Note from Mr. W: this tutorial was inspired by worksheets on this webpage, credited to Nick Peers-Dent. Permission to adapt Mr. Peers-Dent’s work is pending.

1. What is Biodiversity?

Biodiversity (short for “biological diversity”) is a measure of the number and variety of organisms in a particular area. In this century, as our growing human population increasingly exploits more and more of our planet’s surface, biodiversity has plummeted planet-wide — a topic we’ll address in the last module in this course.

Biodiversity can be approached from a variety of angles.

1. Ecosystem/habitat diversity is the number of habitats or ecosystems in an area. All other things being equal, an area with a variety of habitats — such as streams, forests, ponds and meadows — will have more biodiversity than an equal-sized area with just one habitat.

2. Genetic diversity is the amount of genetic variability within a particular species. Species with more genetic diversity are thought to be more adaptable, and more resilient to environmental change.

3. Species diversity is the focus of this tutorial. It involves two components: species richness, and species evenness. See if you can figure out what these concepts mean by completing the exercise below.

[qwiz style=”width: 650px !important; min-height: 400px !important;” random=”false”]

[h]Introducing Species Diversity

[i]

[q] Consider 3 communities of wildflowers, shown below as Communities A, B, and C. Think about what it means to be rich, and determine which of the communities shown below has the lowest species richness (enter A, B, or C)?

[textentry single_char=”true”]

[c*] A

[f] Nice job. Community A has the lowest species richness. That’s because species richness is a measure of the the number of species in a community. Community A has 3 species of wildflowers.  Communities B and C each have 4. Hence A has the lowest richness.

[c] Enter word

[c] *

[f] No. Species richness is a measure of the the number of species in a community. Count the number of species in each community, and choose a different answer.

[q] One meaning of the word “even” is “equal in value.” Keeping that definition in mind, which of the communities shown below has the lowest species evenness?

[textentry single_char=”true”]

[c*] C

[f] Awesome. Community C has the lowest species evenness. That’s because species evenness is a measure of how similar the species in a community are in their relative abundance. In Community C, there are 9 individuals of species 1; and 1 individual from species 2, 3, and 4. The other communities have a much more even spread of abundance from species to species (and are therefore more even).

[c] Enter word

[c] *

[f] No. Species evenness is a measure of how similar the species in a community are in their relative abundance.  Count the number of each type of flower in each community, you’ll find the answer.

[q]The number of species in a community is that community’s species [hangman]. That’s why community A, with 3 species, is the community that’s lowest for this measure.

[c]richness

[q]The relative similarity of the number of individuals of each species in a community is referred to as species [hangman]. That’s why community C, with so many more individuals of species 1 than individuals of species 2, 3, or 4, is the community that’s lowest for this measure.

[c]evenness

[q multiple_choice = “true”]To determine which community has the highest overall species diversity, you need to plug numbers into a formula. But knowing what you know about species richness and species evenness, which community would you guess has the highest species diversity?

[c]A

[f]No. It’s not A. For now, (before we’ve learned the formula), find the community that’s highest in both species richness and evenness.

[c*]B

[f]Fabulous. B is the community that’s highest in both species richness and evenness, which gives it the highest species diversity. Later, we’ll see how to determine species diversity mathematically.

[c]C

[f]No. C has high species richness, but low evenness. For now, (before we’ve learned the formula), find the community that’s highest in both species richness and evenness.

[q]Before going on, make sure you have a good understanding of these two components of species diversity.

  • Species richness: the number of species in an area under study.
  • Species evenness: how evenly distributed the members of any one species are in their numbers.

[x]

You’ve learned above that community A has the lowest species richness, and community C has the lowest species evenness. But which community, A or C, has the higher species diversity? For that, you need a formula, and that’s what we’ll address below.

[/qwiz]

 

Understanding the Simpson Biodiversity Index

Ecologists have combined measures of species richness and species evenness to create a variety of indices to measure a community’s overall species diversity. In this module, we’ll learn how to use one of these indices: the Simpson’s Diversity Index. 

The formula has a couple of variations. We’re going to go with the one that’s on the College Board’s AP Bio exam formula sheet. Here it is:

In this formula, n represents the total number of organisms of each individual species, and N represents the total number of organisms in the entire community. The Greek letter ∑ means “the sum of.”

