Simpson Index (D) - a measurement that accounts for the richness and the percent of each subspecies from a biodiversity sample within a zone. The index assumes that the proportion of individuals in an area indicate their importance to diversity.

Shannon-Wiener index (H) - Similar to the Simpson's index, this measurement takes into account subspecies richness and proportion of each subspecies within a zone. The index comes from information science. It has also been called the Shannon index and the Shannon-Weaver index in the ecological literature.

There are some underlying assumptions that all the measures of biodiversity have in common:

1. The categories are well known.
   In most cases, such as with our study, this assumption is not violated since the classification of subspecies that we use is accepted world-wide. However, this may be difficult if everyone does not use the same classification system (e.g., some people may put similar individuals into several subspecies categories while others use one category).

2. All categories are equally different.
   Categories are equally different from each other category. This is not always true since two subspecies from the same genera are treated the same as two subspecies from different families.

3. Use a measure of species importance.
   Usually, one uses the number of individuals, percent coverage, relative density or biomass. The choice usually depends on the ease of measurement. In our case, we use the number of individuals of each subspecies observed.

4. The community under study is well defined.
   The relative importance of an individual category will vary greatly depending on the definition of the extent and makeup of the community. The communities in our studies are the zones.

The most common measures of biodiversity are Species richness, Simpson's index and Shannon-Wiener index.

Species Richness

This is the simplest of all the measures of subspecies diversity. All you do is count of the number of subspecies found in a community (e.g., the number of the subspecies found in the Meadow Fields zone).

However, this does not indicate how the diversity of the population is distributed or organized among those particular subspecies. For example, if there were 4 different subspecies observed in Zone 1 and Zone 2 the richness would be equal. This does not indicate what percentage of the abundance there were of each subspecies. In Zone 1, 80% of the abundance could have been Blue Jay while at Zone 2 there could have been an even 25% of each subspecies.

Simpson's Index

A measure that accounts for both richness and proportion (percent) of each species is the Simpson's diversity index. It has been a useful tool to terrestrial and aquatic ecologists for many years and will help us understand the profile of biodiversity across our Zones.

The index, first developed by Simpson in 1949, has been defined three different ways in published ecological research. The first step for all three is to calculate P_i, which is the abundance of a given subspecies in a zone divided by the total number of subspecies observed in that zone.

1. Simpson's index: D = sum(P_i²)
   The probability that two randomly selected individuals in the zone belong to the same subspecies.

2. Simpson's index of diversity: 1 - D
   The probability that two randomly selected individuals in a zone belong to different subspecies.

3. Simpson's reciprocal index: 1/D
   The number of equally common subspecies that will produce the observed Simpson's index.

D is influenced by two parameters - the equitability of percent of each species present and richness. For a given species richness, D will decrease as the percent of the species becomes more equitable. The researcher must observe the species patterns carefully to interpret the values effectively.

Shannon-Wiener Index

This diversity measure came from information theory and measures the order (or disorder) observed within a particular system. In ecological studies, this order is characterized by the number of individuals observed for each subspecies in the sample plot (e.g., zone on our site). It has also been called the Shannon index and the Shannon-Weaver index.

Similar to the Simpson index, the first step is to calculate P_i for each category subspecies. You then multiply this number by the log of the number. While you may use any base, the natural log is commonly used. The index is computed from the negative sum of these numbers. In other words, the Shannon-Wiener index is defined as:

H = -sum(P_ilog[P_i])

Using species richness (S) and the Shannon-Wiener index (H), you can also compute a measure of evenness:

E = H/log(S)

Evenness (E) is a measure of how similar the abundances of different species are. When there are similar proportions of all subspecies then evenness is one, but when the abundances are very dissimilar (some rare and some common species) then the value increases.