Download MEASURING BIODIVERSITY (REVISED) If you take a walk outdoors

General Ecology Lab (BIO 160)
Spring 2009
Dr. Jim Baxter
MEASURING BIODIVERSITY (REVISED)
If you take a walk outdoors, say along the American River or in the foothills of the Sierra
Nevada, you’ll encounter a tremendous variety of species – plant, animal, fungal, and microbial.
Even at this very local scale and in a brief walk, you’re likely to come across dozens if not
hundreds of different species. Each of these species has evolved to succeed in its particular
niche and is carrying out important ecological functions that affect many other species and even
ourselves. On a much larger scale, Earth’s ecosystems support an amazing diversity of
species. To date, scientists have identified and named approximately 1.4 million species
worldwide – and we’re still discovering new species every day. It is estimated that the total
number of species on planet Earth may be as much as 10 million!
Over the years, ecologists have discovered that the number of species in any particular
community can vary from very few species to many hundreds. For example, tropical rainforests
are considered to harbor some of the highest numbers of species per unit area on the Earth
whereas tidal marshes have relatively few species. These differences among communities
pose interesting questions. For example, why do some communities have more species than
others? What factors influence how many species a community has? How can so many
species coexist? What are the functional roles of the different species in an ecological
community? Which species are vital for maintaining certain ecosystem functions? The central
importance of these and other questions have prompted ecologists to study the concept of
biological diversity or biodiversity for short. Biodiversity describes the sum total variation of life
forms across all levels of organization – from genes to ecosystems.
Species abundance
Because biodiversity is a broad and complex concept, a variety of measures have been
created to measure it empirically. Most commonly, biodiversity is measured at the level of the
species. The simplest measure of species diversity is species richness, which is a count of
the number of species in a given area. Generally, species richness is determined for taxonomic
communities or functional groups, such as the plant community (taxonomic) or zooplankton
(functional) community. Another measure of species
diversity is species evenness, which is a measure of
Species 1
Species 2
how equitable the species in a community are in their
Species 3
abundance. For example, a community with high
Species 4
Species 5
evenness would be one in which all species are more or
less of equal abundance, whereas one with low
evenness would be one in which the community has one
or a few dominant species and many rare ones. To
illustrate this concept, four imaginary communities with
the same richness (5 species) are shown in Figure 1;
A
B
C
D
species evenness decreases from left (Community A) to
Community
right (Community D).
Figure 1. Abundance of species in four imaginary communities (A - D)
containing five species each.
Although there are some quantitative measures of evenness, an informative graphical
approach to describing evenness is to plot a rank-abundance curve. In this approach, species
are plotted in sequence from the most to the least abundant along the horizontal (x) axis, with
their abundances typically displayed in a log10 format on the y-axis. The advantage of a rankabundance curve is that both species richness and evenness are displayed together in a single
graph and any differences in these measures among communities can be quickly compared.
General Ecology Lab (BIO 160)
Spring 2009
Sea star present
Sea star absent
Log Abundance
For example, imagine two rocky intertidal
communities along the coast of California in which a
keystone species (i.e., sea star) is removed from one
community but not the other. A rank-abundance
curve of these two communities provides two basic
pieces of information about the species diversity of
these two communities (Figure 2). First, species
richness of the community with sea stars present is 30
and with sea stars absent is 17. Second, as indicated
by its steeper negative slope, the community with sea
stars absent has lower species evenness than the
community with sea stars present.
Dr. Jim Baxter
0
5
10
15
20
25
30
35
Species rank
Figure 2. Rank-abundance curves for two rocky intertidal communities in
which sea stars are either present or absent. The intertidal community
with sea stars present has both higher richness and evenness than the
community with sea stars absent.
Although these two measures are commonly used, ecologists have developed several
diversity indexes that combine both richness and evenness together into a single index of
diversity. These species diversity indexes are often used when comparing the diversity of one
community to another and rely on abundance or frequency data of species in a community. Two
commonly used diversity indexes are: Simpson’s diversity index and Shannon’s diversity index.
