Environmental Engineering Reference
In-Depth Information
several diversity indices in which the contribution of each species is weighted by its relative abundance.
Three such indices are widely used in ecology: Simpson's index, Margalef index and the Shannon-
Weaver index. Simpson's index is
1
ª
º
§
n
2
·
S
¦
D
«
i
(10.7)
¨
¸
»
2
N
«
»
©
¹
¬
¼
i
1
in which
n
i
is the number of individuals of the
i
-th species, and
N
is the total number of individuals in the
sample. For any particular number of species in a sample (
S
), the value of
D
can vary from 1 to
S
, depending
on the evenness of species abundances.
The Margalef index is defined as the total number of species present and the abundance or total
number of individuals. The higher is the index, the greater the diversity. The Margalef index
M
is given
by (Margalef, 1957):
N
(10.8)
The Shannon-Weaver index, developed from information theory and integrating the species richness
and evenness of the abundance distribution, is given by (Krebs, 1978):
MS
(
) / l
e
nn
S
¦
H
i
ln
i
(10.9)
NN
i
1
The Shannon-Weaver Index provides no information on the total abundance of the bio-community. For
instance, samples from two sites have the same number of species, the distributions are also the same but
the density of individuals for site one is 10 ind/m
2
and for site two is 100 ind/m
2
. Eq. (10.9) gives the
same values of
H.
The difference in population density for the two cases is large, but it is not reflected by
the values of
H
. Considering both the abundance and biodiversity, the following bio-community index is
suggested (Wang et al., 2009):
S
nn
¦
BH N
ln
ln
N
i
ln
i
(10.10)
NN
i
1
Macro-invertebrates census data from 9 sites along the East River in south China can be used to
illustrate these different methods of presentation, as listed in Table 10.1 (Wang et al., 2008). The East
River is 562 km long and has a drainage area of 35,340 km
2
. The river is one of the three major rivers of
the Pearl River system—the largest system in South China. The Fenshuba Dam is a hydropower project
on the river dividing the upper and middle reaches of the river and is 382 km from the river mouth.
Figure 10.41 shows the variation of the species richness,
S
, number density of individual invertebrates,
N
, Shannon-Weaver index,
H
, and the bio-community index,
B
, from upper to lower reaches along the
course. In general the richness,
S,
the density,
N
, Shannon-Weaver index,
H,
and the bio-community
index,
B
, of benthic invertebrates reduce from the upper to the lower reaches. The Fenshuba Dam causes
instantaneous fluctuation in flow discharge and velocity, which strongly impact the invertebrates.
Therefore, only one species,
Palaemonidae
, which may survive the fluctuation, was found at the site
downstream of the dam. The impact of velocity fluctuation becomes weak further downstream from the
dam and exhibits no influence on the benthic invertebrates at a distance of 80 km from the dam.
In the lower reaches the channel has been regulated with relatively uniform width and the banks have
been hardened with concrete and stones. Flow velocity in the channel is more uniform than the upper
reaches and the substrate consists of only sand. The sand bed is compact, which provides no space for
benthic animals to live and no shelter for the animals to escape current. The richness, number density,
and biodiversity and bio-community indices in the lower reaches are very low or zero. Humans have
reclaimed river bays, riparian lakes and wetlands, and sluggish and backwater zones, which caused loss
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