Chao Presence/absence

This is the second of the series of estimators of species richness. The output presents a series of estimates using 1, 2, 3 and so on samples. Thus to get sensible estimates you must ensure that all the data comes from samples obtained from the same community.

If Number of Random Selections (R) is set > 1 then the sequence samples are selected R times at random from the complete set of samples and the mean estimate calculated. As this removes sample order effects it is generally to be recommended.

By looking at the progressive change in the estimates it is possible to assess if sufficient samples have been taken to stabilise the estimate.

A simple plot of the way the estimate changes with the number of samples used is shown by clicking on the graph button in the upper left hand corner of the window.

These methods are discussed by Colwell & Coddington (1995). A present there are no clear indications as to which is superior and they all should be used with caution.

Developed by Chao (1984) this estimator is

S = S(obs) + G/2M,

where G is the square of the number of species that only occur in one sample (L),

M is the number of species that occur in only two samples

and S(obs) the observed species number.

The variance is calculated as for the Chao quantitative estimator with L replacing a and M replacing b.