Target Grade Level / Age Range:

Grades 9-12

Estimated Time:

50 minutes

Purpose:

Students will identify the invertebrates present in healthy soil, understand the role that each play, and begin to understand the interactions between each.

Materials:

Activity Option 1

  • 1 stereoscopic microscope with camera and monitor OR multiple microscopes – 1 per team (at least 10-15x)
  • Graduated slide(s) – with gradations at 1/10 th of 1 millimeter
  • Glass bottom dish(es)
  • Soil sample
  • Water
  • Graduated beaker
  • Micro pipette or Pasteur pipette

Activity Option 2

  • Soil sample
  • Scissors
  • Elmer’s glue
  • Duct tape
  • 2-liter soda bottle
  • Mesh laundry bag
  • 70% isopropyl alcohol
  • Microscope

Essential Files (maps, charts, pictures, or documents)

Vocabulary (with definitions)

  • Macrofauna – moles, worms, ants, pill bugs, etc.
  • Mesofauna – invertebrates between 0.1mm and 2mm in size, which live in the soil or in a leaf litter layer on the soil surface.
  • Microfauna – really tiny: nematodes, between 0.1mm and 0.005mm
  • Microorganisms – a microscopic organism, especially a bacterium, virus, or fungus.
  • Bioturbation – the disturbance of sedimentary deposits by living organisms.
  • Middens – heaps of biological material left behind by the earthworms

Background – Agricultural Connections (what would a teacher need to know to be able to teach this content)

Amoeba, ciliates, and flagellates belong to a group of organisms known as protozoa – single celled but with a nucleus. They reproduce asexually and sexually. They are typically 10 µm in diameter and live in soil pore spaces of that size or larger. Flagellates have one to four whip-like projections called flagella that aid in locomotion; ciliates have many small hair-like projections (these can number in the thousands) that help them move. Amoeba move by use of pseudopods – extensions of their flexible membrane pushed forward by internal body fluids. They therefore continuously change shape as they move about the soil environment. Amoeba require relatively moist soil to be active, and research suggests that amoeba spend much of their live as cysts since soil conditions are often too dry.

They are mainly predators, feeding on bacteria and some fungi, but they also process fine organic particles, either as free-living organisms or inside the gut of a host organism such as termites. Bacteria can protect themselves from some protozoan predation due to their ability to inhabit soil pores too small for amoeba to reach.

Vorticella are a genus of one-celled protozoa characterized by a ciliated, bell-shaped body and a slender stalk. They are characteristically found in wet soils. They sweep finer organisms and organic matter into their elementary feeding canal. They contract and expand as they feed and are especially sensitive to external vibrations.

Nematodes are typically 1mm in length and 0.05 mm in diameter. They reproduce sexually. Nematodes are mostly parasites (of plants and other soil microbes) and predators; their role in causing diseases of agricultural crops makes them a frequent pest for farmers, but they also play an important role in the decomposition process. Nematodes are large and therefore inhabit films of water surrounding large soil pores. They can form cysts to survive drought periods.

Rotifers are a small group of soil animals sometimes called wheel animalcules. Rotating ciliate give the appearance of a rotating wheel. They have a mastax between their mouth and stomach that mixes and grinds the food they eat. Twin propellers or wheel organs of the organism sweep food particles into its gullet by vortex feeding or swirling organisms into its mouth. It has a foot and is commonly attached to soil particles unless it is moving to a new location. Rotifers are often associated with polluted water.

Mite is a common name for an arthropod order that includes many species of animal and plant pests. But most live as free-living soil organisms. There are four common soil orders of mites depending on body parts and shape. They are normally less than 1mm in length. Mites are commonly carnivores that attack other soil organisms and are secondary decomposers helping break down organic debris such as leaves, twigs, and stems. Their colors can range from brown to red to white to clear. Some species of mites have very small hairs on their limbs that aid in attaching them to soil particles and other debris. The hairs also help detect vibrations in the soil which can aid in catching prey.

