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F3.4 · Trace food-energy flow in human populations

Learn to trace food-energy flow in human populations through clear examples and targeted practice.

Ontario Grade 12 Biology

Population Dynamics

From producers to people—and how to interpret the numbers

A person eating beans and a person eating beef both obtain chemical energy from food. But the energy reaches them by different routes. Beans store energy in plant tissue that people eat directly. Beef comes from an animal that obtained energy by eating plants or other feed. Tracing these routes helps us understand food-energy flow in human populations. In this lesson, a population means a group of people considered together. Energy values in the examples are invented for practice, not measurements of a real population.

What you will learn

  • Trace food energy from producers through food chains to humans.
  • Explain why less energy is available at each feeding step.
  • Use simple energy calculations to compare food pathways and population totals.
  • Identify what a food-energy model can and cannot tell us.

1. SBI3U bridge: energy moves through food chains

In ecology, a food chain shows who obtains food from whom. A producer, such as a plant, makes organic food molecules using light energy. A consumer obtains energy by eating other organisms. Humans are consumers. They may eat producers directly, such as grains, or eat animals that consumed plants or other animals.
Food contains chemical energy: energy stored in molecules. When an organism uses food, some energy supports life processes, such as movement and maintaining the body. Much of the energy is transferred to the surroundings as heat, or remains in material that is not eaten or digested. Energy is not recycled through a food chain in the way matter can be; it flows through the system and is eventually dispersed as heat.
A trophic level is a feeding position in a food chain. Plants occupy a producer level. An animal that eats plants occupies the next consumer level. Humans may occupy different positions depending on what they eat. A person eating a plant is feeding at a shorter chain than a person eating an animal that ate plants.
producer→consumer→human\text{producer} \rightarrow \text{consumer} \rightarrow \text{human}
  • A food chain traces energy through feeding relationships.
  • Humans can obtain food energy directly from plants or indirectly through animals.
  • Energy is transferred between organisms, but some is unavailable at each step.

2. The biological system: where food energy goes

Consider a crop harvested for food. The crop contains chemical energy in its tissues. If people eat the crop, some of its energy enters the human food pathway. If livestock eat the crop first, only some of the crop energy becomes energy in animal growth that people later eat. The animal also uses energy for its own life processes, and some energy leaves as heat and waste.
This does not mean that every food pathway has the same energy values. The amount transferred depends on the organisms, the food, and how the system is measured. The useful course-level idea is that energy available in edible tissue at one feeding level is less than the energy originally present in all food at the level below.
A population-level trace can follow a larger route: energy in crops or other producers, energy in food animals if present, food harvested and distributed, and food eaten by people. Some harvested food is not eaten by people. Some edible food is discarded. These are losses from the human food pathway, even though the energy may remain in waste or be used by decomposers.
A food-energy account is a simplified model. It tracks energy amounts across chosen steps. It does not describe every person’s diet or every ecological detail. To make a fair comparison, the same units and time period must be used for each value.
Eavailable<Efood belowE_{\text{available}} < E_{\text{food below}}
  • Eating plants directly creates a shorter food-energy route than eating animals raised on plants.
  • Not all energy in a crop becomes edible animal tissue or food eaten by people.
  • Compare values only when their units, boundaries, and time periods match.

3. Evidence, calculations, and limits

Energy flow can be represented with measured or estimated energy values for food produced, lost, and eaten. A joule is a unit of energy. Large food-system totals may use kilojoules, where one kilojoule equals one thousand joules. When a problem gives values, treat them as data for that problem. Do not assume that an example value describes a real community.
One useful calculation is the fraction transferred between two steps. Divide the energy at the later step by the energy at the earlier step, then multiply by one hundred to express the result as a percentage. The result describes those two steps and those data. It is not a universal transfer rate.
A population total can be found by multiplying a per-person amount by the number of people, provided both refer to the same time period. For example, a daily amount multiplied by a population gives a daily total. If the time periods differ, convert them before comparing.
Models leave things out. A simple calculation may not include differences among foods, changes in food storage, unequal access to food, or variation in people’s diets. It also cannot show by itself why a population has a particular diet. Report what the calculation supports, and avoid extending its conclusion beyond the supplied evidence.
transfer percentage=ElaterEearlier×100%\text{transfer percentage}=\frac{E_{\text{later}}}{E_{\text{earlier}}}\times 100\%
  • Transfer percentage compares energy at two stated steps.
  • Population totals require a matching per-person time period.
  • A calculation describes its data and assumptions, not every real population.

4. Reading a food-energy trace

Start by naming the beginning and end of the pathway. Then identify each feeding step between them. Mark energy that enters the next step and energy that does not. This prevents a common error: treating all energy in a crop as if it becomes food for people.
Next, attach units and a time period to every number. If the question asks how much energy reaches people, use the value for food eaten by people, not the value for crops harvested. If it asks for a transfer percentage, use the energy values at the two steps named in the question.
Finally, state the scope of the result. A calculation may show that one pathway delivers a greater fraction of its starting energy to people in a particular example. It does not establish that every population has the same food system or that the example is a complete description of food production.
  • Trace each link in order from source to people.
  • Distinguish food produced, food available, and food actually eaten.
  • State units, time period, and limits when reporting a result.

