Extract from Patricia Roberts-Pichette and Lynn Gillespie, 1999.
When developing a methodology to undertake research in plant communities it can often be difficult to determine how many quadrats are needed to best sample the subject area. A species accumulation curve can be prepared specific to your sample community and is a useful tool to help determine the number of quadrats needed for your monitoring project.
To develop a species accumulation curve for ground vegetation the accumulated number of species (i.e. the number of new species found in each successive quadrat added to
the total already found) is plotted on the y axis against the quadrats (in the order tallied) on the x axis (Figure 1). When the points are joined, the curve characteristically rises abruptly because many new species are added with each new quadrat and then levels off as fewer species are added with each additional quadrat.
As added effort in sampling more quadrats yields fewer new species, the need for data about additional species must be weighed against the effort required to get that data. Sampling is sufficient when no or very few species are added with each successive quadrat after the curve starts to flatten. Sample size (minimum number of quadrats) may be estimated with reference to the area where flattening starts, known as the “break point.”
The break point may be estimated by the “10% rule” i.e. when 10% increase in area yields less than 10% new species. To locate the break point, draw a line from the zero point through the point representing 10% of the species and 10% of the sample area (represented by the white dot in Figure 1) and extend it. This gives you the slope for determining the break point. Drawing a second line parallel to the first, touching the curve gives you the break point (adapted from Oosting, 1956; see also Barbour et al, 1999).
For ground vegetation species, the recommended minimum number of 1m by 1m quadrats is ten more than the break point. If you don’t have ten more than the break point, complete five to ten more quadrats, prepare a new curve with breakpoint and determine if the minimum number has now been reached. If not, continue completing additional quadrats and creating new curves until the minimum number has been reached.
The example in Figure 1 displays 24 species after 30 samples, so the white dot represents y=2.4 and x=3. A line is drawn between the xy intersection and the white dot. Another line is drawn parallel and used to determine the break point, which in this example happens at approximately 12 samples. Therefore the recommended minimum number of quadrats for this monitoring project would be 12+10 = 22.
From this the rule of thumb could be used: once the number of new species found per quadrat has levelled off, continue monitoring for another 10 quadrats.
This piece has been slightly adapted from Terrestrial vegetation biodiversity monitoring protocols by Patricia Roberts-Pichette and Lynn Gillespie and the whole article can be accessed and downloaded from the following website: https://www.ec.gc.ca/Publications/F98684B9-64E5-40D1-AD7F-2BA9E4138CC9%5CTerrestrialMonitoringProtocolTerrestria lVegetation.pdf
Barbour, M. G., J. H. Burk and W. D. Pitts. 1999. Terrestrial plant ecology. 3rd ed. Toronto, Addison Wesley Canada.
Oosting, H.J. 1956. The study of plant communities: an introduction to plant ecology. 2nd ed. W.H. Freeman and Company, San Francisco.
Roberts-Pichette, Patricia, and Lynn Gillespie 1999. Terrestrial vegetation biodiversity monitoring protocols. EMAN Occasional Paper Series, Report No. 9. Ecological Monitoring Coordinating Ofﬁce, Burlington, Ontario.