The advantage of expressing the formula in the way that’s shown above is that higher values (values closer to 1) mean more diversity, and lower values (closer to 0) mean less diversity. That’s pretty easy to understand. But, truth be told, the expression used by the College Board is actually the “diversity index difference.” It’s the diversity value subtracted from one. The only time this will make a difference will be if you talk to someone who’s not taking an AP Bio class (or when you’re in future biology classes). If you’re already in one of those classes, please make sure that you understand diversity in the way your instructor is teaching it.

Use the quiz below to help you solidify your understanding of what high and low species diversity means in a biological community.

[qwiz]

[h]High and Low Biodiversity

[q labels = “top”]Low species diversity means:”

  • There are relatively _______ successful species in this community.
  • The environment might support only a small number of ecological ____________.
  • The environment might be stressful, and difficult for a species to ____________ to.
  • An environmental shift might force the few species in this environment to become locally ___________.
  • A diagram of this community’s food web would be relatively ____________.

[l]adapt

[fx] No, that’s not correct. Please try again.

[f*] Good!

[l]extinct

[fx] No. Please try again.

[f*] Good!

[l]few

[fx] No. Please try again.

[f*] Correct!

[l]niches

[fx] No. Please try again.

[f*] Great!

[l]simple

[fx] No. Please try again.

[f*] Great!

 

 

[q labels = “top”]High species diversity means:

  • Many available ecological ______________.
  • A large number of ______________ species.
  • An ecosystem that is able to remain ___________ in the face of environmental fluctuation.
  • A diagram of this community’s food web would be ______________.

[l]complex

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]niches

[fx] No. Please try again.

[f*] Great!

[l]stable

[fx] No. Please try again.

[f*] Correct!

[l]successful

[fx] No, that’s not correct. Please try again.

[f*] Correct!

 

[/qwiz]

The hot springs and pools of Yellowstone National Park. The colors are partially caused by the heat-loving bacteria and archaea that live in these pools.

It should be noted that there are some ecosystems that have low levels of biodiversity, but which are still “healthy.” Consider the geothermally heated pools in Yellowstone National Park. In this extremely hot water, where temperatures range between 32° and 79° C (90° – 174° F), you’d find only a handful of species of bacteria and archaea. But that’s about as diverse as things can get in this extreme environment.

The thing to watch for as we try to assess the health of a biological community is whether the biodiversity has fallen from an earlier baseline. On the other hand, a rise in biodiversity can be a sign that measures designed to increase the health of an ecosystem (such as the reintroduction of wolves into Yellowstone) has been successful.

 

 

Calculating Biodiversity using the Simpson Diversity Index Formula

To learn how to calculate biodiversity, let’s walk through an example. You’re comparing biodiversity in three communities. In the table below “Sp.” stands for species, and C1, C2, and C3 are the three communities.

In each community, the same 10 species are potentially present, but in different numbers (including 0). The total number of individual organisms in each community adds up to 100.

Sp. A Sp. B Sp. C Sp. D Sp. E Sp. F Sp. G Sp. H Sp.
I
Sp. J Total
C1 12 9 9 10 9 11 10 11 10 9 100
C2 68 9 3 4 1 3 4 3 2 3 100
C3 32 36 32 0 0 0 0 0 0 0 100

Our formula is:

For any operation like this, I like to create tables (but this can also be done on a scientific calculator). Find out how your instructor wants you to do it, and to what degree they want you to show your work.

We’ll start with community 1.

Step 1: Set up your table. Fill in “n,” which is the number of individuals of each species.

Sp. A Sp. B Sp.
C
Sp. D Sp.
E
Sp.
F
Sp. G Sp. H Sp.
I
Sp.
J
TOTAL (N)
n 12 9 9 10 8 11 10 11 10 9 100
n/N
(n/N)2

Step 2: For each species, divide n by N (the total number of individuals in the community). In this case, N = 100.