Simpson’s diversity index (D) is based on the probability that two individuals chosen
randomly from the same community belong to the same species. The index is calculated as
follows:
D=
1
s
∑p
2
i
i =1
where pi is the proportion of individuals of the ith species to the total number of individuals in the
community: ni/N (ni = the number of individuals of species i; N = the total number of individuals
of all species) and s is the total number of species in the community. Simpson’s index is
increased by having additional unique species (increasing species richness) and/or by having
greater species evenness; it ranges from 1 to s.
Shannon’s diversity index (H’) also combines richness and evenness into a single index of
species diversity and is a measure of the likelihood that the next individual in the sample will be
the same species as the previous sample. The index is calculated as follows:
s
H ' = −∑ pi ln pi
i =1
where pi is the proportion of individuals of the ith species to the total number of individuals in the
community: ni/N (ni = the number of individuals of species i; N = the total number of individuals
of all species); s is the total number of species in the community, and ln is the natural log.
Shannon’s index is also increased by having additional unique species (increasing species
richness) and/or by having greater species evenness. The index can also be scaled so it
ranges from 1 to s by taking the inverse natural log: eH’.
Lab Exercise
In this lab, we will compare the species diversity of two insect communities that occur on
different plant species along the American River. We will collect our insect samples in the field
using sweep nets and then preserve them for counting. To compare the diversities of our insect
General Ecology Lab (BIO 160)
Spring 2009
Dr. Jim Baxter
communities residing on our two plant species, we will compare them using the measures of
species diversity described above.
Orders of insect herbivores we’re likely to find...
Hemiptera – aphids, leafhoppers, cicadas
Coleoptera – beetles (ladybugs, etc.)
Diptera – flies (house flies, midge flies, crane flies, etc.)
Neuroptera – lacewings, antlions
Hymenoptera – ants, bees, wasps, sawflies
Orthoptera – grasshoppers, crickets, katydids
Counting the critters
Since we want to count the abundance of each insect Order, it’s best to do the following:
1. First scan your sample and separate individual insects into similar groups
2. Use the Key to Insect Orders to key your insects taxonomically into Orders
3. Count the number of individuals in each Order in both of your samples.
Data Analysis
Our objective is to compare the diversity of insect communities living on the two plant
species. To accomplish this, we will compare the two insect communities using the following
quantitative statistics/approaches: species richness, species evenness, Simpson’s index,
Shannon’s index, relative abundance, and rank abundance.
Steps:
1. Develop a testable hypothesis about the insect diversity or composition of your two
communities. Do you expect them to be the same or different?
2. Enter the number of individuals of each insect Order into the Excel spreadsheet
provided. Note: your ‘counts’ will be transferred to the appropriate sheets in your Excel
file to calculate and plot your diversity measures.
3. Plot (bar graph) the abundance of each species in your two insect communities.
4. Calculate and plot (bar graph) the means (± SE) for species richness, Simpson’s index
(D), and Shannon’s index (H’ & eH’) for each community.
5. Compare the species diversity of your two communities by conducting a t-test on your
species richness and Simpson’s index data.
6. Plot (scatter plot) a rank-abundance curve for each insect community.
ASSIGNMENT (10 pts; Due next week): Interpreting Your Results
The questions below require you to complete the data analysis section above. To receive
full credit, you must include the results above (graphical and statistical) to support your answers.
1. Which insect order/s was/were dominant on each plant? Which were rare?
2. Did your two insect communities differ in the relative abundance of species? If so, how?
3. Did your two insect communities differ significantly in species diversity? If so, how?
Note: Be sure to address both richness and evenness and to include any statistical
and/or graphical results.
4. Why do you think you got the results you did? Was your hypothesis supported? Be
sure to provide an ecological interpretation of your results.