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Evaluating Biodiversity in the Field
A discussion of biodiversity requires an understanding of several terms and concepts.
Biodiversity refers to the population heterogeneity of a community, or the number and variety
of species in a given area. A population is a group of individuals of the same species
occupying a given area at a given time. A community is the sum of all the population
interacting in a given area at a given time. Species diversity is widely viewed as a
combination of species richness and species evenness. Species richness is the total number
of species present in the community. It was formerly used to describe the diversity of a
community; it is not generally considered inadequate because it fails to take into account the
evenness of those species. Species evenness is the relative distribution of individuals among
the species present in a community. Evenness contrasts with dominance, and is maximized
when all species have the same number of individuals. In this lab, we will examine the
Simpson’s Index.
For example, consider 2 communities of ants. Each community contains 3 species.
Community A contains 1000 ants of one species and 100 ants of each of the other 2 species.
Community B contains 400 ants of each of the 3 species. The communities have the same
species richness (3), but Community B has greater species evenness. As species richness
and evenness increase, the measure of biodiversity increases.
Why is biodiversity important? Biodiversity has been a hot topic for a variety of
reasons. A number of new medicines have been discovered by screening thousands of
organisms for anticancer or antiviral properties. One source for these different organisms is
the tropical rain forests. Rain forests are comprised of a wealth of species, few of which have
properties well-known to modern science. One common argument for preserving biodiversity
is that if we fail to do so, the organism with “the cure for cancer” may become extinct before
we realize its utility. While potential pharmacological uses of unknown biological chemicals
provide the most compelling and “newsworthy” rationale for preserving biodiversity, there are
also many other industrial uses for chemicals that may be isolated from undiscovered or littleexamined species.
In addition, there are other strong arguments for maintaining diversity. Current
agricultural practices commonly involve planting acres of a single genotype of a crop plant.
This is done because some traits of the genotype are superior to other strains. However, if
that genotype is susceptible to a pest insect or disease, the entire crop can be lost. In wild
populations, there is often enough genetic variability that a few individuals will be resistant to
the pest or disease, and will survive. But these survivors may not be the ones with the best
fruit properties or resistance to another disease. Maintaining the genetic diversity of our crop
plants and their wild relatives provides breeders with the “raw material” needed to select for
traits in response to new agricultural challenges.
Diverse communities are generally less susceptible to perturbation than otherwise
similar but less diverse communities. One of the reasons for this is that in diverse
communities there are often species with overlapping niches. If one is lost, the other still
serves a similar role in the functioning of the system.
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And finally, there is an aesthetic value to biodiversity. We find variety appealing.
Many people enjoy seeing diverse organisms. Zoos, gardens, and forests are popular places
because they provide us with the opportunity to see organisms we may not normally see.
So if biodiversity is important, how can we quantify it? Clearly, data must be collected
from the community, and then it must be analyzed. What samples need to be collected?
What is the best method for collecting them? How many samples do we need to effectively
represent the community? What method is best for analyzing the data? Scientists use small
pilot studies to predict the answers to these questions before they spend a lot of time and
money on a large-scale project.
Simpson’s Index of Diversity
As species richness and evenness increase, diversity increases. Simpson's Diversity
Index is a measure of diversity which takes into account both richness and evenness.
Simpson’s Index (D) measures the probability that two individuals randomly selected from a
sample will belong to the same species (or some category other than species). Here is the
formula:
Where ni is the number of each individual species, N is the total number of individuals, and D
ranges between 0 and 1. With this index, 0 represents infinite diversity and 1, no diversity.
That is, the bigger the value of D, the lower the diversity.
To calculate Simpson's Index for a particular area, the area must first be sampled. The
number of individuals of each species present in the samples must be noted. For example,
the diversity of the ground flora in a woodland, might be tested by sampling random quadrats.
The number of plant species within each quadrat, as well as the number of individuals of
each species is noted. There is no necessity to be able to identify all the species, provided
they can be distinguished from each other.
As an example, let us work out the value of D for a single quadrat sample of ground
vegetation in a woodland. Of course, sampling only one quadrat would not give you a reliable
estimate of the diversity of the ground flora in the wood. Several samples would have to be
taken and the data pooled to give a better estimate of overall diversity.
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Then, plug in the data into Simpson’s Index:
In this example, Simpson’s Index = 0.3
Pre-Lab Questions
1. Why would we want to rate the amount of biodiversity in a location (see first page of
the lab)?
2. Examine the table above and calculate the diversity index for the three samples (show
your work).
Sample 1 = ______________
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Sample 2 = ______________
Sample 3 = ______________
3. Use the results from the previous question to describe the biodiversity of each sample.
Now that you are familiar with the Simpson’s diversity index, let’s get to the lab:
Purpose:
We will attempt to evaluate the biodiversity between several micro-habitats on the
school campus. These micro-habitats will be within the same general ecosystem; however,
by assessing the number of different species that are observed, we will attempt to make
some conclusions about their relative importance.
Methodology:
We will use a sampling technique involving a quadrat which is 1 m2. These quadrats
come in all different sizes; however, for our purposes, this size will be the most appropriate.
Your task will be to determine the number of different species (morphotypes) in your
quadrat. YOU MAY NOT KNOW THE NAME of the species; however, you can designate it a
plant species “a” or animal species “x” with a short description to help identify it later. Count
the number to the best of your ability, proceeding in this manner for up to 8 different species.
Results:
Design a data table below where all the data from this microhabitat can be recorded
while you are in the field that resembles the following table:
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Additionally, record the following information:
General description of microhabitat:
General description of weather:
Specific Time of Day: _______________________________
Date: ____________________________________________
Analysis
Calculate the Simpson Index on the data from your quadrat. Show your work in your
lab notebook. Then, share this information with the rest of the class (on the board); make a
chart to show the information for the index for the other possible habitats (we will do this
together).
Sources of Error
What factors might have led to distorted data collection? What might be problematic
about only visiting your quadrat one time vs. more consistent visits for a longer stretch of
time?
Conclusions:
Compare the different regions with all of the class data. Which area was the most diverse?
Why do you think this was?
If we had come outside on a different day, would the results have been different?
Biodiversity Impacts Study
Read the following: http://www.scientificamerican.com/article/how-biodiversity-keeps-earthalive/ and answer the following questions (in complete sentences):
1) How does biodiversity affect biomass? Why might biomass be important to track?
2) In the second paragraph, what key term (from the last unit) does increased biodiversity
allow for to maximize the amount of resources utilized in a given ecosystem area?
3) What is the biggest driver of biodiversity loss?
4) On campus at Byron, how might you suggest we improve biodiversity? Should we
spend the money? Explain.
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