Describing Populations
Applying Descriptions of Populations
In cases where the populationglossary term (opens in a new window) size is fairly limited, such as on an island, population size can be measured directly by counting the number of individuals. For larger populations, population size must be estimated using sampling techniques. Sampling techniques vary based on the organism and location being studied.
One common sampling technique for measuring plant populations is to use a quadrat and extrapolate to the rest of the population. A sampling of at least 1% of the total area is often sufficient to make accurate predictions. In quadrat sampling, small sections of land are selected, and the plants within that area are either counted or the percent cover calculated. The percent cover is the measure of how much space a plant occupies. It can be estimated by measuring the size of the plant’s shadow. Percent cover is an important demographic because larger, older plants have different roles in the population than do smaller saplings.
The size of the quadrat depends on the type of plant being studied. Larger plants require larger quadrats for an accurate calculation. The perimeter of the quadrats adds sources of error because counting plants on the borders can be subjective. Minimizing perimeter is one consideration, but the lay of the land is another important consideration. Plants often tend to grow in clusters around water or nutrient sources. The quadrat arrangement must take into account rivers, slopes, and other factors. One common strategy to account for these gradations in area is to use transects rather than quadrats. A transect is a long line rather than a circle or rectangle. A long rope can be laid out, and plants that fall within the range of the rope are enumerated.
One common sampling technique for measuring animal populations is the capture-recapture method. A set number of animals is captured, tagged, and released. After some time, another set of animals from the same population is captured. A certain percent of these will have tags from the previous capture. This count can be used to determine the total size of the population.
Example:
Population ecologist is interested in determining the size of a squirrel population. In the first trial, she captures and tags 100 squirrels. These squirrels are released back into the general population. After a month, the ecologist comes back to the same area. A month should be sufficient time for the tagged squirrels to reintegrate into the overall population. In this second trial, she captures 100 more squirrels; 20 of these were tagged from before.
The second trial is a random sampling of the population. In this random sampling, 20% of the squirrels were tagged. We can extrapolate that 20% of the entire population is tagged. From the first trial, we know that 100 squirrels are tagged. Therefore, 100 is 20% of the total population size; the total population is 500 squirrels.
These calculations are based on the following equation:
Using the data from the study above:
Solving for x:
The equation for calculating total population size using this technique can be rewritten as:
STEM and Describing Populations
Tagging animals can help ecologists measure population densityglossary term (opens in a new window). Ecologists can also collect data on these animals while tagging them. Often the age, health, and other characteristics of captured animals are recorded. Returning to the same population to tag and study animals over longer periods of time can provide valuable information about the changes in the population over time.
Electronic tracking devices can provide even more information on population demographicsglossary term (opens in a new window). Tracking devices are able to track an animal’s movements using a global positioning system, sometimes called a GPS. The device records interactions between tagged animals using proximity sensors and sends the data to researchers’ computers via satellites. Some newer devices are solar powered to increase battery life. Tracking devices may be attached to the animals as collars, as leg wraps, or as shell attachments. New technology for these devices is focused on making them smaller and less intrusive.
Teacher Note
This formative activity requires students to interpret data and relate it to the topics being learned. Students could work independently and then share and discuss their answers as a group.
