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Alien Invasion
Teaching Guide

As you learned in segments like "The Silken Tree Eaters," alien invaders are not beings who have space-hopped down to Roswell, New Mexico. They are animal and plant species that are transported to regions outside of their natural habitat. Within a new environment, these invaders may be free of the natural controls like predators and disease that had previously regulated their population numbers. Without this control, the aliens can rapidly overpopulate an area, destroying the region's established ecological balance.

In this activity, you'll observe an actual problem that is faced by tropical fish enthusiasts. When new fish or plants are introduced into a stable and maintained fish tank, unwanted organisms may inadvertently be transferred along with the new arrivals. Here's an activity that demonstrates both the transfer process and the problem faced by owners of tropical aquariums.


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This activity page will offer:

  • An operational definition in alien invasion
  • Hands-on activity in the inadvertent transfer of organisms
  • An opportunity for microscopic observation


  • Two large test tubes, labeled "A" and "B"
  • Dropper pipette
  • Distilled water
  • Sprig of Elodea (also called Anacharis)*
  • Forceps
  • Microscope
  • Slides
  • Coverslips
  • Sterile Cotton Ball

    Teacher Note:
    Prior to this activity, sprigs of Elodea should be maintained in a standard, non-sterile aquarium tank that has sufficient aeration to support a rich aquatic community. A tank overgrown with algae is ideal. You can also obtain these or similar plants at most tropical fish stores.


  1. Use a clean pipette to transfer a drop of distilled water onto a glass slide.
  2. Gently position a coverslip over the drop.
  3. Examine this drop with a microscope using both low and high power magnifications. Record the presence of any observed organisms. If applicable, use classroom resources to identify these organisms. Include a sketch of what you see.
  4. Fill both sterile test tubes "A" and "B" two-thirds full with distilled water. Make sure not to contaminate either of these tubes with unwanted organisms.
  5. Use forceps to transfer a sprig of Elodea to test tube "A".
  6. Set both test tubes aside. Both tubes should be exposed to sufficient light to meet the photosynthetic needs of the plant.
  7. Plug the mouth of each tube with a sterile cotton ball.
  8. Each day examine the appearance of the tube walls, water, and plant surface. Record the appearance of any additional life forms.
  9. Use a sterile pipette to remove several drops of water from each tube. Examine these droplets using a microscope. Note the appearance of any microorganisms.
  10. After recording your observations for one week, compare and contrast the appearance of the water in both tubes "A" and "B".


  1. Why did this activity require two test tubes, if only one was needed for the Elodea sprig?
  2. What happened to tube "A", into which the Elodea sprig was introduced?
  3. What happened to tube "B"?
  4. What types of organisms were transferred along with the Elodea sprig?


On Your Own
Contact a local pet store that maintains tropical fish. Discuss the different types of species that are common invaders to local fish tanks. Identify the preferred means of dealing with each of these unwanted organisms. What types of techniques are biological controls? Which ones are chemical controls?

Sterile Transfer
Suppose you wanted to introduce Elodea, and only Elodea, into a new environment. How could you improve the transfer technique so that few if any unwanted organisms were imported into the new environment?

Art Extension
Imagine if you were the size of a tiny protozoan species. How might the surface of an aquarium plant appear to you? Think about the nooks, crannies and other types of microorganisms you'd uncover. Use classroom resources to investigate the flora and fauna common to such environments. Working in a team, create a classroom mural that depicts this microscopic universe.


Noxious, Invasive, and Alien Plant Species
Government site that addresses alien species infringing on wetlands.

Anacharis-Elodea densa
Description and great image of Elodea sprigs.

Basics of Fish Care
Basic fish care with a section on quarantine of newly acquired individuals from the Minnesota Branch of the American Association for laboratory Animal Science Inc. website.

For more Web links on this topic - visit our Resources Section.


The activities in this guide were contributed by Michael DiSpezio, a Massachusetts-based science writer and author of "Critical Thinking Puzzles" and "Awesome Experiments in Light & Sound" (Sterling Publishing Co., NY).

Academic Advisors for this Guide:

Corrine Lowen, Science Department, Wayland Public Schools, Wayland, MA
Suzanne Panico, Science Teacher Mentor, Cambridge Public Schools, Cambridge, MA
Anne E. Jones, Science Department, Wayland Middle School, Wayland, MA

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