Also commonly called a "plot," a quadrat sample is a biological sampling technique used to estimate population density and study species diversity. A set of consistently sized frames are randomly distributed throughout a study area. All of the study plants or animals that fall within the quadrat are then counted, compared with the other quadrats and generalized to describe the nature of the study area. Quadrat samples are used to study plant life and slow-moving animals such as nesting birds, mussels and certain insects.
Single-Species Quadrat Samples
In a single-species study, quadrat samples are used to gather detailed information about the population density, health, use of resources and distribution within the study area. For example, quadrats are frequently used in long-term monitoring programs to assess the health and well-being of species recovering from polluted ecosystems, such as the Mississippi freshwater mussel. Because monitoring large areas is impractical, representative sample quadrats are designated for long-term observation, and the data gathered is generalized to apply to the whole region.
Multiple-Species Quadrat Samples
Quadrats can also be used to determine the distribution of various species within an area. Plots in which multiple species are counted provide general information on biodiversity; for example, the diversity of insects in a population is often measured via quadrat sampling. Quadrat frames are randomly placed, the insects within each are collected and the number of each species is counted. Researchers then compare the relative distribution of insects within each quadrat and between quadrats in the sample area to determine the general representation of insects in the region being studied. Additional information about health and behavior can be gained by further analysis of the quadrat sample data.
Quadrat samples provide information about how plants distribute themselves in a landscape. For example, several specimens of a plant species might be found clustered in one quadrat while they are sparsely distributed in others; this suggests that there might be important differences between the quadrat areas that are worth investigating.
Quadrat samples are likewise useful for studying nesting behavior, distribution of sedentary species (such as muscles and oysters) and slow-moving species diversity for animals such as insects. Quadrat samples can either represent in-place studies, in which species such as oysters are counted within the quadrat area but not disturbed, or collected samples in which all of the animals within the quadrat are removed for study in a lab. The second method is most commonly used with moderately fast animals such as insects, or for in-depth studies where lab work is preferable to field work, such as a controlled study on changes in diet.