A simple food-energy route

StepExampleWhat happens to energy
ProducerCropStores chemical energy in tissue
Possible consumerLivestock eating cropsUses some energy; some becomes animal tissue
Human populationPeople eating crops or animal foodReceives energy in food eaten
Outside the food eaten by peopleHeat, waste, or discarded foodEnergy does not reach people as eaten food

Worked example

Example 1: Compare direct and indirect routes

In an illustrative model, 12,000 kJ of crop energy is available. People eat crops containing 3,000 kJ. Alternatively, livestock eat the crops, and people later eat animal food containing 900 kJ. Find the percentage of starting crop energy reaching people by each route.
  1. Identify the starting amount
    Both pathways begin with 12,000 kJ of crop energy, so use that same starting value for each percentage.
    Estarting=12,000 kJE_{\text{starting}}=12{,}000\ \mathrm{kJ}
  2. Calculate the direct route
    Divide the crop energy eaten by people by the starting crop energy. Multiply by one hundred to convert the fraction to a percentage.
    3,00012,000×100%=25%\frac{3{,}000}{12{,}000}\times 100\%=25\%
  3. Calculate the route through livestock
    Use the energy in animal food eaten by people as the later-step value. The result is smaller in this illustrative model.
    90012,000×100%=7.5%\frac{900}{12{,}000}\times 100\%=7.5\%
Answer: In this model, 25% of the starting crop energy reaches people through direct crop consumption, while 7.5% reaches people through the livestock pathway.
Check: The two percentages use the same starting amount and the appropriate energy eaten by people for each route.

Worked example

Example 2: Calculate a population total

A hypothetical population has 4,000 people. In a model, each person eats food containing 8,500 kJ of energy per day. Find the population’s total food energy eaten in one day.
  1. Match the quantities
    The per-person value is daily, and the requested total is for one day. Multiplying by the number of people keeps the time period consistent.
    8,500 kJ/person/day×4,000 people8{,}500\ \mathrm{kJ/person/day}\times 4{,}000\ \mathrm{people}
  2. Multiply
    The person units cancel, leaving energy per day for the population.
    34,000,000 kJ/day34{,}000{,}000\ \mathrm{kJ/day}
Answer: The modeled population eats food containing 34,000,000 kJ of energy per day.
Check: The result is a population total, not an amount for one person.

Worked example

Example 3: Find a missing transfer amount

A simplified food pathway begins with 20,000 kJ in producers. The model states that 15% reaches the next food step. How much energy reaches that step?
  1. Convert the percentage to a fraction
    A percentage means an amount out of one hundred. Use 15 hundredths of the starting energy.
    15%=0.1515\%=0.15
  2. Find the transferred energy
    Multiply the starting energy by the fraction transferred. This gives the energy at the next step in this model.
    20,000 kJ×0.15=3,000 kJ20{,}000\ \mathrm{kJ}\times 0.15=3{,}000\ \mathrm{kJ}
Answer: The model assigns 3,000 kJ to the next food step.
Check: The result is less than the starting energy, as expected for a 15% transfer.

Common mistakes and how to avoid them

Assuming all energy in plants reaches people, even when livestock are part of the pathway.
Correction: Track the energy at each feeding step. Only the energy present in food later eaten by people counts as reaching people in that pathway.
Calling energy that is not eaten or transferred to the next food step recycled energy.
Correction: Energy flows through the system. Some leaves the food pathway as heat or waste rather than returning as food energy.
Multiplying a daily per-person amount by a population and reporting it as a yearly total.
Correction: Keep the time period attached to the units. Convert days to years only if the question asks for a yearly total.
Treating a practice example as a measured fact about a real population.
Correction: Use the values only as stated. Identify them as illustrative unless evidence establishes that they are real measurements.

Lesson summary

  • Human food energy can come directly from producers or through other consumers.
  • At each feeding step, only part of the energy becomes food available at the next step.
  • Transfer percentages and population totals are useful when units and time periods match.
  • A model supports conclusions only within its stated data and limits.

Check your understanding

Question 1

A food route begins with 10,000 kJ in crops. People eat crop food containing 2,000 kJ. What percentage of the starting energy reaches people in this example?
  1. 2%
  2. 20%
  3. 50%
  4. 80%
Show answer and explanation
20%
Divide 2,000 by 10,000 and multiply by 100. The result is 20%.

Question 2

Why does a pathway through livestock generally have less energy available to people than a direct crop pathway, all else equal?
  1. Livestock create energy from nothing.
  2. Some crop energy is used by livestock or does not become edible animal tissue.
  3. People cannot obtain energy from plant food.
  4. Energy increases each time an organism eats.
Show answer and explanation
Some crop energy is used by livestock or does not become edible animal tissue.
Some energy is used by the animal or is not present in the tissue people eat, so less is available along the longer pathway.

Question 3

A model gives food energy per person per day. What must you do before comparing it with a yearly population total?
  1. Remove the units.
  2. Convert the time period and account for the population size.
  3. Assume the daily value is already a yearly total.
  4. Add the percentage sign to the energy value.
Show answer and explanation
Convert the time period and account for the population size.
The time periods and population scope must match before the totals can be compared.

Key terms

Chemical energy
Energy stored in molecules, including molecules in food.
Consumer
An organism that obtains food by eating other organisms or their products.
Food chain
A model showing feeding relationships and the path of energy through organisms.
Population
A group of people considered together for a particular question or model.
Producer
An organism, such as a plant, that makes organic food molecules using light energy.
Trophic level
An organism’s feeding position in a food chain.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation F3.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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