Name: _______________________
Ecosystems Lab
Lab Part One: Estimation of the number of plants in each ecosystem
Imagine how difficult it would be to count every bacterial cell in a forest floor or every single plant in a prairie. These are
challenges that an ecologist faces every day. Ecologists cannot determine exact amounts of biomass for such large
ecosystems, but they can estimate these amounts by taking equal-sized samples from different areas of the ecosystem and
then estimating.
In the next section, you will use a sampling technique that ecologists use to estimate the total amount of green
plant biomass in two different ecosystems. Since green plants are producers that capture and store energy from the sun,
they form the base of a food/energy pyramid for an ecosystem. Once the green plant biomass in each of the ecosystems
has been estimated, you will investigate whether the amount of producers results in differences in the number consumers
in each ecosystem and the number of trophic levels each of these ecosystems can support.
Follow the instructions from the instruction sheet to sample and count the number of grasses (green cubes) in the
grassland and in the tundra. Then, calculate both your average and the class average:
Table one: amount of grass (energy units) in grassland vs. tundra
Sample
Grassland
1
2
3
4
My group’s average
Class average
Average ecosystem amount
(class average x 50)
Tundra
1. Do you think that the grassland and the tundra will support the same amount and type of organisms? Explain
your answer:
Lab part 2: How many things can survive in that ecosystem?
In a food chain, energy is transferred from one organism to another. The food chain begins with plants which
capture energy from the sun and converts the energy to food through photosynthesis. The grasshoppers, bullfrogs, water
snakes, and red-tailed hawks are consumers which must eat other organisms to survive.
In ecology, the trophic level is the position that an organism occupies in a food chain – what it eats and what eats it. As
you have learned, the greatest amount of energy in an ecosystem is found in trophic level one with the producers and
energy decreases at each trophic level as you move up the food chain; the transfer of energy is not an efficient process
because all living things need energy to carry out life processes. When a consumer eats another organism, only a small
fraction of the energy taken in is used for growth. About 50% of the food is not digested or absorbed and is passed out as
waste. About 40% of the energy of the food is lost in the form of heat or used by the organism for cellular respiration —
making energy for daily life. Only about 10% of the food eaten is used for growth and is therefore available as energy to
the next trophic level in the food chain. This produces a pyramid of energy and a pyramid of numbers in an ecosystem.
The greatest amount of energy and the largest population is at the base of the pyramid with the producers. The least
amount of energy and the smallest population is at the top of the pyramid in the top level consumers. Only about 10% of
the energy from the food eaten is used for growth. Because of the loss of energy moving up the food chain, each trophic
level can support fewer organisms. In today’s lab, you will be investigating why more or fewer things can survive in
different ecosystems.
1. According to the visual on the side, when a snake eats a mouse, how
many J of the original 1,000,000 does it actually get??
2. A hawk requires ten times more energy than a grasshopper- why might that be?
Number of
Organisms
Available
Energy Units Available to
Next Level Consumer
grass
x
grasshoppers
bullfrogs
water snakes
1 energy unit
available per unit of
grass
x
5 energy units
available per grasshopper
x
10 energy units
available per bullfrog
x
10 energy units
available per water snake
Number of
Organisms
Supported
Total Energy
Absorbed/
Consumed
(Transfer to the first
column
in the next row)
÷
10 energy units
needed per
grasshopper
=
÷
30 energy units
needed per
bullfrog
=
÷
30 energy units
needed per
water snake
=
÷
55 energy units
needed per
hawk
=
grasshoppers
bullfrogs
water snakes
H
hawks
3. Why are the numbers different between grassland and tundra? That is, what is the relationship between the amount of
biomass (number of energy units) and the number of organisms that an ecosystem can support?
4. Draw the two sets of numbers on the energy pyramid shown on the side:
grassland
tundra
5. Based on your observations of both ecosystems, why is it beneficial for all levels of consumers in a food chain if there
are a large amount of producers?
6. Why do higher-level consumers such as the hawk need a larger hunting territory than organisms that are lower in the
food chain such as a grasshopper?
7.
8.
If the amount of grass changes, how does this affect each consumer?
Based on your data, why do you think there is a much greater biodiversity of organisms in tropical biomes with
lots of rain, compared to biomes with less rainfall and more extreme temperatures?
Calculating the Total Plant Biomass in an Ecosystem
a. Place two 50 cm pieces of masking tape 100 cm apart on the table.
b. Connect the ends of pieces of tape with two 100 cm pieces of tape
to form a rectangle
that is 50 cm by 100 cm.
Pour the beans representing the plant energy units of the ecosystem
into the taped-off area of the lab table. Spread the beans as evenly as possible within the taped
edges.
2.
Obtain 4 biomass/energy unit samples from the ecosystem as
follows:
a. Carefully toss the sampling frame into an area of the
ecosystem.
b. Collect all of the beans inside the sampling frame. Do not
collect any beans covered by the sampling frame (see
arrows). Place the beans in beaker “A”.
c. Repeat the sampling technique three more times by tossing the frame into areas that have not
been sampled in the ecosystem. Place the collected beans in beakers “B,” “C,” and “D.”
4. Count the energy units of each sample of beans by counting the number of beans in each beaker
(each bean = 1 energy unit). Record the number in your chart
Table one: amount of grass (energy units) in a grassland vs. a tundra
3.
Sample
1
2
3
4
My group’s average
Class average
Average ecosystem amount
(class average x 50)
Grassland
Tundra
5. Calculate the total and average energy units of the samples collected. Record on the table on your
worksheet.
6. Calculate the biomass of the entire ecosystem. The “Average Plant Biomass”
calculated in Table B is only 1/50 of the entire ecosystem as shown in the
picture to the right.
Use the following formula:
Average Sample Biomass _________ x 50 = _________ Total Plant
Biomass*
*Be prepared to share this information in Step 8 below
7. Return all of the beans to the ecosystem beaker.
As a class, share group data for total biomass from Step 6 above for both Ecosystem 1 and Ecosystem 2.
Average the results to determine the “Average Total Plant Biomass” of the class for each Ecosystem