Compare Primitive Communities to Climax Communities (2023)


16.1: Community Environmental Lab

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  • Lab adjusted past Staci Forgey and Dr. James Holden, Tidewater Community College biology faculty, with permission from Dr. William Edwards, biology faculty at Niagara University.

    Learning Objectives

    • Describe the processes of primary succession and secondary succession
    • Explicate what makes a community and an ecosystem different
    • Describe the differences between abiotic and biotic factors
    • Explain why disturbances play an important role in the progression of succession
    • Define a climax community
    • Describe why most areas will non brand it to a climax customs
    • Describe the establish communities present later on glacial succession and how they change the environment
    • Explain the stages of succession of milk
    • Describe how pH changes every bit milk goes through successional stages
    • Explain the difference betwixt gram negative and positive bacteria
    • Draw and describe the shapes of leaner
    • Formulate a hypothesis based on background data

    Compare Primitive Communities to Climax Communities (1)

    A SlideShare element has been excluded from this version of the text. You can view it online here:

    (Video) Communities

    Ecological Succession of Bacteria in Milk

    The communities within ecosystems develop over time, from very simple species assemblages, to complex, rich ecosystems.
    In this process, called succession, each
    species facilitates changes in surround which allow new species to come into the ecosystem.
    Equally the community becomes more and more complex, the biodiversity of the ecosystem also increases.
    Both biotic and abiotic processes tin reset the succession process.
    That is, events cause past both the community itself, and outside events can render the customs to an earlier succession state.
    The gradual changes in the community are both orderly and predictable in many ecosystems.
    The summit or near complex, advanced community that can develop in whatever abiotic surround is called the climax customs.
    The pic beneath describes the developing communities as a series of steps, each of which can be driven against the succession process by disturbances:

    Compare Primitive Communities to Climax Communities (2)


    1. What types of events could “reset” a succession process? Proper noun at to the lowest degree one biotic and one abiotic disturbance.

    One example of a severe disturbance, reducing the land to bare ground, is the passage of a glacier.
    Though glaciers have not covered this office of N America for ten m years, at that place are parts of the continent that are even now becoming uncovered past receding glaciers.
    One area is the pacific northwest.
    From Juneau to Glacier Bay, many glaciers which take previously fallen directly into the ocean are at present leaving bare soil which has not been exposed for more than fifty thousand years.
    Because the glaciers retreat very slowly, we can watch communities change across time in a single snap shot.
    Accept the glaciers of Glacier Bay National Park, midway between Juneau and Anchorage, Alaska.
    The glaciers at that place have been retreating since the explorer Vancouver’south commencement expedition in 1794.
    Since then, the retreat has covered over 100 km, including new coastline, meadows and mountains:

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    Compare Primitive Communities to Climax Communities (3)


    1. Explain how this retreat volition outcome in different communities along the glacier’s retreat, though in like environments.
      Hint: what is the deviation between exposed soil at point A and signal B.

    Compare Primitive Communities to Climax Communities (4)

    As the glacier retreats, it leaves nutrient poor soil which can only support simple plants such as liverworts, lichens, and other primitive plants.
    As they photosynthesize and die, we meet them enter the decomposer pathway and increase the quality of the soil for afterwards plants.
    Nonetheless, this dull glacier retreat is a unique situation.
    To fix an experiment to test our understanding of succession would require hundreds of years, longer than a scientists lifetime.
    However, some organisms and communities proceed at a much faster rate, inside your own refrigerators. The process of milk decomposition from a community of bacteria tin exam the same processes and theories in a much more reasonable fourth dimension frame.
    This substitution of a simpler and faster customs for experimental purposes is called a ‘model’ system.

    Milk is a highly nutritious food containing carbohydrates (lactose, or milk saccharide), proteins (casein, or curd), and lipids (butterfat). This loftier level of diet makes milk an excellent medium for the growth of bacteria.
    Pasteurizing milk does not sterilize it (sterilizing kills
    bacteria) but merely destroys pathogenic bacteria, leaving many bacteria that can multiply and these bacteria volition begin to grow and bring about milk spoilage. Biologists take discovered that equally milk ages, changing weather in the milk bring almost a predictable, orderly succession of microorganism communities.

    In this laboratory practice, y’all will discover successional patterns in several types of milk.
    Yous will record changes in the environmental conditions of the types of milk as they age.
    These changes are a result of changes in the bacterial communities.
    Here are s
    ome of the major leaner found at diverse stages.

