SKILL FOCUSInitiating and planningPredictingIdentifying variablesPerforming and recordingAnalyzing and interpretingConducting researchInvestigation 14•A
486 MHR • Unit 5 Population Dynamics
Pre-lab Questions
What is the natural food supply of micro-organisms
such as paramecium?
Describe the life cycle of paramecium.
How would lower pH levels affect populations of
organisms that inhabit aquatic ecosystems?
How would changes in food availability affect
populations of organisms that inhabit aquatic
ecosystems?Problem
How can you determine the impact of density-dependent
factors (such as food supply) and density-independent
factors (such as pH) on the growth rate of paramecium
populations?Prediction
Predict the effect of changes in pH level and food
supply on the growth rate of paramecium populations.CAUTION: Handle hydrochloric acid with care.
Wash your hands after each observation.Materials
glass jars of uniform size thermometers
culture medium (skim dilute hydrochloric acid
milk powder) pH hydrion papers (or other
distilled water indicators that provide
paramecium cultures accurate pH readings)
droppers microscopes
heating sources methyl celluloseProcedure
1.For this investigation, your class should be split into
several groups. One half of the class should focus
on testing the effects of a density-dependent limiting
factor (food supply), while the other should
investigate the effects of a density-independentlimiting factor (pH). Each student group will run one
control and one or more experimental set-ups.
2.Identify the specific factor or variable (such as pH
or food supply) to be investigated. This will be your
independent (manipulated) variable. Select the range
of factors to be tested. (For example, one group
could maintain one control culture at neutral pH and
an experimental culture at a slightly lower pH while
the other groups maintain cultures at different pH
levels. That way, a broad range of pH levels can
be analyzed.)
3.Maintain all cultures at constant temperature and
at uniform, medium light conditions. Avoid direct
sunlight, drafts, and contamination of cultures.
Ensure that all glassware is clean and
uncontaminated by soap or other chemical residue.
Leave each culture open to the air, but ensure that
water levels remain constant by adding room-
temperature distilled water as required.
4.To start, add about 5 g of skim milk powder to
250 mL of distilled water. Inoculate each set-up with
an identical volume of paramecium culture (about
three or four full pipettes or droppers). The control
set-up for each group should be kept at neutral pH
and be provided with the same food supply and
starter paramecium culture. In fact, you may wish to
maintain each set-up for three to four days with a
standard food supply and neutral pH, to stabilize
each paramecium population before you begin to
manipulate the environments.
5.If you are investigating food supply as a variable,
add different amounts of food to your experimental
set-ups (the class should work with a range of
0 – 20 g of food per experimental culture).
6.If you are investigating pH as a variable, add varying
quantities of dilute HCl (start with 0.1 mol/L HCl) to
produce a range of pH, from neutral to about pH 3.
Maintain a constant pH in each set-up throughout
the experiment.Paramecium Populations
Cultures of small organisms, such as microscopic paramecia, can be used to study
how various abiotic and biotic factors affect population growth patterns. These
unicellular protozoa are ideal for studying changes in population size because they
have a relatively short reproductive cycle and cultures can be easily maintained,
measured, and manipulated in a laboratory environment. This investigation
focusses on how density-dependent factors and density-independent factors
affect the growth rate of paramecium populations. These observations will allow
you to suggest how abnormal pH levels in natural ecosystems, resulting from
acid precipitation or other types of contamination, might have negative effects
on organisms living in aquatic environments.