−) or no effect (0). (+, +) is mutualism, (+, −) is predation or parasitism, (+, 0) is commensalism and (−, −) is interspecific competition.

Summary

10% Rule of Energy Flow

energy of the next trophic level ≈ 10% of the level below

the other 90% or so is used in respiration or passes through decomposers, and in the end leaves as heat

Matter Cycling vs Energy Flow

matter: cycles (producer → consumer → decomposer → producer) / energy: one way (Sun → living things → heat)

matter is used again, energy is not

CSAT-style problem, adapted

Which statement about energy flow and matter cycling in an ecosystem is INCORRECT?

①Energy starts at the Sun and flows in one direction.

②The amount of energy decreases at higher trophic levels.

③Matter such as carbon and nitrogen cycles within the ecosystem.

④Energy cycles within the ecosystem just as matter does.

⑤Most of the energy at each level leaves as heat through respiration.

▸ ④ Energy cycles within the ecosystem just as matter does.

Matter such as carbon and nitrogen cycles between living things and their surroundings, but energy flows one way: in from the Sun and out as heat.

Energy scattered as heat cannot be used again, so it does not cycle. The incorrect statement is ④.

🎯 Exam Points

①Energy flows one way, Sun → producers → consumers, and finally leaves as heat

②About 10% of the energy passes to the next trophic level; the rest becomes heat through respiration and other processes

③Ecological pyramids include pyramids of energy, biomass and numbers, and of these only the energy pyramid can never be upside down

④Matter such as carbon and nitrogen cycles, but energy does not

⑤The more complex the food web, the more easily other prey can stand in when one species declines, so the ecosystem is more stable

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Last updated 2026-10-17

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