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For an introduction to the species accumulation curve go here.

The species accumulation curve is very important for knowing how long you need to survey/sample to collect enough data for your biodiversity measurement study. Based on the aim of your study, you can ideally plan out the length of your study and therefore the time and money it is going to take to undertake sampling/surveying sufficiently.

 

This page will give you a simple step by step procedure for graphically carrying out the species accumulation curve.

 

1)Let's say we want to measure the diversity of beetles in a particular study site. We start out by collecting data on the number of species new to the study that we find in that area and enter it into a table. In one column we put the number of species we find that are new to the study, in the other we put the time taken to find them. Usually hours are used to denote the time but any unit such as minutes or days etc can be used as long as it is accurate and is a reasonable unit to use taking into consideration your estimates for the total length of the survey.

 

Time (in hours)

Number of species new to the study

1

3

2

4

3

3

4

5

5

4

6

3

7

3

8

2

9

1

10

0

11

0

12

1

13

0

14

0

15

0

 

2) We convert the number of species to the cumulative number of species in another column. A cumulative count will mean that every cell in the “Number of species new to the study” column will take the value of that original cell plus the value of the preceding cell. So the first cell will be 3, the next will be 4 + 3 (The value of that cell plus the value of the cell immediately preceding it), i.e 7, the third cell would be 10 and so on.

 

Time (in hours)

Cumulative number of species new to the study

1

3

2

7

3

10

4

15

5

19

6

22

7

25

8

27

9

28

10

28

11

28

12

29

13

29

14

29

15

29

 

3) We convert the following into a graph where “Cumulative number of species new to the study” is plotted on the x axis, against “Time in hours” as the y axis of the graph.

 

species accumulation curve how to

4) As we can see initially the curve starts climbing very quickly. That's because when you start the study, you get many more species new to the study than you do later on in the study. Obviously that's because when you initiate the study all the species you see are new. Also all the “common” species, i.e. The species that are the most abundant in the area are found relatively easily and quickly. When you finish finding all the common species, that's when you reach your first plateau in the curve (check out the next figure). So once you reach the plateau you can be pretty sure you've found all the common species in the study area. The second plateau starts when you find what is quite probably a slightly rare species, (as the rarer a species the more difficult it is to find it). Thus after the first plateau more and more time is required to get the next species, it's not as quick as it was in the start of the study. And there will be a new plateau after each rare species that is new to the study that is discovered after that, but each plateau will most probably be longer than the one before it and each species that is new to the study will therefore take that much more time to find than the one before it.

 

Species Accumulation Curve Explained

 

So depending on the aim of the study  one can stop the survey after the first plateau is reached, as that is the stage at which we can be reasonably sure that all the common species have been found. Of course there is no way to know for sure if you have found all the species available at the site even after several plateaus have passed. Logically due to time and money constraints the survey has to stop at some point or another and cannot go on for ever.

 

Once a few plateaus have passed, the curve can be analysed to estimate the time required to find the next species. This is done through methods such as non-linear regression.

 

About the Author of this Article

References -

Southwood T. , and P. Lenderson. Ecological Methods - Published by Blackwell Science Ltd.