Name: ____________________________________
Date: ____________
Partner’s Name: ___________________________
Eating at a Lower Tropic Level
Purpose:
To compare human food needs at different trophic levels.
To construct a Biomass pyramid.
Note: You may work with a partner on steps 1-6. You must do steps 7-10 individually.
Background:
In this exercise, you will compute numerical values for human energy needs based on diets at different trophic levels. You
must show all work in your calculations.
The Situation:
Monty Mead, neighborhood soybean farmer, raises guinea hens for food and insect control. Guinea hens will eat
grasshoppers and other insect pests and ticks. They also acts as “watch-birds” by making noise when intruders approach
their territory. Farmer Monty allows the hens free range in his fields during the day and provides roosts for them at night.
Assumptions:
1. The farmer lives on 1 hen/day for a year.
2. One hen eats 25 grasshoppers/day.
3. 1000 grasshoppers have a mass of 1 kg.
4. 1 grasshopper eats 30g soy/day.
5. One human would eat about 600 grasshoppers/day.
6. Dry soybeans contain about 3.3 kcal/g.
Assignment:
1. Calculate the number of grasshoppers a hen needs to eat in a year.
2. How many grasshoppers are required to feed the hens the farmer needs to eat in a year?
3. What is the total mass (in kg) of the grasshoppers required to feed all the hens in (2) for one year?
4. What is the total mass (in kg) of soybeans required to feed all the grasshoppers in (3) for one year?
5. If Farmer Monty decided to eat grasshoppers instead of hens, how many people (including himself) could be fed by the
grasshoppers that the hens need for one year? (it is interesting to note that it is believed that early Native Americans
were able to collect 90kg grasshoppers/hr per person when they were abundant)
6. Farmer Monty needs to consume 3000 kcal/day. If he ate only soybeans instead of hens or grasshoppers, how many
people (including himself) could the farmer’s crop feed?
7. Draw a Biomass pyramid for this scenario (you may have to look this up in the notes/online resources) based on the
information you have at this point. Why do most food chains not have a fourth or fifth trophic level?
Please respond to #8-10 on a separate sheet of paper.
8.
Should people eat at a lower trophic level? In a paragraph or two, discuss two pros and two cons of this suggestion
(use a separate sheet of paper).
9.
On average, cows produce 19kg of protein/acre/year and soybeans produce 200kg of protein/acre/year. Based on
this information and your earlier analysis, discuss why people in less-developed countries usually eat at a lower trophic
level than those in more-developed countries.
10. What about omnivores? Coyotes, rats, and humans can eat at several trophic levels. How does this make them
successful?
Energy Through Ecosystems
The amount of available energy at each trophic (feeding) level decreases as it moves through an
ecosystem. As little as 10 percent of the energy at any level is transferred up to the next level.
In the energy pyramid below, calculate the amount of energy that is passed up from one trophic level to
the next, assuming only 10% of the energy from the previous level is available for the next level. For
each trophic level, circle all the words that apply to identify each organism as either a producer or
consumer and as either an autotroph or a heterotroph. If the organism could be considered a predator
and/or prey, circle those words also.
Circle all words that apply:
Calories transferred to eagle:
producer
prey
consumer
heterotroph
predator
autotroph
Calories transferred to snake:
producer
prey
Calories transferred to grasshopper:
Calories available from grass:
10,000
heterotroph
producer
prey
producer
prey
consumer
predator
autotroph
consumer
predator
heterotroph
autotroph
consumer
predator
heterotroph
autotroph
Questions
1. In the energy pyramid, which level contains the least amount of energy?
2. If only 10% of the energy from one trophic level passes up to the next level, what happens to the
90% energy that is not passed on?
3. If the grass at the bottom of the pyramid is contaminated with pesticide, how might the food chain be
affected?