ISLAND BIOGEOGRAPHY
Introduction
Biogeography is the study of large-scale patterns of distribution and abundance of
organisms. One of the longest-standing observations of early naturalists (and, later, of
biogeographers) is that islands typically contain fewer species of any particular group of organisms
than do continents. Furthermore, the number of species on an island is often a function of the area
of the island and its distance from a continent. In the 1960s, ecologists Robert MacArthur and E. O.
Wilson developed a graphical model to explain these observed species richness patterns on islands.
Their model focused on four variables:
(1) island size
(2) island distance from the mainland
(3) the colonization rate, or the rate at which new species arrive on the island
(4) the extinction rate, or the rate at which species currently on the island die off.
Methods
In this lab we will simulate the process of colonization and extinction of islands of different
sizes and, if time permits, different distances from a “mainland.” Our islands will be egg cartons,
and each cup in the egg carton will represent a niche that can be filled by a single species. Large
islands contain 12 niche spaces, while small islands contain only 6 niche spaces. We’ll use pingpong balls as our species, and they can colonize islands or go extinct as described below.
1. 1.
Obtain a large egg carton and a small egg carton from the lab supplies. Number the
cups in the small carton from 1 to 6. For the large egg carton, number the cups on the left
side from 1 to 6, and also number the cups on the right side from 1 to 6. You might want to
use separate colors to mark the large carton. Designate one side of the large carton as the
‘heads’ side and the other as the ‘tails’ side (we’ll use these during the extinction phase of
the experiment).
2. 2.
Either on the floor or the lab bench, mark off a distance of one meter. At one end
place the large egg carton; the other end should be marked with a piece of masking tape (or
it can be the end of the lab bench).
3. 3.
Obtain 4 ping-pong balls from the lab supplies. You will simulate colonization by
standing behind the tape mark and attempting to bounce the ping-pong balls into the egg
carton. Toss one ball at a time; each ball must bounce on the ground once before hitting the
egg carton. Only balls that fully settle into a cup in the carton count as becoming
established. Make note of the number of colonists (new species) on data sheet 1.
4. 4.
After you’ve tossed all four balls, you’ll simulate extinction. Toss a coin to decide
which side of the carton will be subject to extinction. Then toss a dice. If no ping-pong ball
occurs in the corresponding cup, then no extinction occurs. However, if a ping-pong ball
does sit in the corresponding cup, then that species has gone extinct; remove the ping-pong
ball forthwith! Again, note the results (either a zero for no extinction or a one if a species
goes extinct) in data sheet 1.
5. 5.
Repeat steps 3 and 4 for 30 rounds. Be sure to record the data in each round!
6. 6.
Repeat steps 3-5 after replacing the large egg carton with the small egg carton. Note
that here you do not have to flip a coin during the extinction simulation!
7. 7.
(IF WE HAVE TIME): Repeat steps 3-5 after moving the large egg carton to a
distance of 2 meters from the masking tape mark.
DATA ANALYSIS
1. 1.
Look through the second column of data sheet 1 (Initial # of species) and count the
number of rounds in which this value was zero. Multiply this number by 4 (the number of
balls thrown each round) and enter this value into the appropriate row in column 2 (# of
Colonization Attempts) of data sheet 2. What this tells us is the total number of attempts you
made to colonize the large carton when it had zero species already there.
2. 2.
Using only the rounds from part 1 above, add up all the corresponding values in
column 3 (# of New Colonists) of data sheet 1. Enter this total into the appropriate row of
column 3 (# of Successful Colonizations) of data sheet 2.
3. 3.
Repeat steps 1 and 2 for rounds in which the initial number of species is (in order) 1,
2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12.
4. 4.
Look through column 4 (Total # Species after Colonization) of data sheet 1 and
count the number of rounds in which this value is zero. Record this count in the appropriate
row of column 4 (# of Extinction Attempts) of data sheet 2. NOTE: DO NOT multiply this
by 4! We only do a single extinction event each round.
5. 5.
Using only the rounds from part 4 above, add up the corresponding values from
column 5 (# of Extinctions) of data sheet 1. Enter this total in column 5 (# of Successful
Extinctions) of data sheet 2.
6. 6.
Repeat steps 4 and 5 for rounds in which the total number of species after
colonization is (in order) 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12.
7. 7.
Perform steps 1-6 separately for the large island, the small island, and (if we do it)
the large distant island.
8. 8.
Each group should turn into me a copy of their data sheet 2.
LAB WRITE UP
We will compile our class data – if there is no time in class, I will post in on our website.
Using this data, you’ll need to do the following:
1. 1.
Calculate the mean colonization rate and mean extinction rate for each initial
number of species. To do this, simply use the formulas below:
a. a.
Colonization rate = (# of Successful Colonizations) / (# of Colonization
Attempts).
b. b.
Extinction rate = (# of Successful Extinctions) / (# of Extinction Attempts).
c. 2.
On the same graph, plot colonization rate vs. initial number of species AND
extinction rate vs. initial number of species. You can do this in Excel by setting up 3
columns (initial # of species in column A; colonization rate in column B; extinction rate in
column C), highlighting these three columns of data, and doing a scatter plot with
connecting lines.
d. 3.
You should have separate graphs for the large island, the small island, and (if we do
it) the far large island.
e. 4.
On each graph, determine where the two lines cross. This will be the equilibrium
number of species, as predicted by the MacArthur-Wilson model.
f. 5.
Do your results conform to the predictions of the M-W model? Why or why not?
g. 6.
Looking at the last 5-10 rounds for each of your islands, does it look like they have
reached an equilibrium species richness? How does this compare to the predicted
equilibrium richness from part 4 above? If they differ, why might this be?
Round
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
DATA SHEET 1 (LARGE ISLAND)
Initial # of
# of New
Total # of
Species
Colonists
Species After
Colonization
0
# of Extinctions
Round
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
DATA SHEET 1 (SMALL ISLAND)
Initial # of
# of New
Total # of
Species
Colonists
Species After
Colonization
0
# of Extinctions
Round
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
DATA SHEET 1 (LARGE ISLAND AT 2 METERS)
Initial # of
# of New
Total # of
Species
Colonists
Species After
Colonization
0
# of Extinctions
Initial # of
Species
0
1
2
3
4
5
6
7
8
9
10
11
12
Initial # of
Species
0
1
2
3
4
5
6
Initial # of
Species
0
1
2
3
4
5
6
7
8
9
10
11
12
DATA SHEET 2 (LARGE ISLAND)
# of Colonization # of Successful
# of Extinction
Attempts
Colonizations
Attempts
# of Successful
Extinctions
0
0
DATA SHEET 2 (SMALL ISLAND)
# of Colonization # of Successful
# of Extinction
Attempts
Colonizations
Attempts
# of Successful
Extinctions
0
0
DATA SHEET 2 (LARGE ISLAND AT 2 METERS)
# of Colonization # of Successful
# of Extinction
Attempts
Colonizations
Attempts
0
# of Successful
Extinctions
0