Download Diversity Index and Environmental Assessment

Diversity Index and Environmental Assessment
Lab developed by Nancy Nicholson
INTRODUCTION
Scientists usually use the term “DIVERSITY” as a synonym for “variation,” especially if
discussing the concept in an evolutionary context. For ecologists, use the idea of diversity
in a different technical sense. They subdivide the concept as
Diversity (inventory balance of an ecosystem or subunit of an ecosystem)
Richness – number of species/kind of organism present
Evenness – the abundance of each species or kind
These two measurements are first recorded as a tally of the ecological unit (forest, lawn,
stream, coral reef) under study. This tally is technically called a frequency distribution
because it contains the “laundry list” of organisms in addition to their numbers. From this
information, several formulae might be used to calculate a single number or value that
represents the DIVERSITY INDEX. This index can be used to compare ecological units
that may be of very different composition: microbial or large organisms, terrestrial or
aquatic.
The Diversity Index we will use is called the Shannon-Weiner Index, symbolized by Hs.
It is relative independent of sample size and is not strongly affected by the way
organisms are classified. For instance, an investigator could choose to classify the
organisms in a community by food habits or growth forms or leaf shape instead of
species names. In this system, a set of images on a sheet of paper can serve as a field data
tally and the information coded according to image for entry into a spreadsheet for
calculation of the Index value.
As you can see from the sample calculations below, Sample one has a higher diversity
than Sample Two. With this information in hand, the investigator can now ask what are
the observed factors that would contribute to the domination of Sample One by a single
species (or form) while Sample Two has an even distribution of just two of the species.
There might be natural stress factors (temperature, light, moisture, soil conditions)
present that limits the number of species that can occupy a site.
The Shannon-Wiener Index can be calculated using logarithms to the base 2 or base 10:
we will use logs to the base 2. Your hand calculator is also a handy tool for calculating
the index, as you can type in values in sequence, use the function feature to find the log
and fill out a data sheet table if you do not have an electronic spreadsheet already set up.
S
Hs = -  (Pi) (ln Pi) , where
i =1
Hs – symbol for the diversity in a sample of S species or kinds
S - the number of species in the sample
Pi – relative abundance of ith species or kinds measures, = ni/N
N – total number of individuals of all kinds
ni - number of individuals of ith species
ln – log to base 2
These are sample computations using a table that follows the formula above.
Sample One
Species 1
Species 2
Species 3
16
16
16
Sample Two
44
2
2
Sample One
Species #
1
2
3
ln Pi
(Pi) (ln Pi)
Pi = ni/N
16 16/48 = 0.3333 -1.0986 (0.3333) (-1.0986) = -0.3662
16 16/48 = 0.3333
-0.3662
16 16/48 = 0.3333
-0.3662
Sum
-1.0986
1.0986
Hs
Sample Two
Species # Pi = ni/N
1
16 44/48 = 0.9167
2
2 2/48 = 0.0417
3
2 2/48 = 0.0417
ln Pi
(Pi) (ln Pi)
-0.0870 (0.9167) (-0.0870) = -0.0796
-3.1780
-0.1324
-3.1780
-0.1324
Sum
-0.3444
0.3444
Hs
The higher index for sample one indicates that it has a higher uncertainty of composition.
That is, if you were to select two individuals at random from the first sample, it is more
uncertain that they will be the same than if you try the same selection in the second
sample. Both sample are equally rich in species (3), but sample one is even and the more
diverse. Sample tow is uneven in its species make up.
If factors controlling distribution of species at a site can be determined to cause the
different compositions of the samples, then we are in a position to propose that Sample
One is a candidate for “environmental degradation.” The actual determination needs to be
made in the light of other observations. If the stresses on the sampled community are
caused by human activity, we could talk about “degraded” habitat. However, an
agricultural field might fit the characteristics of Sample One. This illustrates how careful
observation of the context of data can determine the type of hypotheses developed about
a site and whether or not they can be confirmed.
If we sample a site in our lawns or in the Western Woods, we might come up with fairly
high diversity index values. Before we complete the study, we need to look at those
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values, determine if we have native or introduced species and look up what is known
about our study organisms.
METHOD
Sample the available lawn space with the Frisbee or bucket lid method of the Clover Lab.
Your data count at each throw will be recorded on a sheet with pictures of the species’
leaves. It is suggested that you use 24 throws if using a Frisbee and 12 throws if using a
bucket lid. You will calculate the Diversity index Hs using an Excel Spreadsheet or set up
a data table as in the sample calculation.
Write up the lab according to the format for the Clover Lab. Address the following
questions in your discussion section:
1) What is the diversity of your sample compared to diversity for the whole class?
2) How many of the species in your sample could qualify as “weeds?”
3) Briefly describe one question you would ask about the differences or sameness
noted in between the calculations. In other words, is your team sample of about 1
square meter enough to indicate the diversity in the Alumnae Hall lawn area
(about 1000 square meters)?
4) Identify two ecosystems on the Miami/Western campus that could be evaluated
according to this Index. Explain your choices briefly (future possible
investigations).
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