A number of bird species are believed to be sensitive to the density of shrubs, so this characteristic is frequently measured by people monitoring or mapping potential habitat for these species. Density is the number of things (such as shrubs) per unit area. It is important to be able to distinguish the individual things being counted, so it is more usual to measure the density of easily identified individuals such as trees or shrubs, or animals, and not species that form clumps, such as grasses.
Almost any size or shape of area (quadrat) could be used for measuring density, but results can vary with quadrats of different shapes and sizes. It is important to apply consistent boundary rules about whether a plant is in or out of the quadrat, especially for long narrow quadrats, which have more edge than square or round quadrats. Some options are:
- count only one out of every two plants that are on the boundary
- count all plants that touch the boundary, but only for two sides of the quadrat
- count plants that have more than 50% of their area in the quadrat
