Environmental Engineering Reference
In-Depth Information
information about species identity. A very rich community could contain a high
number of invasive species, which would be negative from an ecological value
perspective.
Species Richness: The number of species is frequently easier to communicate to
decision-makers than composite index numbers representing species diversity,
and so is frequently used to describe wetland systems. Sampling for species
richness can be difficult; pilot studies and species-accumulation curves should
be evaluated prior to final sampling (see above). Raw, or observed, species
richness straight from the field tends to underestimate the true species richness
of a site or system. In order to correct for sampling deficiencies (bias or loss of
precision), species richness estimators have been created. Most estimators use a
process of generating numerous estimates from randomized resampling of data
with different numbers of samples and calculating the mean estimate from the
resampling (Michalcova et al.
2011
). According to Magurran (
2004
), the rich-
ness estimators with the least bias and highest accuracy are the Chao2, Jack1 and
Jack2 methods. Several statistical packages can be used to calculate estimators,
including the R package vegan (Oksanen et al.
2011
) and EstimateS (Colwell
2009
). In practice, the data set with actual observations is entered, yielding an
output with several estimates of species richness based upon different
estimators. The user then must choose which estimator performs the best for
the given data set by examining the output. Some estimators are more conserva-
tive than others or will better mirror the observed species accumulation curve.
Species Diversity: Diversity indices are numbers generated from information
about species richness and how evenly-distributed the abundance of different
species is within the community. These indices provide more information, but
they can be open to interpretation and difficult to communicate to decision-
makers. There are three general types of diversity: alpha, beta, and gamma.
Alpha diversity is the diversity of a single point or site, and is the type most
commonly used. Beta diversity is the difference in community composition
(change in species and their abundance) over a series of samples. Gamma
diversity is the species pool, or the set of species present in the larger regional
landscape, and can be important in determining the potential set of propagules
available for a restored or disturbed wetland site. There are numerous published
diversity indices for describing alpha diversity (see Magurran (
2004
) for a
thorough review), although only a few are widely used. Each index is based
upon the proportion of total abundance (p
i
) that each species comprises within
the community.
- Simpson's Index: This widely-used metric is simply the sum of squares of all
species proportions, where S
¼
number of species, and p
i
is the proportion of
species i:
X
S
p
i
D
¼
i
