Population size, density, & dispersal

What is a population?

In ecology, a population consists of all the organisms of a particular species living in a given area. 

Demography: describing populations and how they change

Ecologists are 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 size—NNN. 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

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.

1. 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.

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.

2. 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.

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:

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.
Clumped Dispersion—Are dispersions that the individuals aggregate in patches through out the surrounding areas. They may be influenced by behavior, mating, and resource availability. Ex. Wolves live in clusters so that hunting is easier for them.
Uniform Dispersion—They are dispersions that the individuals are evenly distributed. This could be influenced by social interactions such as territoriality. Ex. King penguins experience uniform dispersion when they are caring for their eggs making sure that they are not too close to their neighbors.
Random Dispersion—It is the type of dispersion in which the position of each individual is independent to the other individuals. Ex. Dandelions in a field, this is random because of the may its seeds are distributed (carried by the wind; it varies).