Biology 20 - Investigation
Energy Flow in Food Chains
1. INTRODUCTION
The energy used and lost at each trophic level in the ecosystem ensures
that the amount of stored chemical potential energy decreases at higher
and higher levels of the food web
2. PLANNING AND DESIGNING
Problem
How much wheat must be grown to support one hawk for a year?
Background Information
Daily Energy
Requirement (kJ)
Energy Stored in
Tissues (kJ)
Hawk
330
4000
Weasel
80
400
Mouse
20
100
Wheat Plant
5.5
12
Organism
Prediction
Experimental Design
Materials
‰
‰
‰
paper
pencil
calculator
3. PERFORMING AND RECORDING
Procedure
1.
Calculate the yearly energy requirement in kilojoules for one
hawk, one weasel and one mouse.
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Biology 20 - Investigation
Energy Flow in Food Chains
2.
Calculate the number of weasels needed to feed the hawk for one
year. Calculate the number of mice needed to feed one weasel for
one year. Calculate the number of wheat plants needed to feed one
mouse for one year.
3.
Calculate the total numbers in the weasel, mouse, and wheat
populations produced each year to support one hawk. (assume that
hawks eat 30% of the weasel population, weasels eat 30% of the
mouse population, and mice eat 20% of the wheat plants)
4.
Given the amount of energy each organism stores in its tissues, and
the number of organisms needed to support one hawk for one year,
calculate the amount of energy stored by each trophic level.
5.
Calculate the area required to support the hawk for one year
(assume that each wheat plant grows on a patch of soil 5 cm2
6.
Prepare an energy pyramid to summarize your calculations.
7.
Construct a pyramid of numbers based on your calculations.
8.
Construct a pyramid of biomass for the food chain. (assume 1
hawk = 1 kg, 1 weasel = 0.1 kg, 1 mouse = 0.025 kg, and 1 wheat
plant = 0.005 kg)
Evidence
Yearly Energy Requirements
Organism
Energy (kJ/a)
Hawk
Weasel
Mouse
Wheat
Yearly Organism Requirements
Organism
Food Organisms
Hawk
weasels per year
Weasel
mice per year
Mouse
plants per year
Organism per Trophic Level
Organism
Weasels
Mice
Wheat Plants
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Total Number
Biology 20 - Investigation
Energy Flow in Food Chains
4. ANALYZING AND APPLYING
Analysis
1.
Which trophic level receives the most energy?
2.
Which trophic level the least energy?
3.
How do secondary consumers (like weasels) supplement their
energy sources?
Synthesis
1.
2.
To achieve a more realistic estimate of the energy flow through
this food chain, scientists have to consider more about the biology
of an organism than just its trophic level.
a.
What do hawks eat in addition to weasels?
b.
Do all of the weasels and mice have to be alive at the
same time?
c.
How many organisms of each kind must be present in
the ecosystem at one time?
d.
What happens to energy that is not transferred from
trophic level to trophic level?
e.
What role do decomposers plan in the energy flow of
an ecosystem?
Again using the wheat field example, construct a food web for the
system. Include any other organisms that could logically be part of
this ecosystem.
a.
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How would the pyramids of energy, numbers, and
biomass be similar to those you constructed for the
simpler system?
Biology 20 - Investigation
Energy Flow in Food Chains
b.
3.
How would the pyramids of energy, numbers, and
biomass be different from those you constructed for
the simpler system?
What general conclusions can you make based on your data?
(Formulate your answer in terms of energy relationships and
energy transfers. Include the concept of pyramids of energy,
numbers and biomass.)
Evaluation
1.
2.
Make the following assumptions about the food chain in the table:
a.
Hawks take only 20 weasels each year; the rest of
their food is mice.
b.
The weasel's diet is only 50% mice; the rest of its
energy comes from other prey.
c.
Mice consume only 5% of the wheat plants
Revise your pyramids of energy, numbers and biomass based on
these new assumptions.
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