FIG. 46.3

How many butterflies are there in a given population?

BACKGROUND Individuals in many populations of butterflies (and other animals) most often live and die within the patch of habitat where they were born. Population sizes of such animals can be estimated by a technique called mark-and-recapture, in which organisms are captured and released on two successive days. We assume that the population size is about the same on the second day as the first day. We also assume that the captured and released animals mixed with the population and do not avoid recapture on the second day.

METHOD Butterflies, such as the Monarch, are captured in an area, marked with an identification sticker on the wing as shown here, and then released. The number of marked butterflies is recorded. The next day, butterflies are caught and the number of marked and unmarked butterflies is recorded. Butterflies with marks are the recaptures.

To find the population size (N), we take the total number of marked butterflies and unmarked butterflies caught on the second day (C) and divide by the number of recaptures (R). We multiply this number by the number of butterflies marked on the first day (M), as follows:

N = (C/R) × M

RESULTS Let’s say we capture 100 butterflies on the first day, mark them, and release them. On the second day, we capture 120 butterflies, 30 of which are marked and the rest unmarked. We conclude that the population size is (120/30) × 100, or 400 butterflies total in the population sampled.

RELATED METHODS Mark-and-recapture techniques have proved extremely useful in estimating population sizes of organisms from polar bears to disease victims. There are refinements to the mark-and-recapture approach that take into account the movement of individuals between populations. These methods involve taking multiple samples and adding variables to the equation. Many animals have distinctive color markings, such as the blotches on humpback whale tailfins and frog skin-spotting, and so can be recorded without having to be marked.

SOURCE Lincoln, F. C. 1930. “Calculating Waterfowl Abundance on the Basis of Banding Returns.” Cir. U.S. Department of Agriculture 118:1–4.