Biodiversity of Freshwaters - Freshwater Ecology: Concepts and Environmental Applications - page 184

equally represented the numbers of each species are, which is called even-
ness or equitability. For example, consider two habitats, both with 10
species. If one habitat has approximately even numbers of each species, it
will have a high evenness. If the other is dominated by one species and has
one or a few representatives of the rest, it has the same species richness but
lower evenness. This can be visualized in Fig. 10.2. Ponds B and F have
the same richness, but pond F has a greater evenness. Some indices include
both evenness and richness. One commonly used measure of diversity that
includes both evenness and richness is the Shannon-Weaver measure of di-
versity, H

(Example 10.1):

S

H p j ln p j

j 1

where there is an assemblage of organisms with S species and p j is the pro-
portion of species j (i.e., the number of individuals of species j divided by
the total number of organisms in the assemblage). The summation sign at
the beginning of the equation means that the proportions of all the species
in the community are summed. The equation is most often calculated with
the natural log, but others use log 10 or log 2 . Here, I use the natural log,
but care should be taken when comparing studies to be certain that the
index was calculated the same way. H increases with more species and

with greater evenness. One way to estimate evenness is to divide H by

B

A

C

D

F

E

G

H

FIGURE 10.2 A diagram of diversity in two sets of ponds. When A-D are considered
versus E-H, both have approximately the same overall diversity. When

diversity (within-
habitat diversity) is measured, ponds in the A-D group have lower diversity than those in the
E-H group. When diversity (between-habitat diversity) is measured, ponds in the A-D

group have higher diversity than those in the E-H group.