Biology: Sealed Jars – An Ecosystem Experiment
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Period
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Introduction
Ecology is the study of interactions among organisms and between organisms and their environment.
While some ecologists focus primarily on one type of organism and how these organisms interact with
their environment (i.e. “population ecologists”), other ecologists are more interested in “the big picture”
or how entire ecosystems function together (i.e., “ecosystem ecologists”). As you might have
guessed, ecology is a very wide-ranging science.
In this experiment, you and your lab group will focus on ecosystem ecology. Because it is challenging
for students in a high school (or college!) lab course to study large, complex ecosystems, we will be
using a model ecosystem in this activity. Our ecosystem will be a completely sealed 4-liter (1 gallon)
jar containing a variety of living and non-living materials. If properly constructed, plants, animals, and
other organisms in “ecosystems in a jar” can survive for weeks or months (or sometimes longer!).
After examining an ecosystem jar that has already been set up (the control jar) your group’s task will
be to manipulate a single variable with two goals in mind: 1) to extend the survival time of all
organisms in the jar, and 2) to monitor four responding variables in the jars – number of snails,
number of duckweed leaves, number of Daphnia (animals), and Elodea biomass (plant). For example,
perhaps your group will change the amount of plants in the jar, or modify how long the jar receives
sunlight each day. At the end of the unit, each class will determine which manipulated variable
appeared to cause the best response in the responding variables, and which jars appeared to have
survived the best.
Pre-lab questions. Use full sentences to answers these question.
1. Read the introduction above.
2. Get out your 3-1 homework and notes from class. You will need them for this assignment.
3. What is a drawback/con of the “modeling” research method, as described in class, according to
your notes?
Con:
4. In this experiment we will be observing, experimenting with, and modeling a simple pond
ecosystem to explore requirements of ecosystems and how they work. Explain how these jars
allow us to use each of those ecological methods (see page 65 and your notes)
a. Experimenting.
b. Modeling.
5. In this experiment we will be making changes in a model pond ecosystem and predicting
responses to those changes.
a. Why would it be difficult to make changes to and predict results in a real-life pond
ecosystem?
b. Why are models therefore useful? Explain fully.
6. Make a list of 8 species that could be found in a lake ecosystem. Be specific – for example,
instead of listing “fish”, list the type of fish (e.g. Chinook salmon). Be sure to include a wide variety
of living organisms. Visit https://www.eopugetsound.org/species or search online for “Pacific
Northwest Pond Organisms”.
a. Which organisms on your list provide energy or nutrients to other organisms?
b. What would you expect to happen if all the plants on your list died? Explain.
Jars as an Ecosystem – and the Control Jar.
In this experiment, 4-liter sealed glass jars will be used as model ecosystems. As a class we will
brainstorm what an ecosystem needs to survive. Mr. Welman will then explain how he incorporated
each of these into the control jar. During his explanation, take detailed notes in the space below. You
will need these notes later as you construct your group’s ecosystem jar.
Manipulated variables
There are dozens (hundreds?) of things you could change about the control jar that might affect the
biomass of the Elodea, the number of snails, the duckweed, the Daphnia and prolong the survival of
all organisms in the jar. During class we will brainstorm variables that could be manipulated by your
group in this experiment. Each group will have a different manipulated variable. Use the space below
to record the list developed in your class.
Experimental Design Part I: Setup
1. Meet with your group and discuss which of the manipulated variables brainstormed in class you
think will cause the best response in the responding variables (Elodea biomass, number of snails,
daphnia, number of duckweed leaves). Once your group has chosen a variable, ask Mr. Welman
for approval. Before calling Mr. Welman, each person in your group should be familiar with
the chosen manipulated variable and should be able to explain why your group is changing
this variable.
Our manipulated variable is: ________________________________ Stamp: ____________
2. Write a formal hypothesis for each of the responding variables. Be sure that you follow the
appropriate format for hypotheses and that each hypothesis contains sound reasoning to explain
your prediction. Fill in each of the four blanks with the same manipulated variable.
Hypothesis for how __________________ will affect Elodea biomass.
If we…
then the Elodea biomass will…
because…
Hypothesis for how _________________ will affect the number of Duckweed leaves
If we…
Then the number of Duckweed leaves will...
Because…
Hypothesis for how _________________ will affect the number of Daphnia
If we…
Hypothesis for how _________________ will affect the number of snails
If we…
3. Create a data table in your Chromebook that will allow you to record the following for the first day
and ONE MORE TIME over the next few days. Print and attach to this page.




Elodea biomass in grams
 Number of daphnia
Number of Duckweed leaves
 Number of snails
Dissolved oxygen
Observations (space should be large enough for a several sentences both times you sample).
4. Obtain a 4-liter glass jar, and add to it all of the ingredients listed in the control jar section above,
except for your manipulated variable. Everything you do in this step should be identical to how the
control jar was created (see your notes above). Do not seal the jar.
5. Add your group’s manipulated variable to the jar, being sure that you have changed it from the
control jar in the way you discussed with Mr. Welman. All group members must agree that the
manipulated variable has been done appropriately. Do not seal the jar yet.
6. Record today’s date, and your four responding variables in Table 1. Record general comments
about your ecosystem jar. Most students find it helpful to describe what the jars look like generally
and in detail; consider answering these and below in your data table:
a. What does it smell like?
d. How big/small/active are the
b. What does the water look like?
daphnia? What are they doing?
c. What do the snails look like?
e. What do Elodea plants look like?
The more information you record, the easier a time you will have when you write your conclusion
at the end of this lab. Describe anything else you see.