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Research Methods

Species-Area Curves

How do you know if you have collected enough samples of the plant community? You want to collect enough samples so that you don’t miss a lot of species, but since time and energy are limited you usually don’t want to sample too many more plots that necessary. Scientists use the species-area curve to answer the question of how many plots to sample.

Number of Plots Sampled
Cumulative Area Sampled (m²)
Mean Plant Species Richness
1
1
6
2
2
12
3
3
16
4
4
18
5
5
19
6
6
20
7
7
20.5
8
8
21
9
9
21.25

Table 1.

As the cumulative area of a study site sampled increases, the number of species detected increases and then begins to plateau (see Figure 1 below). Once that plateau is reached, you can generally be confident that an appropriate area has been sampled in order to adequately capture the majority of species present in a community. A species-area curve plots the average number of species detected against the cumulative area sampled.  For example, table 1 lists data from a study that used 1.0 m2 plots where all plant species present in each plot were identified. As the cumulative area sampled increases the mean (average) plant species richness also increases. In this case plot 1 contained 6 species; plot 2 contained 18 species, so the mean (average) number of species was 12, and so on. You can see in figure 1 below, that the rate of increase in the number of species begins to plateau after 6 plots are sampled and is pretty stable once 10 plots are sampled, indicating that this is probably enough samples to accurately represent the composition and diversity of this plant community. Different habitats and plant communities will have different species area curves, so just because 10 plots were enough in this site doesn’t mean this is true for all sites. Some sites may require fewer plots, others more.

species area curve

Figure 1. This species-area curve shows that the number of species detected increased a lot until about 6 plots were sampled. After six plots were sampled, it was less likely that more species would be detected. By the time 10 plots were sampled, it was even more unlikely that more species would be detected; indicating that enough total area of the study site had been sampled to give a reliable picture of the plant species composition and diversity. A different site would probably have a different species-area curve, meaning you may need to sample fewer plots or more plots.

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