Specific Guidelines on Sample Unit Size for Density
The size of sample units is a critical decision in determining density, so that we can get precise data without spending too much time counting. If too small, the many empty quadrats (or transects) will give the collected data a skewed or Poisson distribution, and conventional statistical inferential techniques cannot be applied. Conversely, if quadrats are too large, counting all individuals becomes time consuming and there is a greater likelihood of sampling errors originating from careless counting.
Sample Unit Size for Density Sampling
- increasing sample unit size will reduce the number of 'empty' quadrats so that collected data assumes a normal distribution.
- most efficient sampling is achieved by using size that results in 20% 'empty' quadrats (or transects).
- if only one size is used, increase sample unit size is there are more 'empty' quadrats than quadrats with one individual.
- if only one sample unit size is used, aim for an average density of 4 - 10 plants/quadrat.
- use nested quadrats to measure important specieis exhibiting different abundance or size characteristics.
- have sample units no larger than necessary, to avoid problems associated with careless counting and excessive time requirements.
- use the same size sample unit throughout the inventory or monitoring program and avoid comparing data collected from different size quadrats.
References and Further Reading
(Note: pdf files require Adobe Acrobat (free) to view)
Bartlett, M.S. 1948. Determination of plant densities. Nature 162:621.
Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Sons, New York. pp 142-148.
Barman, F.H.A. 1953. The statistical efficiency of sample plot size and shape in forest ecology. Ecology 34:474-487.
Eddleman, L.E., Remmenga, E.E., and R.T. Ward. 1964. An evaluation of plot methods for alpine vegetation. Bulletin of the Torrey Botanical Club. 91:439-450.
Greig-Smith, P. 1983. Quantitative plant ecology. Blackwell Scientific Publications, Oxford.3rd ed. pp. 28-31.
Heady, H.F. 1958. Vegetational changes in the California annual type. Ecology 39:402-406.
Lyon, L.J. 1968. An evaluation of density sampling methods in a shrub community. Journal of Range Management 21:16-20. (pdf)
Mueller-Dombois, D., and H. Ellenburg. 1974. Aims and methods of vegetation ecology. John Wiley Sons, New York. pp. 69-70.