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Research Methods
How many samples is enough?
You can use density, biomass, or species diversity of your target organism to determine whether or not you have collected enough samples. Here we give an example using density, but the basic process and calculations are the same for what ever measure you decide to use. One thing to notice is that depending on the measure of interest (density, biomass or diversity) your minimum number of plots to sample may be different. In the examples below, the minimum number of plots for determining average density was 5, for average biomass it was 12 plots, and for species diversity it was 3 to 5 plots.
Using Earthworm Density To Determine The Number Of Plots Needed
Density is the number of earthworms in a given area. We will use sample plots to determine a reliable estimate of the average density of earthworms for a given site.
Sample plot |
# of earthworms |
Sample plot |
# of organisms |
1 |
12 |
6 |
11 |
2 |
8 |
7 |
7 |
3 |
3 |
8 |
9 |
4 |
15 |
9 |
3 |
5 |
25 |
10 |
10 |
You will use average cumulative density to determine whether you have collected enough samples for a reliable estimate of the earthworm population in your site.
Average Cumulative Density = (Total # of organisms collected)/(Number of sample plots)
In this example, you can see how the average fluctuates a lot until you have sampled a minimum of 5 or 6 plots. Then it drops gradually until abut 9 or 10 plots. In general, once your average stabilizes, indicated when the graph more or less become flat, you can be confident that you have collected enough samples.
You can approach determining the number of plots sampled in several ways. For example, you may want to do a “pilot” study to determine the number of plots to use in your study; or you may simply want to collect as many plots as possible given your time and resources and then use this type of graph to confirm that you feel you have collected enough plots; or you could be calculating an average density as you go. If you use this method, be sure you use enough to be sure you have reached a stable average. For example, in our graph above, if you had stopped at 5 or 6 plots you would not have had enough plots to really see the average stabilize. In fact, the little drop we see until we sampled 9 or 10 plots suggests that the average is only then beginning to truly stabilize. Perhaps a few more plots are required to give us confidence that that is the case.
Using Earthworm Biomass To Determine The Number Of Plots Needed
The same method can be used with earthworm biomass (ash free dry grams per plot) or earthworm diversity (number of species per plot).
Average Cumulative Biomass = (Total ash-free-dry grams earthworms collected)/(Number of sample plots)
Notice that in this example, the average biomass did not start to stabilize until a minimum of 12 or 13 plots were sampled, and perhaps even more are needed. When you have many different species it may take more samples for average biomass to stabilize.
Using Earthworm Species Diversity To Determine The Number Of Plots Needed
The graph above is also called a species-area curve and is often used when measuring plant community diversity (read more about sampling plants). As the number of plots sampled increases, the average number of species detected increases until a plateau is reached indicating that an appropriate number of plots have been sampled in order to adequately identify the majority of species present in a community. In this example, as the number of plots sampled increases the average number of earthworm species (species richness or diversity) also increases (see table below). You can see, the average number of species begins to plateau after 3 plots are sampled and reaches a maximum after 5 plots are sampled, indicating that this is probably enough samples to accurately represent the composition and diversity of this earthworm community. Different habitats will have different earthworm species assemblages, so just because 5 plots were enough in this site doesn’t mean this is true for all sites. Some sites may require fewer plots, others more.
Plot |
Number of species found |
Mean Earthworm Species Richness |
1 |
3 |
3 |
2 |
2 |
2.5 |
3 |
6 |
3.6 |
4 |
4 |
3.75 |
5 |
5 |
4 |
6 |
2 |
3.6 |
7 |
3 |
3.6 |
8 |
7 |
4 |
9 |
3 |
3.8 |
10 |
1 |
3.6 |