ISLAND BIOGEOGRAPHY
Lab 7
Reminders!
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•
Bring memory stick
Read papers for Discussion
Key Concepts
•
•
•
Biogeography/Island biogeography
Convergent evolution
Dynamic equilibrium
Student Learning Outcomes
After Lab 7 students will be able to:
1. explain the principles of island biogeography
and relate them to terrestrial habitats.
2. demonstrate the complex interactions of longterm ecological processes in island
biogeography including immigration and
extinction using graphs and apply these ideas
to broader evolutionary concepts.
Book Chapters
•
I.
Campbell: Chp. 54.4
BIOGEOGRAPHY
Biogeography is the study of the spatial and
geographical distribution of organisms. The
central question of biogeography is, “Why do we
find these organisms at this location?” The
answer to this question includes both large scale
answers (the community of species found at a
given location is a function of the species pool
from the larger surrounding region) and small
scale answers (local competition and habitat
conditions determine community composition), as
well as, both historical (the community
composition is a function of species and
conditions that were there previously) and
modern answers (the community composition is
strictly a function of recent processes). When
discussing the composition of a community, we
can analyze it at many taxonomic levels (e.g. –
genus, family, etc.), not just at the species level.
Biogeography also deals with functional questions
of why certain areas tend to have similar or
different communities. For instance, Figure 1
Figure 1. Desert grasslands from the Gobi Desert in Asia
and the Chihuahuan desert in New Mexico. Can you tell
which one is which?
shows desert habitats from very distant locations
in Asia and North America.
Despite very
different geographical locations, they support very
similar communities of organisms. Similarly, the
organisms within those communities may be very
distantly related, but show convergent evolution
toward a similar ecological role. For example,
Figure 2. The Tasmanian wolf is more closely related to a
kangaroo than a wolf.
7-1
Laboratory 7
the recently extinct Tasmanian wolf (Figure 2)
was a predatory animal very similar to North
American and Eurasian wolves. However, it is a
marsupial and is more closely related to
kangaroos than wolves (yep, it’s got a pouch and
everything).
II. ISLAND BIOGEOGRAPHY
The theory of island biogeography was developed
by Robert MacArthur and Edward O. Wilson and
published in their book on Island Biogeography in
1967. Many early naturalists, including from
Captain Cook’s voyage, noticed a relationship
between the size of an island and the number of
species it supports (species richness), with larger
islands supporting more species.
In the
Galapagos Islands, MacArthur and Wilson
documented a similar pattern (Figure 3). This
trend is true not just for plants in the Galapagos,
but also for bird species in Hawaii, bat species in
the Caribbean, amphibians and reptiles in the
West Indies, and others.
Island Biogeography
dynamic equilibrium because the composition of
species on the island might change over time, but
the total number of species generally remains
about the same.
Imagine a new island that appears off the coast of
the mainland. Species that colonize this island
will come from the pool of species found on the
mainland (or whatever is the source of new
migrants). As the number of new immigrants
increases, the rate of immigration will slow down
because there will be fewer species remaining in
the pool of potential immigrants that could
migrate to the island. Immigration reaches zero
when all the species from the mainland pool have
reached the island. Initially, extinction rates on
this new island will be random. As more and
more immigrants arrive, however, extinction rates
will increase. This is because there are a larger
number of species on the island that could
potentially go extinct, as well as, because habitat
space is being filled up and more and more
species on the island are competing for fewer and
fewer resources. If we plot immigration rates and
extinction rates on the same graph, we can find
the equilibrium number of species at the point
where the two curves intersect (Figure 4).
Figure 3. The relationship between island size and the
number of plant species. (Image from Campbell and Reece
2005).
In MacArthur and Wilson’s view, the number of
species on an island is a dynamic equilibrium
between immigration and extinction rates. The
balance between the number of new species
arriving and the number of species going extinct
on the island will determine the equilibrium
number of species on the island. This is called a
Figure 4. The relationship between immigration and
extinction rates on the equilibrium number of species on an
island. (Image from Campbell and Reece 2005).
We can look at the effect of immigration on the
equilibrium number of species in more depth by
including the effect of distance from the pool of
7-2
Island Biogeography
Immigration rate
potential colonizers (Figure 5). Islands that are
close to the mainland will attract immigrants at a
higher rate for the simple reason that it is easier
for organisms to travel that shorter distance.
Near
island
Far
island
Immigration or extinction rate à
Laboratory 7
Near
island
Small
island
Large
island
Far
island
Equilibrium number of species
Number of resident species
Figure 5. The relationship between immigration rates and
the number of resident species.
We can also look at the effect of extinction on the
equilibrium number of species in more depth by
considering the size of the island. Larger islands
will have lower extinction rates because they
presumably have more space and a wider variety
of habitats so could support a larger number of
species (Figure 6).
Extinction
rate
Small
island
Large
island
Figure 7. Equilibrium species numbers with different
extinction and immigration rates. Immigration curves in red
and extinction curves in black. See also figures 5 & 6 for
more on immigration and extinction curves. (Image from
Campbell and Reece 2005).
