The desert soil succession activity will yield various types of organisms depending upon eggs and cysts present in the soil collected.

This activity also deals with succession in a microsystem. However, this time all the organisms present in the microecosystem will be known, because they will be added from pure cultures provided by the teacher.

By manipulating environmental factors such as temperature, light, and food, students are able to see in a matter of weeks the results of change that can take years in larger ecosystems in nature.

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Background Information:

Allow one lab period for students to observe the organisms in the "pure" cultures so that they become familiar with the appearance of each type. Set up the culture dishes and add the various organisms prior to the beginning of the actual sampling. Allow at least a span of 4 to 6 weeks sampling time.

A set of six dishes is needed for each lab group, but remember that the same sets can be used for different classes. Be sure the water is conditioned, either tap water left out over night or boiled water.

Make a picture key of the various organisms available for the students. They sometimes forget what the "critters" look like since the sampling dates are a week apart.

Student requirements:

Students are responsible for collecting data and reporting group averages to the teacher on each sampling day. Students prepare a formal lab report, complete with graphs of data collected. This is used as the evaluation tool for this activity.

In this investigation, you are to set up six culture dishes containing various protozoans. You are able to place these dishes in areas of specific temperature and light conditions. Over a period of time, you are to determine the order of the stages of succession in the dishes. You are to describe succession as it occurred in each dish.

Purposes:

1. To observe the order of stages of succession in cultures containing bacteria, protists, and microinvertebrates.
2. To determine the factors causing succession.
3. To learn population sampling techniques.
4. To observe food chains in a microecosystem.

Materials:

Medium size culture dishes - 6 Conditioned water Droppers - 6 Cooked wheat grain Stereomicroscope Microscope (compound light) Glass Slides Cover slips Graph paper

Paramecium culture Mixed rotifer culture Blepharisma culture Eudorina culture Peranema culture Euglena culture Amoeba culture

Procedure:

• Half fill each of six culture dishes with boiled pond or spring water. Label the dishes A through F. With a dropper , add 10 drops of each culture to each of the six dishes. Stir each culture prior to transferring it to one of the six culture dishes.

• Mark the fluid levels of each culture dish with tape. To dishes A, B, and C, add three grains of cooked wheat. Do not add any grains to dishes D, E, or F. Place the culture dishes in the proper environment (see table below). Add conditioned water to each dish as needed to keep the fluid level constant.

• Add food (wheat grains) to the appropriate culture dishes (A, B, and C) on days 7, 14, 21, 35, and 42. Sample the cultures prior to adding the food.

• 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. Count the number of each species assigned to you by your instructor. Each member of the group will be responsible for counting a different group of organisms.

• 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. Record the numbers of each species for which you are responsible on the data table for your group.

• 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. Remember: you must sample all six dishes on each sampling day. 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.

Food and Environmental Conditions for Each Culture Dish

Culture Dish	Food	Environmental Conditions
A	3 grains	Unlighted refrigerator (remove bulb)
B	3 grains	Lighted shelf in classroom (18-27 0C)
C	3 grains	Lighted incubator (28-32 0C)
D	none	Unlighted refrigerator
E	none	Lighted shelf in classroom
F	none	Lighted incubator