Bean Bugs

OOne of the basic tasks of field biology is finding out how many organisms live in a study site. There are several ways of determining the size of a population of organisms. (A population is a group of organisms of the same kind that lives and reproduces in a particular area.) Some populations, such as a clump of elm trees on a lawn, are relatively small and can be censused by simply counting the trees. Other populations, however, such as the grass plants in a lawn or lady bugs in a meadow, are relatively large and their numbers must be estimated because counting them would take too long. The quadrat census technique suggested in this activity is appropriate for estimating large populations of organisms that move very slowly or do not move at all.

To estimate the size of a large population, you can count all the individuals in a small, measured area of the study site and then multiply that count by the number of small areas in the whole study site. This small area is called a quadrat. The best size for a quadrat depends on the size and abundance of the organism being counted. Averaging the counts from several randomly selected quadrats gives a better estimate of the number of organisms per quadrat, because populations are often not distributed uniformly throughout an area. Wire squares with a set size are used as the quadrat to address the challenge of estimating the number or individuals in a population too numerous to count.

Outdoor Biology Instructional Strategies (OBIS) is an outdoor program that offers young people fun and challenging opportunities to investigate ecological relationships in their local environment. OBIS was developed in the 1970s at the Lawrence Hall of Science to help adults (teachers, parents, community leaders) take young people outdoors to experience ecological principles in their local area. We have made several of the original 97 OBIS activities available digitally. These activities, which increase environmental awareness, can be used individually or in combination. These simple experiences then act as a gateway for children and adults to explore and understand their local environment.