Use the information on capture-recapture method to calculate the total number of organisms in a habitat. First…

Analyze the question: This is the capture-recapture (Lincoln-Petersen) method for estimating population size. Marked individuals are released and mixed back into the population. A second sample is then taken, and the proportion of marked organisms in it is used to estimate the whole population.

Concept:

Total population (N) = (First capture × Second capture) ÷ Number marked in second capture

This works because the fraction of marked organisms in the second sample should equal the fraction of marked organisms in the whole population:

• Marked in 2nd sample / Size of 2nd sample = Marked in 1st capture / Total population

Solution:

• First capture (marked) = 200
• Second capture = 120
• Marked in second capture = 40

40 / 120 = 200 / N

N = (200 × 120) / 40

N = 24,000 / 40

N = 600

Answer: The total number of organisms is 600, which is option C.

Tip: Remember the formula as "first × second ÷ marked". Check your answer for sense: 1/3 of the second sample (40 of 120) was marked, so the 200 marked organisms should be about 1/3 of the population, and 200 × 3 = 600. Always make sure the answer is larger than both the first and second captures.