Trophic Ecology of Humans

Carolyn Hayes, Barbara Fendley, and Peggy Walker
1991 Woodrow Wilson Biology Institute


Humans eat. This simple statement hides many complexities. When viewed in an ecological context, eating is the way energy and material enter individual organisms. Consideration of how that energy is related to organisms is called trophic ecology. Energy moves from its source in the sun through the photosynthetic organisms that make energy available to living things (producers) and then through organisms that eat other organisms including the photosynthetic ones (consumers). At each step much energy is lost, and it is of great importance to study these energy relationships in order to understand the functioning of communities and ecosystems.


  • Students will analyze the place of humans among the trophic levels by observing what we eat.
  • Students will calculate the caloric content of several foods commonly eaten by humans.
  • Students will calulate the total energy cost to grow, process, transport, store and cook food items.
  • Students will understand the relationship between the energy content of food and the energy cost to obtain it.

Activity I

  1. Bring a lunch to class. You will be able to eat this lunch while engaging in this activity. If class is early in the morning, you may want to bring a breakfast.
  2. Using various calorie counters, determine the number of total calories and the number of grams of fat, carbohydrates, and proteins in your meal. The Calorie as seen on a nutritional label refers to kilocalorie. ( 1000 calories )
  3. The yearly caloric intake can be calculated by multiplying the daily intake by 365.

Analysis Questions

  1. What is the total class kilocalorie intake for the entire week ?
  2. What percentage of the class intake is composed of fats, carbohydrates, proteins?
  3. Which nutrient (fat, carbohydrate, protein) is most abundant in the class diet?

Activity II

  • Bring the nutritional labels from four food items to class. At least one of the labels should be from one of the following: canned fruit, frozen vegetable, potato chips, loaf of bread or canned fish.
  • While working in groups, determine the following information from each food label:
    • kilocalories in the container or package.
    • servings per container
    • kilocalories per serving
    • number of grams of fat, protein, carbohydrate
    • percentage of kilocalories from fat
      [(9 kcal/gm x gm of fat)/# of kcal per pkg] x 100
    • percentage of kilocalories from protein
      [(4 kcal/gm x gm of protein)/# of kcal per pkg] x 100
    • percentage of kilocalories from carbohydrates
      [(4 kcal/gm x gm of carbohydr)/# of kcal per pkg] x 100

Activity III

Now we want to know the amount of energy it costs to produce kilocalories obtained from the food items. You have undoubtedly thought of a variety of energy costs, but they can be roughly classified into costs of agricultural production, costs of processing, and costs of sales. Then in addition, there are the costs of taking the item home and keeping it until it is eaten. We will consider all of the fossil fuel costs these activities accrue.

Using the following table, determine these fossil fuel costs. Multiply the kilocalories in the food product by the factors in the table. Then add the three to get the total energy cost in kilocalories for the item up to the time the consumer buys it.;

Cost of:agricultureprocessingsales
canned fruit31010
frozen vegatable0.21010
potato chips0.74010
loaf of bread0.3(wheat)1010
canned fish81010

  • Agriculture = _______________kilocalories

  • Processing= _______________kilocalories

  • Sales= _______________kilocalories

  • Total = _______________kilocalories

Now calculate the cost in fossil fuel of getting the food home, storing, and preparing it.

Fossil fuel cost of getting the food home considers distance traveled , mode of transportation, kilocalories in a gallon of gasoline, and the kilocalories used by a human when walking or bicycling. Use 8485 Kilocalories as the fossil fuel cost for bringing home your food item. *

To calculate the fossil fuel cost of storing the food consider the appliance used and its energy usage. In a year an average 17 cubic foot refrigerator uses 1,740 kilowatt hours of erergy. A small freezer uses about 1000 kilowatt hours. Choose one means of storage and determine how much energy is actually spent to store the item for three days before being eaten.

1,740 kwatts x 13,543 kcal x ( 3 / 365 ) = ______kcal (storage)

To calculate the fossil fuel used in cooking the item consider the time it takes to cook the item and the energy used by the appliance. First determine the amount of time to cook the item. Cooking time = minutes

For electric stoves divide the cooking time by 60 minutes to get the part of an hour it takes, and then multiply by 2 kw. There are 13,543 kcal/kilowatt hour.

kilowatts x 13,543 kcal/kilowatts hour = ______kcal (cooking)

For gas stoves the energy can be figured in the same way except that the burners output is in Therms. Multiply the fraction of an hour by 0.1 therms. There are 25,202 kcal/therm hour.

Therms x 25,202 kcal/therm hour = ______kcal (cooking)

What is the total cost in kcal for the item being studied?

Total( agri, proc, sales) = ______kcal
Transportation = ______kcal
Storage = ______kcal
Cooking = ______kcal
Total Fossil Fuel Cost = ______kcal

What is the ratio of nutritional kcal to fossil fuel kcal ?

Food Item kcal / Total Fossil Fuel kcal = ______

Since there are 34,800 kcal in a gallon of gasoline, what is the amount of gasoline needed to produce, transport, sell, store, and prepare this food item?

Analysis Questions

  1. Which calculated energy is greater, kilocalories in the food item or kilocalories needed to acquire the food item?
  2. What is significant about the difference in the energy values in question 1 ?
  3. Using the information in questions 1 and 2, predict the long term effect on human populations if this trend continues.

* This number was determined by the Woodrow Wilson Biology Institute 1991 members after conducting a foraging laboratory in Princeton, N J.

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