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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

  1. What seems to have happened to the snake population during this three-week period?
    (According to these calculations, the population decreased by about 223 snakes)
  2. 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.)
  3. 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.)
  4. 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)

 

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