Population size, density, & dispersal

What an ecological population is. How scientists define and measure population size, density, and distribution in space.

Key points

  • A population consists of all the organisms of a given species that live in a particular area.
  • The statistical study of populations and how they change over time is called demography.
  • Two important measures of a population are population size, the number of individuals, and population density, the number of individuals per unit area or volume.
  • Ecologists estimate the size and density of populations using quadrats and the mark-recapture method.
  • The organisms in a population may be distributed in a uniform, random, or clumped pattern. Uniform means that the population is evenly spaced, random indicates random spacing, and clumped means that the population is distributed in clusters.

What is a population?

In everyday life, we often think about population as the number of people who live in a particular place—New York City has a population of 8.6 million.1^1 Or Monowi, Nebraska has a population of one. Just think—you could double the population of Monowi if you felt like moving there!
In ecology, a population consists of all the organisms of a particular species living in a given area. For instance, we could say that a population of humans lives in New York City, and that another population of humans lives in Gross. We can describe these populations by their size—what we often mean by population when we're talking about towns and cities—as well as by their density—how many people per unit area—and distribution—how clumped or spread out the people are.

Demography: describing populations and how they change

In many cases, ecologists aren't studying people in towns and cities. Instead, they're studying various kinds of plant, animal, fungal, and even bacterial populations. The statistical study of any population, human or otherwise, is known as demography.
Why is demography important? Populations can change in their numbers and structure—for example age and sex distribution—for various reasons. These changes can affect how the population interacts with its physical environment and with other species.
By tracking populations over time, ecologists can see how these populations have changed and may be able to predict how they're likely to change in the future. Monitoring the size and structure of populations can also help ecologists manage populations—for example, by showing whether conservation efforts are helping an endangered species increase in numbers.
In this article, we'll begin our journey through demographics by looking at the concepts of population size, density, and distribution. We'll also explore some methods ecologists use to determine these values for populations in nature.

Population size and density

To study the demographics of a population, we'll want to start off with a few baseline measures. One is simply the number of individuals in the population, or population sizeNN. Another is the population density, the number of individuals per area or volume of habitat.
Size and density are both important in describing the current status of the population and, potentially, for making predictions about how it could change in the future:
  • Larger populations may be more stable than smaller populations because they’re likely to have greater genetic variability and thus more potential to adapt to changes in the environment through natural selection.
  • A member of a low-density population—where organisms are sparsely spread out—might have more trouble finding a mate to reproduce with than an individual in a high-density population.

Measuring population size

To find the size of a population, can’t we just count all the organisms in it? Ideally, yes! But in many real-life cases, this isn’t possible. For instance, would you want to try and count every single grass plant in your lawn? Or every salmon in, say, Lake Ontario, which is 393 cubic miles in volume?1^1 Counting all the organisms in a population may be too expensive in terms of time and money, or it may simply not be possible.
For these reasons, scientists often estimate a population's size by taking one or more samples from the population and using these samples to make inferences about the population as a whole. A variety of methods can be used to sample populations to determine their size and density. Here, we’ll look at two of the most important: the quadrat and mark-recapture methods.

Quadrat method

For immobile organisms such as plants—or for very small and slow-moving organisms—plots called quadrats may be used to determine population size and density. Each quadrat marks off an area of the same size—typically, a square area—within the habitat. A quadrat can be made by staking out an area with sticks and string or by using a wood, plastic, or metal square placed on the ground, as shown in the picture below.
Image credit: modified from Population demography: Figure 2 by OpenStax College, Biology, CC BY 4.0; original image credit: NPS Sonoran Desert Network
After setting up quadrats, researchers count the number of individuals within the boundaries of each one. Multiple quadrat samples are performed throughout the habitat at several random locations, which ensures that the numbers recorded are representative for the habitat overall. In the end, the data can be used to estimate the population size and population density within the entire habitat.

Mark-recapture method

For organisms that move around, such as mammals, birds, or fish, a technique called the mark-recapture method is often used to determine population size. This method involves capturing a sample of animals and marking them in some way—for instance, using tags, bands, paint, or other body markings, as shown below. Then, the marked animals are released back into the environment and allowed to mix with the rest of the population.
Image credit: Population demography: Figure 3 by OpenStax College, Biology, CC BY 4.0; originals: left, modification of work by Neal Herbert, NPS; middle, modification of work by Pacific Southwest Region USFWS; right, modification of work by Ingrid Taylar
Later, a new sample is collected. This new sample will include some individuals that are marked—recaptures—and some individuals that are unmarked. Using the ratio of marked to unmarked individuals, scientists can estimate how many individuals are in the total population.

