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Community Structure and Species Diversity
I. Background
The number of species in a biological community is termed species richness, N. For example, tropical
rain forests have higher species richness than temperate forests.
Species diversity takes into account species’ abundances as well. A community is said to have a high
species diversity if many equally or nearly equally abundant species are present. On the other hand, if a
community is composed of a very few species, or if only a few species are abundant, then species
diversity is low. For example, if a community had 100 individuals distributed among 10 species, then the
maximum possible diversity would occur if there were 10 individuals in each of the 10 species. The
minimum possible diversity would occur if there were 91 individuals belonging to 1 of the species and
only 1 individual in each o f the other 9 species.
High species diversity indicates a highly complex community because a greater variety of species allows
for a larger array of species interactions. Thus, population interactions involving energy transfer (food
webs), predation, competition and niche apportionment are likely to be more complex and varied in a
community of high species diversity. This is still the subject of considerable discussion among
The most commonly used measure of species diversity is the Shannon diversity index, H, (often
referred to as Shannon-Weaver or Shannon-Wiener index) and is related to the concept of “uncertainty.”
In a community of low diversity, we can be relatively certain of the identity of a species chosen at
random. In a highly diverse community, however, it is difficult to predict the identity of a randomly picked
individual. Thus high diversity is associated with high uncertainty and low diversity with low uncertainty.
Mathematically, the Shannon diversity index is:
H = −∑ pj log pj where p = fi/ni
that is, pi is the proportion of the total number of individuals occurring in species i, calculated by taking
the total number (frequency, f) of individuals of species i and dividing it by the total number (n) of
individuals of all species.
An equivalent formula that does not require the calculation of proportions is:
H = (n log n −∑ fi log fi )/n
The value of H is affected not only by the relative abundances of the species but also by the number of
species. The maximum possible diversity for a data set with k categories (or species) is:
Hmax = log k
Therefore, it is often useful to calculate J, the evenness measure, as follows: