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Just a note

Plotting the cumulative number of species observed against time (or area sampled) produces a curve. As sampling proceeds the number of new species found flattens off - or reaches an 'asymptote'. Hence a simple way of estimating the total number of species in a community is to decide at what point the line would level off completely.

One obvious difficulty with this is the number of species you observe is partly a matter of chance and season - which introduces a series of arbitrary steps into your graph. The effect of this can be reduced by randomly reordering the sequence in which species were identified - then re-estimating the asymptote. By repeating this process a number of times a mean asymptote can be estimated, and its standard deviation calculated.

Needless to say, estimating the total number of species in a community from an asymptote conceals some rather crucial assumptions...