Measures of the Health of Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 198

Similarly to habitats, the speciic taxa are assigned a numerical rating. The rating may be based on
measures of species richness (speciic numbers of taxa) or speciic groups (such as Ephemeroptera or
Plecoptera), the composition or percentage of distributions, the presence/absence of speciic tolerant spe-
cies, or trophic/habit measures (such as percentages of clingers, ilterers, or scrapers; Barbour et al. 1999).
Once completed, all of the ratings (habitat, water quality, and biota) are totaled and used to deter-
mine the degree of impairment, as compared to the reference condition. For example, the stream or
river could then be classiied as unimpaired, moderately impaired, or severely impaired, based on
the rating, which could then be used to guide the management or listing of impaired waterbodies.

7.7 BIOLOGICAL DIVERSITY
Biodiversity is a metric of aquatic health that refers to the diversity of the biotic assemblage. The
metric has been applied to diverse species, populations, and habitats.
The most biodiverse communities are those that support a variety of species, but none is dominant.
A homogeneous assemblage, one that, for example, has large numbers of only a few or a single species,
is not diverse (see Simpson 1949). A commonly proclaimed relationship in ecology is that between
biodiversity and stability, where the more diverse an assemblage is, the more stable it will be, which
may be thought of as Mother Nature's way of “not putting her eggs in one basket,” so to speak. Some of
the founders of modern ecology such as Eugene Odum in his Fundamentals of Ecology (Odum 1953)
noted that simpliied communities are characterized by more violent luctuations in population density
than diverse communities. Similarly, Charles (1958) argued that “simple communities were more eas-
ily upset than that of richer ones; that is, more subject to destructive oscillations in populations, and
more vulnerable to invasions.” Elton's (1958) phrase “richer communities” refers to species richness or
the number of species in a community. For example, taxonomic richness is a commonly used compo-
nent of a biological assessment and it is simply a count of the distinct number of taxa within selected
taxonomic groups. Another component of diversity is the relative abundance or equitability, which is
the evenness with which the individuals are spread out among the species in a community.
The relationship between diversity, richness, and relative abundance is shown in Table 7.3, where
the species richness is indicated by the number of taxa. The relative abundance ( H ) may be com-
puted using Shannon's index ( H ), also referred to as Shannon's diversity index (Shannon 1948), and
it is computed from

S

∑

H =− P ln() P

i i

i = 1

where S is the number of species and P is the number of individuals in a species divided by the total
number of individuals.
For this example (see Table 7.3), Community 1 is dominated by a single species and has only
ive species, resulting in a low diversity index ( H ). Community 2, with the same richness, has a
greater diversity since the individuals are evenly spread among the species. Community 3, with
an even greater species richness, also has a low diversity, since it is dominated by a single species.
Community 4 has the greatest diversity, since like Community 3 it has a greater species richness
(than Communities 1 or 2) and the individuals are evenly distributed among those species.
In habitat studies, low evenness or high percentage dominance by a few taxa is an indication that
environmental conditions favor a limited type of organism, which suggests the presence of stressors.
According to Connell's (1978) intermediate disturbance hypothesis, it is not only the stressor or dis-
turbance that impacts biodiversity, but also the frequency of the disturbance. Maximum diversity may
be obtained when disturbances are neither too rare nor too frequent (Figure 7.7). This forms the basis of
some water quality criteria based on the magnitude, frequency, and duration of exposure. For example,
if a system is disturbed too frequently, it may never have time to recover from that disturbance.