DESERT SOIL SUCCESSION


There are many areas in which rainwater collects in the desert. Dry soil obtained from these areas contains many bacteria and fungi as well as the eggs and cysts of a variety of invertebrates.

Teacher notes:

Provide the students with a diagram of common microorganisms (i.e., paramecium, amoeba, algae, chilomonas, etc.) Allow at least a span of 4 to 6 weeks sampling time.

Student requirements:

Students are responsible for collecting data and reporting the various identified organisms to the teacher on each sampling day.
You are to describe succession as it occurs in your culture dish. That is, note the different types of new organisms as they appear and describe how each of their populations change with time.

Students prepare a formal lab report, complete with lists of data collected. This is used as the evaluation tool for this activity.

Materials:
  • Medium size culture dishes - 6
  • Conditioned water
  • Droppers - 6
  • Rice, bean, or wheat seed
  • Stereomicroscope
  • Microscope (compound light)
  • Glass Slides
  • Cover slips
  • Graph paper
Procedure:

  1. Half-fill six culture dishes with conditioned water (water that has been in an uncovered container for a day. This allows the chlorine to dissipate and results in room temperature water that is excellent for growing cultures).
    Next add 2 inches of the dry desert soil to each culture dish.
    Finally, drop five to 10 kidney beans, rice kernels, or wheat seeds on top of the soil layer. This will provide food for the bacteria.
    The dehydrated organisms will soon rehydrate in successive stages.

  2. On the appropriate sampling day, stir the material in the culture dish to be sampled and obtain a sample with a dropper. Prepare a wet mount. Observe the slide with the low power objective lens of your microscope. Identify as many species as you can and note their average number.

  3. To obtain an average number of organisms, do the following. Count the number of each species in each of five fields of view (count in each of the four corners of the cover slip and one view in the center of the cover slip), add the total number, and divide by five. For each species you can identify, record the numbers on the data table for your group.

  4. Each group should turn in a completed data table on each sampling day to the instructor. Tallies of class averages will be compiled by the instructor for use in preparing a final lab report.
    Bacteria will appear in the culture dishes and may reach uncountable numbers. We will note only if the numbers appear to be low, medium, high, or very high.

Time: Students will need a few minutes daily to make observations during the first week. Thereafter, they will need a few minutes twice a week over a period of 3 weeks to make and record their observations.

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