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OBJECTIVE
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
MATERIALS
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:
9/01/01
Total
# tagged in population = 750 snakes
# of snakes in this capture = 50 snakes
# of snakes that are recaptured during this event = 30
9/22/01
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)
QUESTIONS
- What seems to have happened to the snake population during
this three-week period?
- Did the population really change as this calculation suggests?
- If the results of this calculation are not guaranteed,
why bother using it?
- What factors in the real world might compromise the accuracy
of this method?
EXTENSIONS
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?
WEB
CONNECTION
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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