Dear 

Dear Sabhrina

In diversity studies, evenness refers to the similarity of frequencies of the different units making up a population (or sample). It is complementary to richness, which is the number of different units relative to population (or sample) size.

An example is probably more telling than those definitions. Assume you get three population samples (each with N=10 individuals) from three different sites. Sample A contains 5 individuals of genotype 1, 2 of genotype 2, 2 of genotype 3 and 1 of genotype 4. Sample B contains 5 individuals of genotype 1 and 5 of genotype 2. Sample C contains 3 individuals each of genotypes 1, 2 and 3, and 1 individual of genotype 4.

Richness in these samples is 0.2 in sample B ( 2 genotypes out of 10 individuals), and 0.4 (4 genotypes out of 10 individuals) in samples A and C. However, although their richness is the same, evenness in sample C is higher than in sample A, because genotype frequencies are more similar ( or more 'even); they range from 0.1 to 0.3 in sample C vs 0.1 to 0.5 in sample A. Therefore, overall, the population from which sample C comes is assumed to be more diverse than that from which sample A comes.

There are several measures of richness and evenness. The famous Simpson index is basically a measure of richness, whereas the Shannon index includes also a measure of evenness.

Hope this helps

Didier

Dear Dr. Sabhrina Gita Aninta:

In my view, evenness for any diversity analysis (species in a community, alleles in a population, etc.) is meaningful whenever different sets are compared. These may be different samples (e.g. bat communities in different ecosystems) or they may be also comparisons of actual data with theoretical and other sorts of expectations. In population genetics for example, the distribution of actual allele frequencies are often compared against the theoretical expectations of such diversity in neutral populations with the same number of alleles. This is the so called Ewens-Watterson neutrality test between gene diversity (which is a diversity measurement related with evenness and is exactly the Simpson's diversity index) and the number of detected alleles. Unbalances between a parameter (theta) calculated from gene diversity (theta H) and from the number of alleles (theta k) tell something about the historical processes affecting genetic diversity in populations. I'm sure the same can be done in ecological analyses under the appropriate comparisons of actual data or actual data and calculated expectations. I hope this may be useful for you.

Best regards.

Luis Medrano González.

Dear Sabrhina and all

To complement my previous answer - which was basically a definition of evenness: yes, you're right - evenness of all community components does not necessarily translate into optimal diversity. This is because, at the community level, what matters in terms of ecosystem functionning is the functions, more than the species which fulfill those. Since several species within an ecosystem may fulfill the same function(s) ( for instance recycling organic matter), the even frequency of those species is not required as long as the function itself is active. However, one can assume that a more even distribution of species within a community might make the ecosystem more resilient, as then the risk of loosing an essential component of the functionnal network is much lower.

Hope this helps

Cheers

Didier

http://link.springer.com/article/10.1134%2FS1067413611010103

i think similarity means when different habitats have the same species and evenness means when the different species are equally distributed in the habitat

If we have a Pielou index of 0.5, then how do we interpret the situation.

Hi, I would like to answer with a figure:

Both figures have the same number of species.

Figure B have less eveness, because have one species (pinus) that have more individuals that others.

Dear Roman,

Good interpretation

Regards

Mazher

Thanks a lot Dr Roman for the interpretation