WAEC Biology 2011 Papers

Concept:The capture-recapture method uses the proportion of marked organisms in the second capture to estimate the total population size.Explanation:Let the total number of organisms in the habitat be $N$.The ratio of marked organisms in the first capture to the total population equals the ratio of marked organisms found in the second capture to the total number in the second capture.This gives the formula:$$\frac{\text{First capture}}{\text{Total population}} = \frac{\text{Marked in second capture}}{\text{Second capture}}$$Substituting the given values:$$\frac{200}{N} = \frac{40}{120}$$Cross-multiplying:$$N = \frac{200 \times 120}{40}$$$$N = \frac{24000}{40} = 600$$Answer:The total number of organisms in the habitat is $600$.Correct option: C. 600