Species evenness refers to how close in numbers each species in an environment is. 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 possible value of (if every species was equally likely), equal to:
J' is constrained between 0 and 1. The less evenness in communities between the species (and the presence of a dominant species), the lower J' is. And vice versa.
Other indices have been proposed by authors where e.g. Hurlburt's evenness index.
S is the total number of species.