Question

An ecologist catches 25 leopard frogs one night in a pothole marsh. These are marked with a color dye and released. Three days later a second trapping period is conducted – 10 leopard frogs are caught without a mark and 12 are caught that carry a mark.

a) Using the Lincoln-Petersen Index, what is the estimate of population size for the leopard frog population?

b) Suppose some of the frogs caught the first period do not disperse from the release site before the second period, and therefore are easier to capture than unmarked frogs. What effect will this have on the population estimate?

Suppose that the mark gradually washes off over time, so that some marks are gone in three days. What effect will this have on the population estimate?

Answer 1

Answer:

a) Given that

Frogs captured and marked = 25 (M)
Total frogs captured after 3 days = 22 (C)
Number of marked frogs recaptured = 12 (R)

Using the Lincoln-Petersen Index and estimation formula
Estimated number of individuals in the population (N)

N = (M x C) / R
N = (25 x 22) / 12
N = 45.83 or Approximately 46 leopard frogs.

b) In case the number of marked frogs do not disperse in the 3 days of time lapse, the chances of these being recaptured will increase. As we see from the formula that if more marked frogs are recaptured, the estimated count will go down. so much so that, if all 25 marked frogs were to be recaptured, it will indicate that that is the total population of frogs in the ecosystem. Hence we will underestimate the population.

C) Incase the ink used to mark frogs has washed off in the 3 days interval, the captured frogs may be marked but as ink is not there, they will get counted as unmarked. This will give bigger than actual estimate of the population. Because as the recaptured number of frogs reduces, the estimated number goes up,