Lab: The Diversity Index of Harriet Creek Ecosystem
Background Information:
Biodiversity is a buzz word on practically everyone’s lips these days. This term has meaning at a
variety of levels…at the genetic level, at the species level, at larger taxonomic group levels, and at the
community or ecosystem level. Biodiversity has been defined by the U.S. Office of Technology
Assessment as “the variety and variability among living organisms and the ecological complexes in which
they occur.” The most immediate thoughts of people refer to diversity at the species level and this is really
the easiest question to study in the context of an AP Environmental Science laboratory.
Estimates of the number of species worldwide vary tremendously. Already identified and named
are 1.6 to 1.8 million species of plants and animals. But many are not yet identified or named…some
estimates are as high as 26 million unnamed species!
Biodiversity is declining world wide; because of human activities, rates of species extinctions are
higher than at any time in the previous history of Earth. The most important cause of these extinctions is
habitat destruction, fragmentation and degradation.
In order to decide which areas to protect, we must be able to recognize and quantify the
biodiversity of different areas. The purpose of this lab is to provide several choices in activities to measure
biodiversity and assess the health of the Harriet Creek Ecosystem.
About the Diversity Index:
A diversity index is a mathematical measure of species diversity in a community. Diversity indices
provide more information about community composition than simply species richness (i.e., the number of
species present); they also take the relative abundances of different species into account. Consider two
communities of 100 individuals each and composed of 10 different species. One community has 10
individuals of each species; the other has one individual of each of nine species, and 91 individuals of the
tenth species. Which community is more diverse? Clearly the first one is, but both communities have the
same species richness. By taking relative abundances into account, a diversity index depends not only on
species richness but also on the evenness, or equitability, with which individuals are distributed among the
different species.
Diversity indices provide important information about rarity and commonness of species in a
community. The ability to quantify diversity in this way is an important tool for biologists trying to
understand community structure.
A diversity index can be used to assess the diversity of any population in which each member
belongs to a unique species. A community dominated by one or two species is considered to be less
diverse than one in which several different species have a similar abundance.
Simpson’s Diversity Index is a measure of diversity which takes into account the number of
species present, as well as the relative abundance of each species. As species richness and evenness
increase, so diversity increases. The following formula is a means to calculate and quantify biodiversity:
n = the total number of organisms of a particular species
N = the total number of organisms of all species
The value of “D” ranges between 0 and 1. With this index, 1 represents infinite diversity and 0, no
diversity.
As an example, let us work out the value of D for a single sample of ground vegetation in the
beach area. Of course, sampling one area would not give you a reliable estimate of the diversity of the
beach flora. Several samples would have to be taken and the data pooled to give a better estimate of
overall diversity. Please note that there is no necessity to be able to identify all of the species, provided
they can be distinguished from each other. In the example data table below, please notice that the
species have been identified. This is not necessary, as long as you can identify and quantify each
species by its physical characteristics. You could define and name the species yourself, such as “species
A”, “species B”, and “species C”. Please remember that, when you are trying to identify Protistans in the
Harriet Creek samples!
Species
Number (n)
n(n-1)
Sea holly
2
2
Sand couch
8
56
Sea bindweed
1
0
Sporobolus pungens
1
0
Echinophora spinosa
3
6
Total
15
64
N = 15
n(n-1) = 64
Putting the figures into the formula for Simpson's Index:
Simpson's Index of Diversity = 0.7
With an Index of 0.7, the ecosystem is fairly diverse. The closer
the value is to 1, the more diverse the ecosystem, the healthier the ecosystem!
Procedure:
Your task is going to be to collect a sample of water at the Harriet Creek. You will then begin to classify
and count all of the organisms you find in your samples in order to calculate the diversity index, using
“Simpson’s Method”. Begin by building yourself a data table similar to the one above, but with
unlimited rows (accounting for unlimited number of species…in other words, leave the bottom off of it!).
This will be for your personal, as well as your group’s data. You will eventually need another table for
the classroom set, but that can be inserted at a later date. In your data table, you may also want to build a
column for qualitative data, as I’ll be looking for that when I grade this lab as well. In qualitative data,
you can describe movement, color, consumption observed…etc. You will be photographing each
organism you classify with a digital camera. I will have a few to offer you, or you may provide your
own. You may try doing this with your cell phone, but that is challenging.
These photos should be put in your quadrille in a section labeled “key”. This will be a significant
section for this lab, depending upon the print size of your photos…so don’t make them too large! In other
words, in the section you label “Key”, you will have photographic documentation of each species of
organism included in your count, so if your data table has listed “Organism A” as one of the species
you’re counting, in your key, you will indicate “Organism A” looks like this: (insert photo here) This
“identifying and counting” task will take multiple days. You may use my classroom key, or the internet
to help you classify or name your organisms, but remember, that is less important than actually getting a
correct number of each species. Each day, share your data with everyone in your lab group, so that each
of you has the same data in your table. Please keep in mind that while your data tables don’t have to be
identical, but the data does!
When we exhaust our supply of water, or after four days, (whichever comes first) we will be
compiling the classroom set of data on the board in a giant data table. You will calculate the Simpson’s
Diversity Index twice. One calculation will be done with your group’s data, and one with the entire
classroom’s data. In this way, you will see the value of larger samples in accuracy of calculations.
Analysis Questions: Diversity Index Lab
1.
Calculate the Simpson’s Diversity Index of the Harriet Creek for both your group’s data, and the
classroom data. Show all work. Describe the index as either healthy, or unhealthy. Justify. Were
the results of each calculation different? Why, or why not? Which were most accurate? Why?
2.
List the single most abundant species in your classroom data table, and write a plausible explanation
as to why this was the most abundant species.
3.
List the single scarcest species in your classroom data table, and write a plausible explanation as to
why this was the scarcest.
4.
If you conducted your count and diversity index calculations for the following areas, describe how
the values might differ from what you found.



5.
Death Valley, Ca.
El Yunque Rainforest, Puerto Rico
The East African Savanna
Why is knowledge of how to calculate and apply the Simpson Diversity Index beneficial for
someone to know? (besides the fact that you need to know it for the AP test!)