Chapter 14 Population Ecology • MHR 4877.Use a microscope to view samples of your cultures
each day. Estimate the paramecium population of
each culture. Try the following procedure to view
and count the paramecia in your samples:
(a)Squeeze all the air out of the dropper bulb.
Use the dropper to remove a few drops of
your paramecium culture.
(b)Place a drop of methylcellulose and one or
two drops of water on the centre of a glass
microscope slide. (You can also add a few grains
of very fine, washed sand to slow down the
paramecia and prevent the cover slip from
crushing the specimens in your sample.)
(c)Carefully squeeze the bulb to deposit your
culture sample into this mixture.
(d)Gently lower a cover slip over the sample.
(e)View your sample with the low-power objective
lens of your microscope. Scan the sample and
count the number of paramecia in five different
fields of view (view the centre and the area near
each edge of the cover slip). Add up the total
number of individuals counted in all of the
samples and divide by five to determine an
average estimate of the current population size.
Enter your data in a summary data table.
(f )Use the same procedure to observe samples
daily from each set-up and record your estimates
of population size in the appropriate data table.
8.Ensure that your sampling technique can provide an
accurate estimate of the population size without
unduly disturbing the integrity of the ecosystem (and,
if possible, without causing any changes to the
appearance of individual paramecia in each culture).
9.After the data are entered in your summary table,
graph the results. If available, you may wish to use
a computer spreadsheet program to record your
experimental data and generate graphs. Combine
data from other student groups to generate graphs
depicting the class results for each experimental
variable tested.Post-lab Questions
1.Describe any observed changes in the paramecium
population size. Graph the control and the
experimental population sizes over the duration
of your experimental procedure.
2.How do the size and growth rate of the control
culture compare with the populations of your
experimental paramecium cultures? Specifically,
what do your results and the class results suggest
about the impact of a density-dependent factor and
a density-independent factor on the rate of growth
of paramecium populations?3.How did your results relate to your original
prediction?
4.Discuss possible sources of error in your procedure
(relating to such factors as the sampling procedures
used and the ability to control any extraneous
variables) that may influence your results.
5.Were your results consistent with the observations
of other groups?Conclude and Apply
6.What can you conclude about the accuracy of your
original prediction?
7.How could your procedure be improved?Exploring Further
8.Devise a supervised investigation to study
populations of micro-organisms in actual aquatic
ecosystems, such as ponds or streams. Have your
teacher approve your experimental design and
safety precautions in advance. Test the pH of each
sample. Identify other significant density-dependent
and density-independent regulating factors that may
have an impact on certain species of microscopic
organisms in local aquatic ecosystems (such as
rivers or ponds). Indicate your sample locations
on a local map.
9.Conduct research to assess population changes
through the seasons in the ecosystems you have
chosen.
10.What do you think is the mechanism by which
changes in pH increase or decrease the rate of
population growth? That is, why does the pH of its
environment matter to a paramecium, or to any
other organism? How would an effect on an
individual organism translate into changes in
population growth?
11.Which general type of regulating factor — density-
dependent or density-independent — is most
important in regulating the size of paramecium
populations in nature? Or do you think both might
be important, but under different circumstances?
Explain your answer in full.