Population Density Activity
By Crystal Jeter
In an ecosystem, energy flow, resource utilization, survivorship and mortality curves, and productivity are all affected by population density. A species’ population density in an environment is most often calculated in terms of crude density—simply the number of individuals per area (or volume, in the case of some aquatic or marine species), with the assumption that the organisms are evenly spread through the habitat. Ecological density, a more precise but more difficult measure, takes into account unequal use of an area by a population (e.g., a large, forested region may include some open areas not likely to be populated by a strictly forest-dwelling species, such as ovenbirds). Usually, population ecologists find the crude density measure to be sufficient for their purposes.
When a scientist counts a species’ population in a discrete portion of a habitat, the density figure can be applied to the entire habitat to get a reasonable estimate of the total population. For example, if 3 separate square-meter samples of a fairly uniform hectare of grassland turn up population counts of 5, 6, and 7 mole crickets (average of 6 per square meter), then it is likely that the entire hectare contains approximately 60,000 mole crickets. The use of sampling squares, or quadrats, is a common method of estimating population density of species that are either sessile or small and slow-moving. If taking students outdoors to try this technique is not practical for you, expose your students to the technique in the classroom. Here’s how.
National Science Education Standards
Science as Inquiry
- Abilities Necessary to Do Scientific Inquiry
- Understandings about Scientific Inquiry
- Interdependence of Organisms
Science in Personal and Social Perspectives
- Population Growth
Preparation and procedure
- Decide what environment you would like students to sample for their study and what in that environment you want them to count. Have students measure their study sites. The following are some suggestions of environments that may be useful.
- Use a printed fabric as an environment, and elements of its pattern as individual organisms to count. The pattern may be conspicuously symmetrical or seemingly random. A pattern of small repeats may be most practical if students are working individually or in small groups; fabric samples can be more easily prepared and distributed. Decide what feature(s) of the pattern represent species that students will be counting. For example, if you are using a pattern with different-colored dots, you might have students count 1 specific color. These fabric habitats can be easily reused from year to year.
- Spread paper dots made by a hole punch across a sheet of paper. Different-colored dots in different proportions can represent different types of organisms in the same habitat. These species can be distributed across the paper randomly, in clumped patterns, or evenly, to represent various natural situations.
- Spread beads, buttons, candy, or beans across a wide tray. These items work well, as they can also be used to model other population study techniques, such as mark-and-recapture.
- Have students make quadrats to sample their habitats:
- Fold a chenille stem in half and twist the ends together.
- Fold it in half again to crease corners for making a square.
- Bend it into a square; the twisted ends serve as a handle.
- Measure the internal area of your quadrat.
- Instruct students to randomly place their quadrats in the environment and count the number of items of interest inside.
- Encourage students to sample multiple times and calculate the average population density (by taking the average number of items of interest and dividing by the area of the quadrat).
- Discuss with students the ramifications of sample size and number of samples, in terms of accuracy and reliability of the data. Also discuss the limiting factors (cost, time, practicality, etc.) of taking large samples and multiple samples in the real world. Discuss also the assumptions involved in the accuracy of extrapolating the measured density count to get a total population count.
Note: You might have students make and test different sizes of quadrats in their environments to see how sampling size may affect the results of the experiment.
A table such as the following helps students organize data during these activities.
|Dimensions of Quadrat|
|Area of Quadrat|
|Dimensions of Study Site|
|Area of Study Site|
|Number of Individuals of the Target Species||Sample 1:|
|Population Density of Target Species|