ANSWERS:
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)
Calculations
for 9/01/01:
Total
population = (#
tagged in population) x (# of captures)
(# of recaptures)
Total
population = (750)
x ( 50)
(30)
Total
population = 1250 snakes
Calculations
for 9/22/01:
Total
population = (#
tagged in population) x (# of captures)
(# of recaptures)
Total
population = (770)
x (40)
(30)
Total population = 1027 snakes
QUESTIONS
 What seems to have happened to the snake population during
this threeweek period?
(According to these calculations, the population decreased
by about 223 snakes)
 Did the population really change as this calculation suggests?
(No one knows for certain. The only absolute way of finding
out is to count each snake in the sector.)
 If the results of this calculation are not guaranteed,
why bother using it?
(It is impractical to do an actual "headcount" of the
snakes. Although this may not be 100% accurate, it is a
good approximation of the snake population.)
 What factors in the real world might compromise the accuracy
of this method?
(All sorts of factors such as animals learning to avoid
traps, clustered distribution patterns, workers accidentally
releasing captures, traps placed in regions that do not
fairly sample the population, etc.)
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?
(Since you set the traps at the perimeter, you don't have
to disturb the sensitive environment within the sector. It
may be easier to check out traps that are placed along the
sides of a square perimeter than following curved trails within
the sector. The disadvantage is that it may take a while for
the snake to travel to the trap. If the snakes don't move
great distances, you may not collect a fair sample upon which
to calculate the population.)
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?
(Although answers will depend upon which boxes were selected
and exact counting method, the answer should be about 120)
