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The closer the number gets to 1, the more even the quantities of the different species are, ie. if the there are 100 crows and 103 foxes the number will be close to one, whereas if there are a lot more foxes than crows the number will be smaller than one. One is the maximum value, since Hmax is the highest evenness possible in the population.
Pielou's or Species evenness refers to how close in numbers each species in an environment are. Mathematically it is defined as a diversity index, a measure of biodiversity which quantifies how equal the community is numerically. So if there are 40 foxes, and 1000 dogs, the community is not very even. But if there are 40 foxes and 42 dogs, the community is quite even. The evenness of a community can be represented by Pielou's evenness index:
Where is the number derived from the Shannon diversity index and is the maximum value of , equal to:
E is constrained between 0 and 1. The less variation in communities between the species, the higher E is. Other indices have been proposed by authors where eg. Hurlburt's evenness index.
S is the total number of species.