Unit 1 : Characteristics of Living Systems
Modeling a Pond Ecosystem
Purpose:
Explore the interrelationships between organisms in a model pond ecosystem while
designing, executing and evaluating student-made experiments.
Abstract:
Students explore the relationships between the biotic and abiotic parts of a pond ecosystem
by setting up a model using 2-liter bottles. They then design and execute an experiment to
test how the manipulation of one biotic or abiotic factor affects the other biotic components
of the model ecosystem.
Materials:
scissors
light microscopes
pipets
depression slides
pond microorganisms identification charts (http://www.microscopyuk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/wimsmall/smal1.html)
string
fertile soil
rulers
quick germinating seeds such as Wisconsin Fast Plants
digital camera – optional
digital microscope - optional
*Other materials may be necessary depending on the experiments designed by the
students.
Advance Preparation:
Prior to the activity beginning, have students bring in empty 2-liter bottles. Each group
will need a 2-liter bottle. Also, bring in enough pond water (including some silt and
vegetation) so that each group can fill one pond model with about .5 liters of the pond
water.
Sequence of Activities:
1. Have the students build their bottle pond models. Ask the students to cut the top
section of their bottle approximately 10 cm under the shoulder (top curve) of the bottle.
Fill the bottom section of the bottle with approximately .5 liters of the pond water
ensuring that a small amount of silt and vegetation gets into each group’s aquarium. Use
the tip of the scissors to make a small hole in the center of the bottle cap. Thread a 20 cm
piece of string through the hole in the bottle cap. Invert the top section of the bottle so
that it rests on top of the lower section of the bottle. If the top section threatens to sink
into the lower section of the bottle, a few pieces of tape can be used to properly hold the
two sections of the bottle together. The string should hang through the bottle cap and
touch the pond water below. This string will act as a wick moving water from the land
portion of the model to the aquarium portion of the model. Add approximately 10 cm of
soil to the inverted top section of the bottle. Evenly disperse 4-6 fast plant seeds in the
soil and water gently. More directions for building this type of bottle pond model can be
found in the book Bottle Biology (Publisher: Kendall Hunt Publishing ISBN-10:
0757500943 ISBN-13: 978-0757500947).
2. Have students use a pipet to extract a small amount of the pond water from their bottle
aquarium. They should then place a drop of the pond water on a depression slide and
view the water’s contents under low power of the light microscope. Using the
identification charts, have students identify the different types of microorganisms living
in the pond water and make a sketch of each type. If a digital microscope is available,
students can save images from their pond water observations to be used in the
presentations of their experimental results.
3. Have the students in each group discuss possible methods to quantify the population
size of organisms living in their aquariums. The students should discuss the merits and
weaknesses of each method finally deciding which method would be the best to estimate
the number of microorganisms in the aquarium. Counting methods may include taking a
set number of samples from the aquarium and counting the microorganisms in each
(using a single field of view without moving the slide around) or taking a single sample
and moving that slide around to get 5 different field of views counting the number of
microorganisms in each. In any method used it may be helpful to supply a graph grid (to
aid in counting the number of organisms in an area) and Slo-Mo, a commercial product
added to the sample to slow down the invertebrates.
4. Students should water their seeds in the “land” portion of their model each day and,
depending on the type of seed planted, see growth in a few days.
5. As the ecology unit continues and students learn about food webs and cycles, have
students apply this knowledge to their model pond ecosystem. Students can use the
internet to determine if the microorganisms they identified in their aquariums are
autotrophs or heterotrophs and where the organisms fall within a their pond food web.
Students can also examine how the carbon cycle and water cycle play out in their model
ecosystems.
6. After one or two weeks, once the aquarium has settled and the seedlings are growing,
student groups should devise an experiment to test a particular aspect of the model pond
community. Possible aspects to test may be the effect of changing the amount available
sunlight, varying the temperature of the system or altering the pH of the water added to
the system. Because the model has both the top, land portion and the bottom, pond
portion a change to, for example, the water added to the land portion can have
consequences on both the land and pond portions of the model ecosystem. Students
should write out a step-by-step plan for the experiment and clearly identify how they will
collect data throughout the experiment. These data could include quantitative and/or
qualitative data collected on various aspects of the system such as the health of the plants,
the number of microorganisms surviving in the aquarium and/or the types of
microorganisms surviving in the aquarium. If a digital camera and/or digital microscopes
are available, students can use these to take pictures throughout their experiment. After
approval from the teacher, students should carry out their experiments.
7. Upon completion of their experiment, students should interpret and evaluate their
collected data. From their data, students should draw conclusions for their experiment.
These conclusions can then be used to predict possible consequences to other organisms
within the food web and/or effects on the carbon cycle within the model.
8. Have the student groups answer the following questions about their experiment:
a. How effective was your experimental design? Were there aspects of the
design that you would like to have changed given more time or
different resources? Are there different types of data or more data
that you would like to have collected?
b. Explain possible sources of error in the experiment.
9. Student groups should share their results and self-evaluation of their experimental
design with the class. This could be done in an informal manner such as using white
boards or newsprint to share their data or student groups could prepare a more formal
presentation using computer presentation software. At the end of each presentation, other
students in the class should be encouraged evaluate the presenters experimental design
and share suggestions they might have for refining the experiment or additional
experiments that could illuminate other aspects of the interrelationships within the model
pond ecosystem.
Duration – 3-4 weeks