BIOLOGICAL SUCCESSION IN A MICROECOSYSTEM
Type of Activity:
- Hands-on activity
- Inquiry lab
- Group/cooperative learning
- Environmental Studies
This laboratory activity helps students understand the concept of biological succession by simulating the process in a microenvironment with various microorganisms. By manipulating envirnmental 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.
Lab groups of four students are used for this activity. I 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. I usually set up the culture dishes and add the various organisms prior to the beginning of the actual sampling. Allow at least a span of four to six weeks sampling time. We usually sample on four consecutive Fridays (a nice way to end the week!)
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 boiled or pasteurized.
I usually have 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. It's a good idea to laminate a set for each lab table.
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.
This activity is a modification of a laboratory exercise from the Merrill laboratory manual, Probing Levels of Life, in collaboration with a fellow teacher, Lucrecia Herr. This activity helped solve the problem of how to help students understand the concept of biological succession in our ecology lesson for honors biology students. I have also used this lab successfully with other levels of students.
In any biological community, certain species of living things are more numerous than others. Usually a community contains one dominant organism. However, some communities are able to have two dominant species, such as the oak-hickory forest.
Some communities do not remain stable because slight modifications of physical and biological factors alter the habitat. When the habitat changes, the dominant organism(s) can be replaced or reduced in number. When a new species replaces a species as the dominant organism, succession has occurred.
Succession in some communities is scarcely visible because of the length of time it takes for noticeable changes to occur. However, in cultures containing protists and microinvertebrates, it is possible to observe succession. Also, the factors causing the succession can be determined.
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.
- To observe the order of stages of succession in cultures containing bacteria, protists, and microinvertebrates.
- To determine the factors causing succession.
- To learn population sampling techniques.
- To observe food chains in a microecosystem.
Materials (per team of four):
- Medium size culture dishes - 6
- Boiled pond or spring water
- Droppers - 6
- Cooked wheat grain
- Microscope (compound light)
- Glass Slides
- Cover slips
- Graph paper
- Paramecium culture
- Mixed rotifer culture
- Blepharisma culture
- Eudorina culture
- Peranema culture
- Euglena culture
- Amoeba culture
- Half fill each of six culture dishes with boiled pond or spring water. Label the dishes A through F. With a dropper , add ten 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 1-1). Add boiled pond or spring 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.
Some generalizations you should try to make include:
- In which culture dish did the greatest changes in numbers take place? Why?
- Which organisms appeared to rise and decline the most? The least?
- Which environmental factors were necessary for the survival of most organisms?
What is the ideal temperature for survival with or without food?
- Do photosynthesizers survive without food? If so, why?
- Who is eating whom? Construct a food chain or web illustrating this.
- Assuming that no new organisms were added each week when food was added,
- account for the presence of more organisms in the culture dishes.
- Did "new" types of organisms appear in later examinations of the cultures? If so, from where did these organisms come? From where did the bacteria come?
- What causes successional changes to occur in a community?
- Which physical and biological factors could have caused the succession in the culture dishes?
- Why did we use a sampling technique to determine the population numbers of the various species?
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)|
|E ||none||Lighted shelf in classroom|