Biodiversity in ecosystems - Fundamentals of the Physical Environment - page 537

where n = number of species, P i = proportion of the i th
species as a proportion of total cover and log = log base n
(usually log 10 ). The Shannon index is also known as the
Shannon-Wiener index , and incorrectly as the Shannon-
Weaver index. It is derived from the complex mathe-
matical field of information theory and hence its
alternative name of information theoretic index .An
example of the calculation of the Shannon index for the
data in Table 22.2 is given in Table 22.3 , using log 10 .The
most diverse woodland is community A, with five species
of equal dominance. The second most diverse is woodland
B, which has fewer species than woodland A and also
has a more uneven distribution, with oak being domi-
nant. Woodland C, a pine plantation, has the lowest
diversity, being almost a monoculture. The Shannon-
Wiener values which reflect this trend are 0·70, 0·48 and
0·15 respectively. Despite the relative ease with which the
index can be calculated, much discussion of the diversity
and complexity of ecosystems is still based on species
diversity rather than on both species diversity and
evenness ( Plate 22.1 ).

community of n species can have a minimum connectance
of ( n - 1) and a maximum connectance of:

n (n-1)

———

2

Thus a community of four species can have a minimum
of three interactions and a maximum of six. Connectance
is important as an index of how strongly all the species in
the system interact; if it were possible, it would be very
useful to distinguish pairs of species which interact from
those which do not ( Figure 22.1 ).

TYPES OF SPECIES DIVERSITY

The diversity of any geographical area is made up of five
components. The local diversity or within-habitat diversity
is the alpha diversity ( ) , the diversity of a particular
habitat, e.g. a field or a woodland clearing, of size 0.1 ha
to 1 kha. The regional diversity is the gamma diversity ( )
which covers a larger area, generally 1 k ha to 1 M ha and
shows the species diversity of a landscape made up of
more than one kind of natural community. The epsilon
diversity (

Ecological connectance

Other indices of complexity are more difficult to handle,
because of the sophistication of the data required. Thus
connectance in an ecosystem describes the actual number
of interactions between species divided by the number of
possible interactions between species. For example, a

) is the species diversity of a broad region of
differing landscapes, generally 1 M ha to 100 M ha. These
three measures have been termed inventory diversities
( Figure 22.2 ).

Table 22.3 Shannon indices for three woodlands

Species Cover (%) Proportion (P i ) Log P i P i log P i

Woodland A
Oak

20 0·2 -0·70 -0·14

Ash 20 0·2 -0·70 -0·14

Birch 20 0·2 -0·70 -0·14

Alder 20 0·2 -0·70 -0·14

Pine 20 0·2 -0·70 -0·14

- P i log P i = 0·70

Woodland B
Oak 40 0·4 -0·40 -0·16

Ash 30 0·3 -0·52 -0·16

Birch 30 0·3 -0·52 -0·16

- P i log P i = 0·48

Woodland C
Pine 90 0·9 -0·05 -0·05

Birch 10 0·1 -1·00 -0·10

- P i log P i = 0·15