Simpson diversity index = the probability that two randomly sampled individuals belong to different species.

= 1- The probability that two randomly sampled individuals belong to the same species.

= 1- The sum of the number of species of each species divided by the square of the total number of plants.

For example, the individuals of species A and B in community A are 99 and 1 respectively, while the individuals of species A and B in community B are 50. According to Simpson diversity index,

: Simpson index of D community A = 1-(0.99 2+0.0 1 2. ) = 0.0 198.

B community: Simpson index of D B = 1-(0.5 2+0.5 2) = 0.5.

It can be seen that the more species in the community, the more uniform the individual distribution and the higher the index, indicating that the community diversity is good.

Rare species play a smaller role in Simpson diversity index, while common species play a larger role. The species diversity estimated by this method needs more samples. Routledge( 1980) points out that if the number of samples is less than 30, it will lead to underestimation.