Environmental Engineering Reference
In-Depth Information
Fig. 5.7 Two species may have different roles in a community, but similar importance values
(Published with kind permission of
©
M. Kuchta 2014. All Rights Reserved)
Relative value of species A
¼
Value of species A
=
Total value for all species
Frequency, density, and cover each represent a different aspect of the role of a
species in a plant community. If a measure of the overall importance of a species is
desired, then composite measures, such as an importance value (Curtis
1959
) can be
calculated.
Importance Value
¼
ð
relative frequency
þ
relative density
þ
relative cover
Þ=
3
Practitioners must use caution when interpreting composite measures, however,
because two species with very different biological roles in a community can have
the same importance value. If species A has low density but high cover and species
B has high density, but low cover, they could have the same importance value
(Fig.
5.7
).
5.4.2 Assessing Species Richness and Diversity
There are many reasons why it is important to assess wetland plant species richness
and diversity. Levels of diversity can indicate the health or status of wetland
systems (U.S. EPA
2002
). Diversity assessments can be important in establishing
conservation priorities. Sometimes practitioners are interested in the factors that
contribute to high or low wetland plant diversity or richness (e.g., Bedford
et al.
1999
, Michalcova et al.
2011
). In wetland plant science, richness refers to
the number of species found in a defined area. Diversity is defined by a combination
of species richness (number) and the evenness of the distribution of abundance
among species. For example, a wetland site with 142 different species, but 97 -
percent-cover of
Typha
spp. has high richness, but low evenness. Numerous
mathematical formulas have been invented and used to describe how diverse such
a system is. Regardless of the method used, both richness and diversity give no
