Download Species Diversity Biological communities vary in the number of

Species Diversity
Biological communities vary in the number of species they contain and a knowledge of this number is important
in understanding the structure of the community. The number of species in a community is referred to as species
richness. The relative abundance of species is also important. For example, two communities may both contain
the same number of species but one community may be dominated by one species while the other community
may contain large numbers of all species. The relative abundance of rare and common species is called
evenness. Communities dominated by one or a few species have a low evenness while those that have a more
even distribution of species have a high evenness. Species diversity, includes both species richness and
evenness. Communities with a large number of species that are evenly distributed are the most diverse and
communities with few species that are dominated by one species are the least diverse.
For some ecological investigations, it may be useful to measure diversity of one taxonomic group. For example,
if a plant ecologist were interested in studying species of plants, they would measure plant diversity and exclude
other kinds of organisms.
A number of different measures of species diversity have been proposed. This exercise explores two methods
for measuring species diversity of communities: Simpson's Index and the Shannon-Weiner Index.
Simpson Index
If a community with high diversity was randomly-sampled twice, there is a good chance that the two samples
will contain different species. However, if a low-diversity community were sampled twice, it is likely that both
of the samples will contain many of the same species. Simpson (1949) derived a formula based on the expected
outcome of two random samples.
N(N-1)
Ds = ______
ni(ni-1)
Equation 1:
Equation 1
where N = the total number of individuals of all species
ni = the number of individuals of species i
Example: We will illustrate using Simpson's index on a hypothetical community with three species.
Table 1. A hypothetical community with 3 species.
Species No. of Individuals
Beech
32
Maple
18
Oak
12
For this community, N = 32 + 18 + 12 = 62. The calculations using equation 1 are shown below.
Ds =
62 X 61
___________________________
(32 X 31) + (18 X 17) + (12 X 11)
= 3782
____
1430
= 2.64