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Alien Invasion
Teaching Guide

As you saw in the segment "The Silence of the Birds," alien brown tree snakes have destroyed the fragile balance of life on the island of Guam. To best understand this invasion, biologists must estimate the number of snakes in various land sectors. A successful method of estimation involves capture and recapture ratios. To identify a captured snake, an ID tag is implanted on the animal. This device is then used to identify recaptured individuals, a procedure which also helps to illustrate the dynamics of the tagged population.

In this activity, you will use the same technique employed by Guam researchers to estimate the population size of a tagged species. You will then critically analyze your data using a mathematical model.


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This activity page will offer:

  • An introduction to estimating populations using capture/recapture statistics
  • An opportunity to calculate estimated population
  • An opportunity to critically analyze sampling techniques


  • Paper
  • Pencil

The Scenario
You are part of an international project that is studying the population of brown tree snakes in isolated sectors of a Pacific Island. The target animals are captured in traps without injury. A tag is then implanted into each animal to help identify the individual snake when released back in the wild.

Your responsibility is to uncover the total population of brown tree snakes in the capture and release sector. Since the tagging began, there have been 750 captured snakes in a 20,000 m2 sector. You have been a part of two tagging events that occurred three weeks apart.

Here is the data from each of those events:


Total # tagged in population = 750 snakes
# of snakes in this capture = 50 snakes
# of snakes that are recaptured during this event = 30


Total # tagged in population = 770 snakes
# of snakes in this capture = 40 snakes
# of snakes that are recaptured during this event = 30

Population Estimation
Use the following equation to estimate the total population of brown tree snakes on both dates.

Equation for Estimating Population:

Total population = (# tagged in population) x (# of captures)
                                       (# of recaptures)


  1. What seems to have happened to the snake population during this three-week period?
  2. Did the population really change as this calculation suggests?
  3. If the results of this calculation are not guaranteed, why bother using it?
  4. What factors in the real world might compromise the accuracy of this method?


Perimeter Trapping
The sampling techniques used by the Guam researchers uses perimeter trapping. Since the traps are set along the perimeter of a sensitive region (such as a bird nesting ground), the snakes must travel a greater distance to the trap. What are the advantages of this type of trapping as opposed to setting traps throughout the sector? What are its disadvantages?

Count the Boxes
An alternate method for estimating population involves random sampling of blocks within a larger grid matrix.

It represents a sector that is divided into 16 smaller blocks. Select any 4 random blocks. Count the number of "snakes" in each block. You will need to develop a set of counting rules that addresses snakes that span two or more boxes. To estimate the number of individuals in this entire grid, multiply the total number you counted by four. Discuss the advantages and disadvantages of this population estimation technique. Is this technique foolproof? Explain. What assumptions do you need to make when using this method?


Brown Tree Snake Trapping
An extensive overview of the impact of the brown tree snake on Guam.

Policy Statement on Invasive Alien Species
A US policy statement designed to impact the introduction of alien species.

Population Ecology
A rich-resource site on population biology hosted by Virginia Tech.

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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