Quadrat

A quadrat is a square or rectangular sampling frame of known area used to delimit a study plot in ecology and related disciplines. Typically made of metal, wood, or plastic, quadrats range in size from about 0.1 m by 0.1 m to 1 m by 1 m or larger, depending on the organisms and habitat studied. The name derives from Latin quadratus, meaning square.

Quadrats are employed to survey the distribution and abundance of plants and other organisms within a defined area. In vegetation studies, researchers record species presence and estimate percent cover within each quadrat; in faunal surveys, counts can be made for sedentary organisms that stay inside the frame or adjusted with sampling rules for mobile species. Placement is often random to avoid bias or systematic along transects to describe gradients.

Data from multiple quadrats are used to estimate population density (for plants, often as individuals per unit area or coverage percentage), species frequency (the proportion of quadrats where a species occurs), and diversity metrics. Means and confidence intervals are computed across quadrats to infer conditions in the broader area. Edge effects are managed by including partial quadrats or excluding them, depending on the protocol.

Common quadrat variants include frame quadrats, which may contain a grid for estimating cover, and point quadrats, where a pin or point tester is used at fixed locations to identify the species touching the pin. Line quadrats along transects and small-mesh subquadrats are used for fine-scale or habitat-specific sampling.

Advantages of quadrat sampling include simplicity, flexibility, and repeatability for vegetation and sessile organisms. Limitations include sensitivity to quadrat size, placement bias, habitat heterogeneity, and difficulties sampling highly mobile taxa. When used with randomization and appropriate replication, quadrats provide a practical, standard method for ecological surveys.