Sp.
A
Sp.
B
Sp.
C
Sp. D Sp.
E
Sp.
F
Sp. G Sp.
H
Sp.
I
Sp.
J
TOTAL (N)
n 12 9 9 10 8 11 10 11 10 9 100
n/N 0.12 0.09 0.09 0.1 0.09 0.11 0.1 0.11 0.1 0.09
(n/N)2

Step 3: For each species, take the value of n/N and square it. In other words, calculate (n/N)2.

Sp.
A
Sp.
B
Sp.
C
Sp. D Sp.
E
Sp.
F
Sp. G Sp.
H
Sp.
I
Sp.
J
TOTAL (N)
n 12 9 9 10 8 11 10 11 10 9 100
n/N 0.12 0.09 0.09 0.1 0.09 0.11 0.1 0.11 0.1 0.09
(n/N)2 0.0144 0.0081 0.0081 0.01 0.0081 0.0121 0.01 0.0121 0.01 0.0081

Step 4: Add together all the values of (n/N)to get ∑(n/N)2 .

Species
A

Species
B
C C E F G H I J TOTAL (N)
n 12 9 9 10 8 11 10 11 10 9 100
n/N 0.12 0.09 0.09 0.1 0.09 0.11 0.1 0.11 0.1 0.09
(n/N)2 0.0144 0.0081 0.0081 0.01 0.0081 0.0121 0.01 0.0121 0.01 0.0081 0.101

Step 5: subtract ∑(n/N)from 1. (That’s because we’re using the College Board’s formula, which is the Diversity Index Difference).

1 – 0.101 = 0.899. Remember that for this formulation of the Simpson Biodiversity index, values closer to 1 indicate higher biodiversity, while values closer to 0 represent lower biodiversity.

Now calculate the species diversity for community 2. Try to do it on your own, but I’ll break it down step by step in the set of cards below.

[qwiz style=”width: 800px !important; min-height: 400px !important;”]

[h]Calculating Species Diversity: Example 2

[q]Step 1: Set up your table. Fill in “n,” which is the number of individuals of each species.

Sp. A Sp. B Sp.
C
Sp. D Sp.
E
Sp.
F
Sp. G Sp. H Sp.
I
Sp.
J
TOTAL (N)
n 68 9 3 4 1 3 4 3 2 3 100
n/N
(n/N)2

 

[q]Step 2: For each species, divide n by N (the total number of individuals in the community). In this case, N = 100.

Sp.
A
Sp.
B
Sp.
C
Sp. D Sp.
E
Sp.
F
Sp. G Sp.
H
Sp.
I
Sp.
J
TOTAL (N)
n 68 9 3 4 1 3 4 3 2 3 100
n/N 0.68 0.09 0.03 0.04 0.01 0.03 0.04 0.03 0.02 0.03
(n/N)2

 

[q]Step 3: For each species, take the value of n/N and square it. In other words, calculate (n/N)2.

Sp.
A
Sp.
B
Sp.
C
Sp. D Sp.
E
Sp.
F
Sp. G Sp.
H
Sp.
I
Sp.
J
TOTAL (N)
n 68 9 3 4 1 3 4 3 2 3 100
n/N 0.68 0.09 0.03 0.04 0.01 0.03 0.04 0.03 0.02 0.03
(n/N)2 0.4624 0.0081 0.0009 0.0016 0.0001 0.0009 0.0016 0.0009 0.0004 0.0009

 

[q]Step 4: Add together all the values of (n/N)2.

Sp.
A
Sp.
B
Sp.
C
Sp. D Sp.
E
Sp.
F
Sp. G Sp.
H
Sp.
I
Sp.
J
TOTAL (N)
n 68 9 3 4 1 3 4 3 2 3 100
n/N 0.68 0.09 0.03 0.04 0.01 0.03 0.04 0.03 0.02 0.03
(n/N)2 0.4624 0.0081 0.0009 0.0016 0.0001 0.0009 0.0016 0.0009 0.0004 0.0009 0.4778

 

[q]Step 5: subtract ∑(n/N)from 1.

1 – 0.4778 = 0.5222

[/qwiz]

 

Now calculate the species diversity of community 3. Clicking “show me the answer” will show you the completed table with the calculated diversity index.

[qwiz style=”width: 800px !important; min-height: 400px !important;”]

[h]Calculating Species Diversity: Example 3

[q]Step 1: Set up your table. Fill in “n,” which is the number of individuals of each species.

