Ecologists use a variety of approaches to estimate population size and extent

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Accurately measuring the size and extent of a population is harder than you might think. One challenge is determining the boundary of the population. If the area is self-contained and small enough, you might be able to simply count all the individuals of a particular species to get the population size. Biologists performed this type of count, called a full census, on the African elephant population of Samburu and Buffalo Springs National Reserves in Kenya. By monitoring the elephants for 21 months, the biologists learned to recognize each of the 760 individuals in the population, primarily by their unique and distinctive ear markings. But populations are often too large and too mobile for a full census; often the extent of the range of a population is not perfectly known. In these cases, ecologists instead determine the population density, or the number of individuals within a given area (or volume, for organisms living in water), and then extrapolate from these samples to estimate the total population size.

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As you might imagine, estimating population densities is easiest for sessile organisms. Investigators need only count the individuals in a sample of representative locations and extrapolate the counts to the entire geographic range of the population. Individuals may be counted within a measured area called a quadrat or along a transect—a line drawn across an area within the range of the population (often designated by a tape measure marked at regular intervals). By making repeated counts with either of these methods, investigators can use these surveys to make reasonably good estimates of the size of a population.

Counting mobile organisms is more difficult because individuals move into and out of sampling areas, as you saw in the humpback whale example. In such cases, investigators may use the mark–recapture method (Figure 54.5). They begin by capturing, marking, and then releasing a number of individuals. Later, after the marked individuals have had time to mix with unmarked individuals in the population (but before enough time has elapsed for births, deaths, and individual movement to affect the population size significantly), another sample of individuals is captured. This sample is then used to obtain an estimate of the total size of the population in the sampling area. The extrapolation is achieved by applying the equation described in Figure 54.5A, which assumes that the proportion of marked individuals in the second sample (i.e., individuals that were captured and marked in the first sample) is about the same as the proportion of individuals in the sampling area that were captured in the first sample.

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Figure 54.5A The Mark–Recapture Method The method described here is used to estimate animal population sizes for highly mobile species (such as Ixodes scapularis, the black-legged tick). Once a sampling area has been determined, investigators capture, mark, and then release the animals back into the population. The proportion of marked individuals recaptured in a second sample is assumed to be the same as the proportion of the total individuals in that sample to the population size within the area.

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Animation 54.1 The Mark—Recapture Method

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Figure 54.5B work with the data follows below.

work with the data

Figure 54.5B Monitoring Tick Populations

Original Paper: Falco, R. C. and O. Fish. 1988. Prevalence of Ixodes dammini near the homes of Lyme disease patients in Westchester County, New York. American Journal of Epidemiology 127: 826–830.

Lyme disease is a chronic and debilitating condition caused by spirochete bacteria of the genus Borrelia, which infect humans by way of the bite of an intermediate host, the black-legged tick (Ixodes scapularis), also known as the deer tick. (In 1993 I. dammini and I. scapularis were found to be one species.) The incidence of Lyme disease has increased dramatically in the past 20 years, particularly in the northeastern United States. In order to assess the risk of exposure to this disease in Westchester County, New York, investigators measured the abundance of deer ticks in suburban lawns near wooded areas using the mark–recapture method described in Figure 54.5A. (Ticks are typically collected by dragging a white cloth along the ground; the ticks latch onto the cloth in much the same way they would to a passing leg.) By drag-sampling one representative lawn, the researchers collected the data shown in the table.

Original
capture
event		 Second
capture event
(3 weeks later)
Adult ticks captured 180 33
No. of marked ticks 180a 8

aAll ticks captured in the first event were marked with acrylic paint and released.

QUESTIONS

Question 1

Refer to Figure 54.5A. Using the equation and other information described in that figure, estimate the total number of adult ticks in the sampled lawn from the data table above.

The equation in Figure 54.5A states that N = (n1 × n2)/M. In words, the estimated total number of ticks (population size N) equals the total number of individuals captured, marked, and released in the first sample (n1 = 180) times the total number of individuals captured in the second sample (n2 = 33) divided by the number of marked individuals recaptured in the second sample (M = 8). Thus N (estimated population of the sampled lawn) = (180 × 33)/8 = 5,940/8 = 742.5 adult ticks.

Question 2

The lawn was approximately 700 m2 in size. What is the approximate density of ticks per square meter?

Calculate this by dividing 742.5 (the estimated number of ticks in the lawn population, per Question 1) by 700 (number of m2) = approximately 1.06 ticks per square meter.

Question 3

What do you think might be the implications of this study for residents of this neighborhood?

This study was conducted in order to evaluate the risk that residents of this suburban neighborhood have of encountering the tick vectors of Lyme disease in their own yards; a high risk of encounter means that there is also likely to be a high risk of contracting the disease. A density of slightly more than one tick per square meter of lawn suggests that residents of this community are indeed likely to encounter ticks and, accordingly, have a high probability of contracting Lyme disease if they spend time outdoors on their lawns.

A similar work with the data exercise may be assigned in LaunchPad.

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More recently, ecologists have used DNA analysis to determine how many unique individuals might be contained within a population. In some cases, instead of capturing individuals and collecting DNA, ecologists collect the next best thing—tissues, fur, or feces left behind by organisms in their natural habitat to be analyzed for DNA signatures. In a novel twist on sample collection, some ecologists have even trained dogs to locate feces of some hard to find species such as orca whales. With enough samples, the extent and size of populations can then be determined for species that are hard to survey in the wild.