Species Accumulation Curves
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The plot of the cumulative number of species, S(n), collected against a measure of the sampling effort (n) is termed the species accumulation curve. The sampling effort can be measured in many different ways; some examples are the number of quadrats taken, total number of animals handled, hours of observation, or volume of water filtered. As effort increases, gradually more and more of the species living in a habitat will be caught, until eventually only the rarest species or occasional visitors remain unrecorded. When this occurs increased effort will not increase the recorded species number. The species accumulation curve will have reached an asymptote.
If 'Number of Random Selections of sample order' (in the drop-down menu at the top of the program window) is set to 1 (see below) then Species Accumulation will give the simple species accumulation curve for your samples.
See Species accumulation curve tutorial for more information.
For larger numbers of randomisations the average species accumulation curve is plotted for randomly shuffled samples. For example, with 10 randomisations, the order of the available samples is shuffled 10 times and the average calculated. The effect of this procedure is to remove the effect of sample order on the species accumulation curve and hence produce a smoothed curve.
Using species accumulation curves
When a species accumulation curve approaches an asymptote the user knows that sampling effort has been sufficient to collect most of the species present, and also that the asymptotic value is a measure of the total species complement. We assume here that the method is capable of catching all the species - this is not often the case! Asymptotic behaviour will only be observed if the samples all come from a single, reasonably homogeneous, habitat.