Sp. A Sp. B Sp.
C
Sp. D Sp.
E
Sp.
F
Sp. G Sp. H Sp.
I
Sp.
J
TOTAL (N)
n 32 36 32 0 0 0 0 0 0 0 100

 

[c*] [show_me_placeholder]

[f]

Steps 2-4

Sp.
A
Sp.
B
Sp.
C
Sp. D Sp.
E
Sp.
F
Sp. G Sp.
H
Sp.
I
Sp.
J
TOTAL (N)
n 32 36 32 0 0 0 0 0 0 0 100
n/N 0.32 0.36 0.32 0 0 0 0 0 0 0
(n/N)2 0.1024 0.1296 0.1024 0 0 0 0 0 0 0 0.3344

 

Step 5: subtract ∑(n/N)from 1.

1 – 0.3344 = 0.6656

[/qwiz]

 

Now that we’ve calculated the species diversity for each community, let’s step back and think about what we’ve found.

[qwiz style=”width: 800px !important; min-height: 400px !important;”]

[h]Calculating biodiversity: Reflection

[i]

Source: Aspire sustainability. Permission pending

[q]Here’s the data we started with. Note that I’ve filled in the Diversity index difference on the column on the far right.

Sp. A Sp. B Sp. C Sp. D Sp. E Sp. F Sp. G Sp. H Sp.
I
Sp. J Total Diversity index difference
C1 12 9 9 10 9 11 10 11 10 9 100 0.899
C2 68 9 3 4 1 3 4 3 2 3 100 0.522
C3 32 36 32 0 0 0 0 0 0 0 100 0.6656

It makes sense that community 1 has the highest overall species [hangman] because it has the highest number of species, also known as “species [hangman].” All of the species in community 1 are more or less equal in abundance, meaning that community 1 also has the highest species [hangman].

[c]diversity

[c]richness

[c]evenness

 

[q]Interestingly, even though community 3 is the [hangman] in terms of species [hangman], it’s high level of [hangman] puts it above community 2, which has high species [hangman] but low species [hangman].

Sp. A Sp. B Sp. C Sp. D Sp. E Sp. F Sp. G Sp. H Sp.
I
Sp. J Total Diversity index (difference)
C1 12 9 9 10 9 11 10 11 10 9 100 0.899
C2 68 9 3 4 1 3 4 3 2 3 100 0.522
C3 32 36 32 0 0 0 0 0 0 0 100 0.6656

 

[c]lowest

[c]richness

[c]evenness

[c]richness

[c]evenness

[/qwiz]

 

Simpson Index Practice Problems

For each of the problems below, calculate the answer on your own. Then click “Show the Answer” to see if you got it right.

[qwiz style=”width: 800px !important; min-height: 400px !important;”]

[h]Simpson Index Diversity practice problems

[q]PROBLEM 1: An ecologist in a study area in Redwood National Park gathers the following data.

Redwoods Douglas Firs Western Hemlocks Sitka Spruce
84 90 7 42

Calculate the diversity index difference for this patch of forest.

[c*] [show_me_placeholder]

[f]

Redwoods Douglas Firs Western Hemlocks Sitka Spruce
n 84 90 7 42 223 N
n/N 0.38 0.40 0.03 0.19
(n/N)2 0.14 0.16 0.00 0.04 0.34 D

Diversity index different = 1 – D = 1 – 0.34 = 0.66

[q]PROBLEM 2: An African Park contains the following array of mammals.

Hyenas Gazelles Giraffes Lions Elephants Wildebeest
84 90 7 42 15 115

 

Calculate the diversity index for the mammals in this park.

[c*] [show_me_placeholder]

[f]

Hyenas Gazelles Giraffes Lions Elephants Wildebeest
n 84 90 7 42 15 115 353 N
n/N 0.24 0.25 0.02 0.12 0.04 0.33
(n/N)2 0.06 0.07 0.0004 0.01 0.002 0.11 0.14 D

 

Diversity index different = 1 – D = 1 – 0.14 = 0.86

[/qwiz]

Updated, for sync, 8/10/20

What’s next

This tutorial ends this module on Community Ecology. Please choose another module, or use this link to return to the community ecology main menu.