Large islands that are close to the mainland will
support the largest number of species, while small
islands that are far from the mainland, will
support the fewest number of species. Distant
large islands and close small islands will support a
similar number of species.
The theory of island biogeography was originally
developed by researchers working on oceanic
islands. However, the concepts apply to any
isolated patch of habitat. This could include cold
mountain tops separated by warm valleys, ponds
separated by dry land, or separate individuals of a
host species for a parasite. A notable example in
Hawaii is a kipuka. A kipuka is a patch of forest
in which flowing lava has destroyed surrounding
areas making it an island of forest in a sea of lava
rock (Figure 8).
Number of resident species
Figure 6. The relationship between extinction rates and the
number of resident species.
By combining these two graphs, we can
simultaneously examine the effects of both island
size and distance from the mainland on the
equilibrium number of species (Figure 7).
Figure 8. Kipukas on the island of Hawaii.
7-3
Laboratory 7
Island Biogeography
III. LABORATORY EXERCISES
We have so far examined population and
community ecology from a variety of
perspectives. Today you will be examining the
effects of the immigration and extinction of
species on the make-up (species richness) of an
island community.
5.
6.
General Procedures
For this island biogeographical simulation, the
class will be divided into four teams of three or
four students. Collections of small cups will
represent islands, ping pong balls will represent
species, bouncing them into the cups will
represent immigration, and dice rolls will
represent extinction. You will keep a running
tally of immigration, extinction, and the species
richness of your islands, and then generate a graph
as in Figure 7.
Each group will be assigned one of the
following scenarios by your TA. At the end you
will combine the data for your homework
within your section and with some other
sections to have a big enough sample size.
Near Large Islands
The large island will consist of two sets of 6
numbered, plastic cups – one blue set and one
red set – placed next to each other.
1. Place a mark one meter away.
2. Take 5 ping pong balls and from one meter
away, try bouncing them into the cups that
make up the island (they have to bounce at
least once). Try a few practice rounds before
recording data.
3. A ball bouncing into a cup is a successful
colonization of a species onto that island. A
ball that lands on top of the cups is not
successful, nor is a ball that lands in the same
cup as another ball (think competitive
exclusion principle).
4. Everyone on the team should take turns at
colonization attempts to control for any
differences in colonization ability (i.e. –
bouncing skills) amongst group members.
7.
8.
Rotate jobs after each round (1 round= 5
balls)
On Data Sheet 1, record the starting number
of species in that round, the number of
successful colonizations, and the new total
number of species (i.e. # of balls in cups).
Randomly choose either the red or blue die
and roll to determine if extinction occurs. If
there is a ball in the cup with the number
matching the die roll, remove that ball – an
extinction has occurred. If there is no ball in
that cup, no extinctions have occurred.
(A piece of tape wrapped sticky side out can
help you remove the balls).
Record the new number of species on the
island for the beginning of the next round.
Repeat steps 2-7 for a total of 30 rounds.
Each round, attempt colonization with 5 ping
pong balls, regardless of events in previous
rounds.
Near Small Islands
The small island will consist of one set of 6
numbered, plastic cups – either blue or red.
1. Follow directions as for the near large island,
but with only one set of cups and one die for
extinctions.
2. Record data on Data Sheet 1. Note that the
max number of resident species you can have
in Datasheets 1A and 1B is 6 (not 12, so
ignore rows for species 7-12).
Far Large Islands
The large island will consist of two sets of 6
numbered, plastic cups – one blue set and one
red set – placed next to each other.
1. Follow directions as for the near large island,
but place the mark two meters away.
2. Record data on Data Sheet 1.
Far Small Islands
The small island will consist of one set of 6
numbered, plastic cups – either blue or red.
1. Follow directions as for the near small island,
but place the mark two meters away.
2. Record data on Data Sheet 1. Note that the
max number of resident species you can have
in Datasheets 1A and 1B is 6 (not 12, so
ignore rows for species 7-12).
7-4
Laboratory 7
Island Biogeography
Data Compilation
IV. ASSIGNMENT (35 pts.)
For each of the four possible island scenarios
above, you will be generating two graphs (eight
graphs total). One graph of immigration and
extinction rates vs. number of resident species
(as in Figure 4) and another graph of resident
species number vs. time (or numbers of rounds).
Remember to label the figure, the axes, and your
curves, and add a caption for each. To help you
with the data for these graphs, there are data
tables for compiling your section’s data.
Do not leave Lab early. If there is time after
the Class discussion, complete at least the
graphs for Question 1 before leaving. Make
sure you label all axes and add captions. The
Datasheets for question 3 will be a compilation
of several sections and will be provided to you
by your TA within 48hrs of your Lab.
Fill in the datasheet that corresponds to the
scenario you collected data for.
