Ecologists estimate population size by sampling.

Imagine trying to calculate the size of a population of Lime Swallowtails by counting every individual in the Dominican Republic. The task is hopeless. Like that of the Lime Swallowtail, the populations of most species are either too large or too widely dispersed for every individual to be counted. Instead, ecologists estimate population size by taking repeated samples of a population and from these samples estimating the total number of individuals. With an estimate of population size and range, ecologists can then determine population density.

For sessile, or sedentary, organisms like milkweeds in a field or barnacles on a rocky shoreline, ecologists commonly lay a rope across the area of interest and count every individual that touches it, or they throw a hoop and count the individuals inside it. Each such count gives one sample. They then take a number of samples and from them estimate the number and distribution of individuals in the population. For example, suppose 1-m2 hoops are placed in three different spots in a field, encircling respectively 4, 5, and 3 individual milkweeds. We can multiply the average number of individuals (in this case, 4) per 1-m2 sample by the total square meters in the area of interest (in this case, the size of the entire field) to give an estimate of population size.

1007

It is relatively easy to count sedentary organisms along a straight line or within a grid area, but many species move around. How do ecologists estimate population size for turtles in a pond, or mice in a forest? In these cases, a method called mark-and-recapture is useful. Biologists capture individuals, mark them in a way that doesn’t affect their function or behavior, release them back into the wild, and then capture a second set of individuals, some of which were previously marked. Fig. 46.3 illustrates how the mark-and-recapture method can be used to estimate the number of butterflies in an area. If a large number of butterflies are caught, marked, released, and then recaptured, it is possible to make an estimate of the total population size by extrapolating from the proportion of the second sample that is marked.

HOW DO WE KNOW?

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.

image
FIG. 46.3

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.

These methods are simple, and they make a number of assumptions about the population. For example, we assume that the population has not changed in size between our first and second samples. Nonetheless, mark-and-recapture methods have proved effective in estimating the sizes of populations in nature. More sophisticated analyses are available for more complicated situations, as when more than two samples are caught, or when immigration or emigration occurs during the time between samples.