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    Stages of Milk Succession

    1. Psuedomonas
      (gram-negative rods) digest butterfat and give milk a putrid smell

    2. Lactobacillus
      (gram-positive rod) and
      (gram-positive coccus) ferment lactose to lactic acid and acetic acid.
    3. Acidity sours milk and converts casein to curd.
    4. Yeast (fungi) thrive in acidic conditions and metabolize the acids into not-acidic compounds.

    5. Bacillus
      metabolize proteins into ammonia products and enhance the milk’s pH. Spoiled milk odor is very noticeable at this stage.
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    1. What are some advantages to using bacteria as opposed to plants in this experiment?
    2. What factors might speed or delay a successional process? Apply your example to succession in milk.

    Locate the milk samples available in the lab.
    Take a look at the samples and form two conditions you would similar to study.
    Develop a short design for studying the first condition, state the dependent and contained variable, control and experimental group, the hypothesis and whatsoever variables which take been controlled.
    Then develop a short pattern for studying the 2nd condition, land the dependent and independent variable, control and experimental group, the hypothesis and whatsoever variables which have been controlled.

    Only as in whatever of our experiments, you must use constructive scientific method.

    Develop hypotheses (at least 2—one with unlike milk types and one with either temperature or time as an independent variable) that you lot can examination in the process of the milk customs succession.

    We will be performing a gram stain on our milk samples.
    Remember from our microbiology department that leaner tin can exist either gram negative (pink) or gram positive (majestic).
    We will also wait at these bacteria under a microscope to identify their shapes.
    Recall that bacteria tin can be cocci, bacilli, or spirillum.


    1. What hypotheses are you testing? List both here.
    2. What information led you to ask these hypotheses?
    3. Make predictions nearly your hypotheses. i.due east. How will you know if the data supports or refutes your hypotheses?
    4. Be sure to identify the variables you will
      test, and those you volition
      for each experiment.
    5. Prepare a table for information collection.
      Y’all will exist recording the pH, smell, consistency, and bacteria shapes and colors present in your milk samples.

    On each lab demote are several small-scale beakers.
    You will obtain a sample of the milk samples you need for your experimental blueprint and test the pH, colour, consistency, aroma and other characteristics of each sample.
    For each milk sample:

    1. Using the Vernier, take the pH of each flask. Tape your results.
    2. Record the color, odor (sour, putrid), and consistency (coagulation slight, moderate, chunky) for the milk in each flask.
    3. Perform Gram Staining as outlined below.
      Note: We will be using chemicals and open flames. Exercise caution when performing this portion of the lab.

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    • Microscope slide
    • Bunsen burner and tubing
    • Crystal violet (master stain)
    • Iodine solution/Gram’s Iodine (mordant that fixes crystal violet to cell wall)
    • Decolorizer (e.thousand. ethanol)
    • Safranin (secondary stain)
    • Water (preferably in a squirt bottle)


    1. Make a slide with your milk sample to exist stained. Heat ready the sample to the slide by carefully passing the slide with a drop or modest piece of sample on it through a Bunsen burner iii times.
    2. Add together the primary stain (crystal violet) to the sample/slide and permit sit for 15 seconds. Rinse slide with a gentle stream of water for a maximum of 5 seconds to remove unbound crystal violet.
    3. Add Gram’southward iodine for xv seconds – this is an agent that fixes the crystal violet to the bacterial cell wall.
    4. Rinse sample/slide with acetone or alcohol for ~three seconds and
      rinse with a gentle stream of water. The booze will decolorize the sample if it is Gram negative, removing the crystal violet. Yet,
      if the alcohol remains on the sample for as well long, information technology may also decolorize Gram positive cells.
    5. Add the secondary stain, safranin, to the slide and incubate for fifteen seconds.
      Wash with a gentle stream of water for a maximum of 5 seconds. If the bacteria is Gram positive, it volition retain the primary stain (crystal violet) and not have the secondary stain (safranin), causing it to look violet/royal under a microscope. If the leaner is Gram negative, information technology will lose the principal stain and take the secondary stain, causing information technology to announced ruby-red when viewed under a microscope.


    1. Describe the changing sequence of organisms and corresponding environmental changes during succession in the milk samples.
      Which bacteria are in each of your milk samples?
    2. Describe the irresolute sequence of organisms and corresponding environmental changes during succession in chocolate milk. Exercise the results of your investigation friction match your hypothesis?
    3. Compare succession in i or more than types milk. Propose reasons for differences.
    4. Advise another experiment to test the environmental factors and/or organisms changing in your proposed scenario for milk succession.
    5. How could you lot improve your test of the hypotheses?
      Be specific!
    6. Identify what happened to the pH of the milk as time passed.
    7. Infer what the modify in pH means nigh the populations of microorganisms in the milk.



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