Springtail or Collembola is a group of common small animals in soil. They are an order of primitive insects with two main body types – those with thick bodies and indistinct body segments and those with thin bodies and distinct body segments. Some have a spring mechanism on the fourth or fifth abdominal segment that allows them to jump. The mechanism likely allows them to escape from predators. Many springtails lack color and graze on fungi, algae, or other debris. Most have antenna with three to six segments and are used for feeling their way around. Some antenna may also have smelling organelles attached. Organisms with longer antenna tend to be surface dwellers and those with shorter antenna are normally found deeper in the soil profile. Most springtails have very rudimentary eyes. Springtails actively move about in the soil. Springtails are a favorite food for beetles, millipedes, and centipedes.

Earthworms are segmented and are large enough to be easily seen with the human eye. They are active in decomposing organic debris and incorporating organic matter into the soil. Their channels are very important for gases and water movement through the soil profile.

Interest Approach – Engagement (what will you do to engage students at the beginning of the lesson)

Show the video “Bioturbation with and without soil fauna” https://youtu.be/Mxp1nnrUG0Q. Ask students questions about what they’ve seen. Could they identify the organisms that were captured in the video?

Procedures

  1. Present the content in the PowerPoint slide deck titled “Invertebrates in Soil.pptx.” Have students capture important ideas into their notebooks or journals.
    1. Macrofauna
      1. Moles, worms, ants, pill bugs, centipedes, millipedes, beetles
      2. There can be 10.6 million worms per acre of fertile farmland
      3. Roots grown more easily down worm tunnels
      4. Water can infiltrate down worm tunnels
    2. Macrofauna
      1. Role in food chain and ecosystem for earthworms
      2. Create macropores
      3. Redistribute organic matter within soil profile
      4. Make organic matter accessible to microbes
      5. Produce nutrient rich casts
      6. Improve aeration and moisture conditions
      7. Mineralize nutrient from organic matter
    3. Mesofauna
      1. Tiny, between 6 and 0.1mm
      2. Collembola (springtails), mites
      3. Break down decaying matter
      4. Eat fungi, algae, and dead plant matter
      5. Shred plant tissue
    4. Microfauna – protozoa
      1. Really tiny: between 0.1mm and 0.005mm think
      2. Protozoa – release nutrients for plants to use
      3. Protozoa can eat bacteria
      4. Protozoa: amoebae, ciliates, flagellates
      5. Eat bacteria and release nitrogen to plants
      6. Habitat characteristics for protozoa
        1. Aerobic only
        2. Need water
        3. Forest, compost, garden/field, prairie, aerated portions of marsh or wetland
      7. Role in food chain and ecosystem for protozoa
        1. Predators of bacteria and fungi
        2. Decomposers
    5. Microfauna – nematodes
      1. Really tiny: between 0.1mm and 0.005mm think
      2. Nematodes – can transmit soil-borne viruses
      3. Nematodes can eat 5,000 bacteria per minute
      4. Larger nematodes prey on smaller ones
      5. Habitat characteristics for nematodes
        1. Aerobic only
        2. Tolerant of acidic soil
        3. Forest, compost, garden/field, prairie, aerated portions of marsh or wetland
      6. Role in food chain and ecosystem for protozoa
        1. Parasites of plants and soil microbes
        2. Predators
        3. Important in predator/prey relationships and population management
      7. Fungal feeders, bacteria feeders, root feeders
      8. Most are non-pathogenic. Some are (soybeans)
      9. Nematicides and other pesticides can kill populations
  2. Facilitate questions throughout the content and check for student understanding. After students have taken notes, ask them to pick a color or marker (or two) that they feel represents each key element. They should then review their notes and highlight the key elements (color coding if appropriate). NOTE: not all of the information is key to understanding. Students should only select the most important information. However, the important information that they choose may be different for each student.
  3. Once students have highlighted or color coded their key elements, ask them to share those key ideas with a neighbor.
  4. Distribute copies of the worksheet Invertebrates Math.docx. As a full class, walk through the background information and the example. Instruct students to work as individuals or as pairs to complete the worksheet problems 1 & 2. Review their responses for questions 5 & 6.
  5. Activity Option 1:
    1. If you have enough supplies for students to work individually or in teams, they can do this as a lab exercise. If not, you can do this as a demonstration for the entire class to watch.
    2. Using a collected sample of soil, mix approximately 10 grams of soil into 100 milliliters of water. Stir the solution until all soil particles are suspended.
    3. Prepare a dish to use under the microscope. Place a graduated slide on the bottom of the dish.
    4. Draw up the water/soil solution into a micro pipette or Pasteur pipette and transfer the sample to the dish. Ensure that the sample is on the graduated slide.
    5. If doing this as a class demonstration, place the sample under a stereoscopic microscope with camera. If students are using microscopes in teams, place the dish under the microscope.
    6. Slowly increase magnification with the microscope.
    7. Have students try to identify and/or count the number of invertebrates (mites, nematodes, protozoa, etc.) present in the soil.
    8. If microscopes and/or other lab equipment is not available an alternative can be to show this video: Soil Biology (Microfauna) – Terry Tollefson https://youtu.be/VuHznslr8aI
  6. Activity Option 2:
    1. Watch the video. Soil Fauna: Extraction and Identification https://youtu.be/5XRoAZBdKaU
    2. Depending on lab equipment available, select one of the three methods described to repeat. Recommended extraction method is the Tullgren Funnel method.
    3. A simplified version of this can be seen here: https://youtu.be/KnoKvpqeMmA
    4. Cut the soda bottle in half, about four inches up from the bottom of the bottle.
    5. Place the top of the soda bottle upside down into the bottom of the soda bottle.
    6. Cut a circle of the mesh laundry bag about six inches in diameter.
    7. Fold the mesh circle in two places, creating a cone. Use the duct tape to secure it in a cone shape.
    8. Place the mesh cone into the top of the soda bottle so that the tip of the cone rests in the mouth of the soda bottle. Secure the mesh in place with some duct tape or with some glue.
    9. Pour the isopropyl alcohol into the bottom of the soda bottle until it is approximately

