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The species accumulation curve is very important in planning the length of the survey when measuring biodiversity. It is a graph that records the cumulative number of species found in a particular study area as a function of the cumulative effort expended searching for them. The time is usually measured in terms of the number of hours spent searching.

The species accumulation curve for any new study area will initially rise very quickly (as a lot of species new to the sample are discovered in relatively less time at the start of the study), but it's rate of increase will slow down (after all the common species are found, the species new to the sample found beyond that tend to be rarer species that are more difficult to come across). Ultimately the curve tends to stabilize for longer and longer periods, i.e it takes more and more effort to find the next new unrecorded species and cause the curve to rise higher.

species accumulation curve explained

 

In theory when all the species are found the curve cannot possibly rise any more and any more time spent searching will not lead to an increase in the species accumulation curve, thus the species accumulation curve is said to have reached it's asymptote (which basically means the curve has gone as high as it can). In practice it is not always possible to keep sampling until the species accumulation curve reaches its asymptote for a number of reasons, there may be too many rare species and it may take very long periods of time to find them all, one can never truly know whether one has found all the species there are in the study site no matter how much time one spends surveying, monetary or time constraints may allow for surveying to take place only for a certain amount of time and most importantly the aim of the study may not really require the assessment or detection of all the species in the area (for example if one wishes to compare the biodiversity value of two areas i.e just to find out which one has more species than the other, one can do so by keeping survey time, effort and methodology constant in both the places and comparing results. One does not need to find all the species in both areas.)

If the aim of the study is one that does not need the detection of each and every single species in the area, one can stop surveying/sampling after the species accumulation curve has stabilized, as that usually means that the most abundant and common species have been found and in most cases may also mean that most of the species in the area have been found.

 

Though as mentioned earlier even after the curve has stabilized, or formed a “plateau”, one can never be absolutely sure that all the species that can be found in in the area being surveyed have been found. There are methods that can be used to analyse the curve and enable the estimation of the number of additional species that will be found with further effort and the amount of effort it's going to take for that to happen. This can be done by fitting a functional form to the curve such as non-linear regression.

 

How you can use the species accumulation curve in your study

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References

Magurran A. Measuring Biological Diversity