On the immigration data compilation sheet 1A,
the ‘initial number of resident species’ is from
column 1 of Data Sheet 1. The ‘number of
colonization attempts’ is the number of rounds in
which you started with X number of resident
species multiplied by five attempts (ping pong
balls) per round. (For example, if you had 8 of
the 30 rounds start with no resident species in the
cups, the ‘number of colonization attempts’ for
zero number of initial resident species is 8 * 5 =
40). For column 2 of Data Sheet 1A, to get the
‘number of successful colonization events’, sum
up the ‘number of successful colonization events’
from column 2 of Data Sheet 1 for those rounds in
which you started with X number of resident
species.
The extinction rate compilation sheet 1B is similar
to the immigration rate compilation sheet,
however, the ‘number of resident species after
colonization’ is from column 3 of Data Sheet 1.
In 1B, the ‘number of extinction trials’ is the
number of rounds in which you started with X
number of species from column 3. To get the
‘number of extinction events’, sum up the
‘number of extinction events’ from column 4 of
Data Sheet 1 for those rounds in which you started
with X number of resident species.
1. Use your own group data to plot the number
of resident species vs. time (rounds). (1 pt.)
Repeat this for each of the other
simulations using the data from the other
groups within your section (Near-Small,
Far-Large, Far-Small). (3 pts.)
2. Describe each of your four graphs from
Question #1 (e.g. - does the number of
resident species fluctuate, does it level off
quickly, etc.) in a cohesive paragraph
including the following questions:
• For each of your simulations, did the
number of resident species on the island
reach equilibrium? (i.e. – did it achieve a
stable number of resident species?)
• If it did reach equilibrium, how many
rounds did it take to reach that number? If
it did not reach equilibrium, why do you
think it didn’t?
• How do the plots for small islands
compare to large islands (think about the
shapes of the plots, the variability, and the
final equilibrium point)? How do near
islands compare to far islands?
• Is this what you expected according to
what
you
know
about
Island
Biogeography? (8 pts.)
3. For the Near-Large Island simulation, using
the compiled data from several sections
(provided by your TA and end of day or
the next day), plot the mean immigration
and extinction rates vs. number of resident
species on a single graph (see Figure 4).
Do a XY scatter plot. Add polynomial trendlines
for both data sets!! (1 pt.)
Repeat this for each of the other
simulations (Near-Small, Far-Large, FarSmall). (3 pts.)
7-5
Laboratory 7
4. a. What do your four graphs from Question
#3 predict to be the equilibrium number of
species for each of the four simulations?
(2 pts.)
b. Do these predictions match the number of
species you found at the end of 30 rounds
of your sections simulations? (1 pt.)
Why do you think that they do or do not
match? Explain. (1 pt.)
c. How do your sections simulations rank in
terms of the number of resident species
(e.g. large near island: 10 species, rank 1;
large far island: 8 species, rank 2; etc.)?
Island Biogeography
V.
REFERENCES
Blackburn, T.M., P. Cassey, R.P. Duncan, K.L.
Evans, and K.J. Gaston. 2004. Avian
Extinction and Mammalian Introductions
on Oceanic Islands. Science 305:19551958
MacArthur, R.H. and Wilson, E.O. 1963. An
equilibrium
theory
of
insular
zoogeography. Evolution 17(4):373-387.
Use number of initial resident species at end of
the 30 rounds for each scenario. (1 pt.)
Do the predicted ranks based on your
graphs (from class data see Question 1; number
of species at equilibrium) match the actual
ranks (your group data, see above)? (1 pt.)
5. Let’s say hypothetically, that one group in the
class was much better than others at
bouncing ping pong balls into cups. In our
simulation, this would symbolize a higher
immigration rate. Name two ecological or
environmental conditions in which you
might find variation in colonization rates
to islands that are otherwise matched for
size and distance from the mainland.
(5 pts.)
6. Write a cohesive paragraph on the material
you read for your class discussion today
according to the specifications of your TA.
(8 pts.)
7-6
Laboratory 7
Island Biogeography
Data Sheet 1: Island Scenario (circle one for both)
Round
#
1
Column 1
Initial Number of
Resident Species
(# of balls in cups)
0
Column 2
Number of
Successful
Colonization Events
Near / Far
Small / Large
Column 3
Number of Species
after Colonization
(Columns 1 + 2)
Column 4
Number of
Extinction Events
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
7-7
Laboratory 7
Island Biogeography
Data Sheet 1A: Immigration rate compilation table
Island Scenario (circle one for both)
Initial
number
of
resident
species
0
1
2
3
4
5
6
7
8
9
10
11
12
Column 1
Number of
Colonization
Attempts
0
Column 1
Number of
Extinction
trials
Small* / Large
Column 2
Number of
Successful
Colonization
Events
0
*If you have the small island then your max of
resident species is 6
Data Sheet 1B: Extinction rate compilation table
Island Scenario (circle one for both)
Number of
resident
species
after
colonization
0
1
2
3
4
5
6
7
8
9
10
11
12
Near / Far
Near / Far
Small* / Large
Column 2
Number of
Extinction
Events
0
*If you have the small island then your max of
resident species is 6
7-8