Unit 1 · Topic 1.10 Beta

Energy Flow and the 10% Rule

Energy enters ecosystems through producers and flows one way, up the trophic levels.

Practice 2: Visual RepresentationsPractice 6: Mathematical Routines

Part 1 · Hook

Why this matters

A field of grass can feed a herd of grazers, the grazers a few predators, and the predators perhaps one large hunter. Every step up loses most of the energy as heat. That single fact explains why big predators are rare, why there are so few trophic levels, and why eating plants feeds more people per acre than eating meat.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. What is the trophic level of a herbivore?

Primary consumer (level 2)
Producer (level 1)
Secondary consumer (level 3)
Apex predator

Show the answer

Herbivores eat producers.

Correct: Primary consumer (level 2):
Producer (level 1):
Secondary consumer (level 3):
Apex predator:

2. Which setup gives what percent 650 is of 2,500?

650 / 2500 × 100
2500 / 650 × 100
(2500 - 650) / 650 × 100
650 × 2500 / 100

Show the answer

Part over whole, times 100.

Correct: 650 / 2500 × 100:
2500 / 650 × 100:
(2500 - 650) / 650 × 100:
650 × 2500 / 100:

3. NPP is...

GPP minus plant respiration
GPP plus plant respiration
the energy reaching the top predator
the sunlight hitting a field

Show the answer

NPP = GPP - R.

Correct: GPP minus plant respiration:
GPP plus plant respiration:
the energy reaching the top predator:
the sunlight hitting a field:

Part 4 · See it

See it first

An energy pyramid from invented data in kilocalories per square meter per year. Producers 10,000; primary consumers 1,000; secondary consumers 100; tertiary consumers 10. Each level holds about 10 percent of the level below; about 90 percent is used for life processes and lost as heat, as the second law of thermodynamics predicts. Widths are not to scale. — An energy pyramid: about 10% of the energy passes up each level. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

Organisms at each level use most of their food energy for respiration and body heatthat energy leaves as heat and cannot be passed on
Some of each level is never eaten, or is lost in wastethat energy goes to decomposers instead
Only about 10% is stored as new tissueabout 10% is available to the next level up, so energy shrinks at every step

Part 6 · Key ideas

Key ideas

The 10% rule: about 10% of the energy at one trophic level reaches the next. Multiply by 0.10 for each step up.
The first law of thermodynamics: energy is not created or destroyed. The second law: every transfer turns some into heat, so no transfer is 100% efficient.
An energy pyramid always narrows going up; biomass pyramids usually do too.
Transfer efficiency = energy at the higher level ÷ energy at the lower level × 100.

Part 7 · Misconception

A common mistake

The wrong idea: Energy is used up or destroyed as it moves up the trophic levels.

What actually happens: Energy is not destroyed (first law). It changes form, and at each step most of it becomes heat released to the surroundings (second law), which organisms cannot use as food.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. Producers in a meadow store 20,000 kcal/m²/yr. Using the 10% rule, about how much energy reaches the secondary consumers?

Type a number, then its unit (kcal/m^2/yr). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

Two steps up (producers → primary → secondary): $20{,}000 \times 0.10 \times 0.10 = 200$ kcal/m²/yr.

Answer: 200 kcal/m^2/yr

2. Primary consumers in a forest store 3,000 kcal/m²/yr. Write the setup for the energy reaching tertiary consumers by the 10% rule, then give the answer.

Type the setup as on the exam, with brackets and units, then the answer.

Show the answer

Primary → secondary → tertiary is two steps: $3{,}000 \times 0.10 \times 0.10 = 30$ kcal/m²/yr.

Setup: $3000 \cdot 0.1 \cdot 0.1$ ; answer 30 kcal/m^2/yr

Data

Energy at each trophic level in a lake

Energy stored in new tissue at each trophic level in a lake, in kcal per square meter per year (invented data for practice)
Trophic level	Organisms	Energy (kcal/m²/yr)
1: producers	algae	8,000
2: primary consumers	zooplankton	880
3: secondary consumers	small fish	80
4: tertiary consumers	bass	7

3. What percent of the producers’ energy is passed to the primary consumers?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

$\frac{880}{8{,}000} \times 100 = 11\%$ .

Answer: 11 %

4. What is the transfer efficiency from small fish to bass, as a percent?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

$\frac{7}{80} \times 100 = 8.75\%$ , close to the 10% rule.

Answer: 8.75 %

5. Which statement do the data support?

Each transfer is close to 10%.
Each level holds about half the energy of the level below.
Energy increases at the higher trophic levels.
The bass get about 10% of the producers’ energy.

Show the answer

Efficiencies: 880/8,000 = 11%; 80/880 = 9.1%; 7/80 = 8.75%. All near 10%.

Correct: Each transfer is close to 10%.: Right: each step is close to 10%.
Each level holds about half the energy of the level below.: Each level holds about a tenth, not half.
Energy increases at the higher trophic levels.: Energy falls at each level.
The bass get about 10% of the producers’ energy.: Bass get 7/8,000, about 0.09% of the producers’ energy.

6. What percent of the producers’ energy reaches the bass?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

$\frac{7}{8{,}000} \times 100$ $= 0.0875\%$ , less than a tenth of a percent.

Answer: 0.0875 %

Model or diagram

Biomass at each level in a grassland

7. Which pattern does the biomass chart show?

It falls sharply at each level.
It rises at each higher trophic level.
Each level has about the same biomass.
Grazers have more biomass than grasses.

Show the answer

1,500 → 120 → 11 → 1.5 g/m²: roughly a tenth or less at each step.

Correct: It falls sharply at each level.: Right: a sharp drop each step.
It rises at each higher trophic level.: The bars get shorter going up.
Each level has about the same biomass.: The bars differ greatly.
Grazers have more biomass than grasses.: Grasses (1,500) far exceed grazers (120).

Part 9 · Summary

Summary

Energy enters ecosystems through producers and flows one way, up the trophic levels. At each step about 90% is used for life processes and lost as heat, so only about 10% passes on: multiply by 0.10 for each step. This explains energy pyramids, the small number of trophic levels and why eating at a lower level feeds more people. Energy is not destroyed; it leaves as heat.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

Question set for this topic