5.2 Impact of Feeding on Different Trophic Levels - Answers
In this activity, you will explore the energy transfer impacts of feeding on different trophic levels. This will help to explain why meat consumption has a big impact on climate change.
These are the answers.
Teacher only boxThis has amazing effects when students complete the exercise and they realise the energy impacts of eating meat. It always stimulates a lot of debate. Sometimes I have used it early in the course when I discover misconceptions about "being more efficient" to eat meat, otherwise it can be used in topic 5 or 6.
The answer key can be found here Feeding on Different Trophic Levels - Answers . Teachers can choose when to turn this page on for students.
Here is a link to the handout.
A farmer grows soybeans and keeps chickens.
Grasshoppers feed on the soyabeans and the chickens eat the grasshoppers.
For this exercise:
- The farmer eats one chicken per day
- The farmer weighs 100 kg
- An average chicken weighs 3 kg
- The chicken eats 50 grasshoppers per day
- A farmer needs to eat 800 grasshoppers per day to survive
- A grasshopper eats 500 g of soya per year
- 1000 grasshoppers have a mass of about 1kg
- dry soyabeans contain about 5 kJ/g
- A farmer needs about 8700 kJ/day
Soybeans
| Grasshoppers
| Chickens
| Farmer
|
Problem data
- [a] The farmer eats 1 chicken/day
- [b] The farmer weighs 100 kg
- [c] An average chicken weighs 3 kg
- [d] The chicken eats 50 grasshoppers/day
- [e] A farmer needs 800 grasshoppers/day to survive
- [f] A grasshopper eats 500 g/year of soy
- [g] 1000 grasshoppers have a mass of about 1 kg
- [h] Dry soybeans contain about 5 kJ/g
- [i] A farmer needs about 8700 kJ/day
Questions and Answers
- Q: Calculate the number of grasshoppers eaten by one chicken per year
A year has 365 days, and a chicken eats 50 grasshoppers each day [d], so this year a chicken will have eaten 50 * 365 = 18,250 grasshoppers
- Q: Calculate the number of grasshoppers eaten by all the chickens that the farmer eats in one year
18250 * 365 = 6,661,250 grasshoppers
- Q: What is the mass of all the grasshoppers calculated in #2?
Since 1000 grasshopper have a mass of 1 kg [g], it is 6,661,250 / 1000 = 6661.25 kg
- Q: What is the mass of all the soybeans needed for all the grasshoppers calculated in #2?
Since each grasshopper eats 500 g of soy in a year [f], it is 6,661,250 * 0.5 = 3,330,625 kg or 3330.625 tons
6. Q: Now calculate the mass of soybeans a farmer would need to eat in one day to provide him with his daily needs of energy
The farmer has a daily energy need of 8700 kJ [i], and 1 g of soybeans provides 5 kJ [h].
The needed mass is therefore 8700 / 5 = 1740 g or 1.740 kg
7. Q: Calculate the mass of soybeans a farmer would need for one year
If, from the previous answer, the farmer has to eat 1.740 kg of soybeans per day, the yearly need will be 1.740 * 365 = 635.1 kg
8. Q: Compare the mass of soybeans in #7 and #4. How many people could the soybeans calculated in #4 feed?
With the "eaten in 2020" interpretation: from #4 we have that to (indirectly) feed the farmer for one year 3,330,625 kg of soybeans are needed. From #7 we have that each 635.1 kg can directly feed one person. Therefore, with the #4 amount of soybeans one could directly feed 3,330,625 / 635.1 = 5,244 people.
9. pros: reduced land use; reduced water use; reduced carbon footprint; health benefits of reduced meat consumption; more food for more people; cons: potential reduction in protein intake; reduction in certain vitamins, e.g. B12; reduction in iron intake; loss of livelihood;
10. First law of thermodynamics states that energy cannot be created or destroyed but only transferred, as in this food chain; but second law of thermodynamics states that energy is lost from the system at each transfer; only about 10% is transferred; and so eating at lower trophic level increases efficiency of food production/consumption;