Specific Guidelines on Sample Unit Size for Frequency
Size of sample units is a key decision made in frequency sampling because it has an overwhelming influence on frequency values, as well as sampling precision. For example, large-sized quadrats may result in several common species having 100% frequency, even though their actual abundance differs, and repeated sampling of the same area would have little ability to detect change in frequency values. By contrast, small quadrats may repeatedly miss the less common species, even when present in the area being sampled.
The problem of obtaining frequency measures for species of differing abundance may be resolved by several approaches. A series of nested quadrats is often adopted when one species dominates the biomass, cover or density of the vegetation. It may also be possible to use species groups for less abundant or insignificant species.
Also refer to Sample Unit Size in the Sampling Concepts section.
Sample Unit Size for Frequency Sampling
- frequency values are dependent on quadrat size - legitimate comparisons cannot be made for frequency data obtained from different sized quadrats
- increasing quadrat size will increase frequency; smaller quadrats will reduce frequency
- if only one quadrat size is to be used, the most frequent species should occur in 60-85% of the quadrats
- use nested quadrats to measure a variety of species that exhibit different abundance or size characteristics
- if the data follows a binomial distribution, ensure that all important species have 20%-80% frequency. Frequency values 5% or 95% cannot be easily distinguished from 0% or 100%, respectively, by the statistical analysis of frequency data using binomial procedures.
References and Further Reading
Aberdeen, J.E.C. 1958. The effect of quadrat size, plant size, and plant distribution on frequency estimates in plant ecology. Australian Journal of Botany 6:47-58.
Despain, D.W., Ogden, P.R., and E.L. Smith. 1991. Plant frequency sampling for monitoring rangelands. In: G.B. Ruyle. (ed). Some methods for monitoring rangelands and other natural area vegetation. University of Arizona, College of Agriculture, Extension Report 9043. pp. 8-12.
Hironaka, M. 1985. Frequency approaches to monitor rangeland vegetation. In: W.C. Krueger. (chairman). Proceeding 38th Annual Meeting, Society for Range Management, Salt Lake City, UT, February 1985. p. 85.
Hyder, D.N., Conrad, C.E., Tueller, P.T., Calvin, L.D., Poulton, C.E., and F.A. Sneva. 1963. Frequency sampling of sagebrush-bunchgrass vegetation. Ecology 44:740-746.
Hyder, D.N., Bement, R.E., Remmenga, E.E. and C. Terwilliger. 1966. Frequency sampling of blue grama range. Journal of Range Management 18:94-98.
Mueller-Dombois, D., and H. Ellenburg. 1974. Aims and methods of vegetation ecology. John Wiley Sons, New York, NY. pp. 73-76.