      1-inch deep. The mouth of the top of the soda bottle should rest in the alcohol, but not all the way to the bottom of the container.

    10. Gently put the soil on the mesh trying to minimize soil particles falling through to the bottom.
    11. Place the Tullgren funnel under a light bulb that emits some heat. The insects and arthropods (springtails and mites) will crawl away from the light and heat, through the mesh and fall into the alcohol.
    12. Transfer some of the alcohol solution to a glass dish and observe it under a microscope. Have students try to identify and/or count the number of invertebrates (specifically springtails and mites) present from the soil sample.

Did You Know? (Ag facts)

  • There can be 10.6 million worms per acre of fertile farmland.

Extension Activities (how students can carry this beyond the classroom)

  • Have students follow the procedure found in this video: Midden hunting https://youtu.be/205Zav4cjVc to conduct a field count of middens and earthworms in a local field, garden, or pasture.

Suggested Companion Resources (books and websites)

Sources/Credits

Author(s)

Will Fett

Organization Affiliation

Iowa Agriculture Literacy Foundation

Agriculture Literacy Outcomes

  • T1.9-12.b. Describe resource and conservation management practices used in agricultural systems (e.g., riparian management, rotational grazing, no-till farming, crop and variety selection, wildlife management, timber harvesting techniques).

Iowa Core Standards

  • HS-ESS2-2. Analyze geoscience data to make the claim that one change to Earth’s surface can create feedbacks that cause changes to other Earth systems.
  • HS-ESS3-2. Evaluate competing design solutions for developing, managing, and utilizing energy and mineral resources based on cost-benefit ratios.*

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