Example: using the mark-recapture method

Let’s say we want to find the size of a deer population. Suppose that we capture 80 deer, tag them, and release them back into the forest. After some time has passed—allowing the marked deer to thoroughly mix with the rest of the population—we come back and capture another 100 deer. Out of these deer, we find that 20 are already marked.
If 20 out of 100 deer are marked, this would suggest that marked deer—which we know are 80 in number—make up 20% of the population. Using this information, we can formulate the following relationship:
number marked first catch (M)total population (N)\dfrac{\text{number marked first catch } (M)}{\text{total population } (N)}= = number marked second catch (x)total number of second catch (n)\dfrac{\text{number marked second catch } (x)}{\text{total number of second catch } (n)}
MN\quad\quad\quad \:\:\quad\quad\quad\quad\quad\quad\quad\quad\quad \dfrac{M}{N} == xn\dfrac{x}{n}
Next, we rearrange the equation:
N\quad\quad\quad \quad\quad\quad\quad\quad\quad\quad\quad\quad\:\:\: N == nMx\dfrac{nM}{x}
And finally, we plug in the values from the deer example:
NN == (100 total second catch)(80 marked first catch)(20 marked second catch)\dfrac{(100 \text{ total second catch})(80\text{ marked first catch})}{(20\text{ marked second catch})} == 400 deer400 \text{ deer}
This approach isn’t always perfect. Some animals from the first catch may learn to avoid capture in the second round, inflating population estimates. Alternatively, the same animals may preferentially be retrapped—especially if a food reward is offered—resulting in an underestimate of population size. Also, some species may be harmed by the marking technique, reducing their survival. The approach also assumes that animals don’t die, get born, leave, or enter the population during the period of the study.
Alternative approaches to determine population size include electronic tracking of animals tagged with radio transmitters and use of data from commercial fishing and trapping operations.

Species distribution

Often, in addition to knowing the number and density of individuals in an area, ecologists will also want to know their distribution. Species dispersion patterns—or distribution patterns—refer to how the individuals in a population are distributed in space at a given time.
The individual organisms that make up a population can be more or less equally spaced, dispersed randomly with no predictable pattern, or clustered in groups. These are known as uniform, random, and clumped dispersion patterns, respectively.
Image modified from Population distribution by Yerpo, CC BY-SA 4.0; the modified image is licensed under a CC BY-SA 4.0 license
  • Uniform dispersion. In uniform dispersion, individuals of a population are spaced more or less evenly. One example of uniform dispersion comes from plants that secrete toxins to inhibit growth of nearby individuals—a phenomenon called allelopathy. We can also find uniform dispersion in animal species where individuals stake out and defend territories.
  • Random dispersion. In random dispersion, individuals are distributed randomly, without a predictable pattern. An example of random dispersion comes from dandelions and other plants that have wind-dispersed seeds. The seeds spread widely and sprout where they happen to fall, as long as the environment is favorable—has enough soil, water, nutrients, and light.
  • Clumped dispersion. In a clumped dispersion, individuals are clustered in groups. A clumped dispersion may be seen in plants that drop their seeds straight to the ground—such as oak trees—or animals that live in groups—schools of fish or herds of elephants. Clumped dispersions also happen in habitats that are patchy, with only some patches suitable to live in.
As you can see from these examples, dispersion of individuals in a population provides more information about how they interact with each other—and with their environment—than a simple density measurement.

Summary

In ecology, a population consists of all the organisms of a given species that live in a particular area. The statistical study of populations and how they change over time is called demography.
Two important measures of a population are population size, the number of individuals, and population density, the number of individuals per unit area or volume. Ecologists often estimate the size and density of populations using quadrats and the mark-recapture method.
A population can also be described in terms of the distribution, or dispersion, of the individuals that make it up. Individuals may be distributed in a uniform, random, or clumped pattern. Uniform means that the population is evenly spaced, random indicates random spacing, and clumped means that the population is distributed in clusters.

Attribution

This article is a modified derivative of "Population demography" by OpenStax College, Biology, CC BY 4.0.
The modified article is licensed under a CC BY-NC-SA 4.0 license.

Works cited

  1. "New York City," Wikipedia, May 22, 2016, accessed May 25, 2016, https://en.wikipedia.org/wiki/New_York_City.
  2. B. Kell, Population sign, Gross, Nebraska, USA. Wikimedia Commons, March 12, 2007, accessed May 25, 2016, https://commons.wikimedia.org/wiki/File:Population_sign,_Gross,_Nebraska,_USA.jpg.
  3. "Lake Ontario," Wikipedia, May 25, 2016, accessed May 25, 2016, https://en.wikipedia.org/wiki/Lake_Ontario.

Additional references

"Demography." Wikipedia. May 23, 2016. Accessed May 25, 2016. https://en.wikipedia.org/wiki/Demography.
"Mark and recapture." Wikipedia. May 22, 2016. Accessed May 25, 2016. https://en.wikipedia.org/wiki/Mark_and_recapture.
Purves, W. K., D. Sadava, G. H. Orians, and H. C. Heller. "Populations in space and time." In Life: The science of biology, 1038-1040. 7th ed. Sunderland, MA: Sinauer Associates, Inc, 2003.
Raven, P. H., G. B. Johnson, K. A. Mason, J. B. Losos, and S. R. Singer. "Population demography and dynamics." In Biology, 1168-1171. 10th ed., AP ed. New York, NY: McGraw-Hill, 2014.
Ricklefs, R. E. "The estimation of population size is critical to the study of population dynamics." In Ecology, 287-289. 3rd ed. New York, NY: W. H. Freeman, 1990.
Wilkin, D. and B. Akre. "Patterns of populations." CK-12 biology advanced concepts. March 23, 2016. http://www.ck12.org/book/CK-12-Biology-Advanced-Concepts/section/18.26/.
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