BIO 300 Ecology
Fall 2005
1
LAB 7
ISLAND BIOGEOGRAPHY
Today’s Objectives
Today we will look at species numbers on habitat islands, specifically artificial substrates in a
stream. The approach is that of MacArthur and Wilson’s equilibrium theory of island biogeography, to
which you will attempt to put actual figures for immigration and extinction rates.
Introduction
How many species will there be on an island? There are some intuitive answers to that question
– it depends, perhaps, on such qualities as the size of the island and how close it is to the mainland
(where invading organisms come from). At any one time, the number of species on an island is surely
the number which got there minus the ones which have left (by becoming locally extinct). At the
beginning of the settlement of the island, each species which arrives is new to the island, so the
immigration rate is high. As time passes, the immigration rate would decrease since the organisms
which arrive are of species which are already there; only new species are counted in an immigration
rate. Counter to the immigration rate is the extinction rate. At first, the extinction rate is low: there are
few species to become extinct, and pressures such as predation and competition are low on the
uncrowded island. Later, as more species and more individuals settle on the island, extinction becomes
more likely.
These sorts of trends were included in the equilibrium model by MacArthur and Wilson (1963).
In general, as the number of species present on an island increases (the maximum is the mainland
species pool), the immigration rate decreases and the extinction rate increases (see below or Figure
12.10 on page 304 of Krohne). At the intersection of the curves, where the extinction rate equals the
immigration rate, the number of species is at an equilibrium call S (There should be a caret above the
S. It is read as “S hat”).
High
Extinction Rate
R
a
t
e
Immigration Rate
Low
Few
S hat
Number of Species
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This equilibrium number of species can be approached from the other direction too; an island could
have, temporarily, more species than S hat. This can occur when a large island becomes smaller
(caused, perhaps, by a higher sea level or by the construction of a parking lot). The species number is
said to “relax” to equilibrium as species become extinct there.
Another approach to estimating S hat is by a colonization curve. (see Figure 12.12 in Krohne or
Figures 1-3 in Simberloff and Wilson, 1969) This is a plot of the number of species present over time.
The colonization curve is expected to level off as S hat is approached.
All of these concepts can be applied to islands of habitats as well as ordinary islands
surrounded by water. A vacant lot surrounded by city cement, a decaying log in a forest, and a parasite
host can all be studied as islands. Perhaps a key concept is that an island can be a patch of an
unexploited resource. The islands used in this exercise are Hester-Dendy samplers placed in a stream.
These artificial substrates provide surfaces for stream organisms (aquatic invertebrates) that ordinarily
cling to rocks. Immigration and extinction rates and a colonization curve are plotted from the tally of
species on the samplers that have been in place for different lengths of time.
Today’s Procedure
The islands for this lab are Hester-Dendy plate samplers. These are 3” diameter hardboard
plates threaded onto a long eyebolt with hardboard spacers between the round plates. The plates and
washers are held onto the eyebolt with a nut and the whole arrangement can be disassembled. They
have fourteen plates, with a total sampling area of 0.16m2. In some years, I might also use leaf packs
as islands. These are mesh bags into which leaves are places. Every week for the past six weeks, two of
these samplers have been placed in the Mill creek, near I-83 and Mt. Rose avenue. The samplers were
placed in a shallow, but swift flowing section of the stream.
I will collect the samplers from the stream on the day of lab and bring them to campus. Each
sampler can be placed in a large, white plastic bin (white so the dark insects are easy to see) and the
plates removed from the eyebolt. Before placing the samplers in the sorting bins, we will filter the
water from the collection jar to reduce the amount of debris that cause the sorting water to become
murky. Each group will sort through the two samplers from one sample week. A little water in the
bottom of the plastic bin will allow the invertebrates to assume a 3-diminsional shape and will be
easier to identify.
The aim is to sort the organisms to species; it is not however, necessary to identify every
species in the sense of knowing its scientific name (much like the habitat diversity lab you conducted).
I will provide black and white drawings to help you identify which Order of insects an individual
comes from (it is relatively easy). All that is necessary is to know how many species were present for
each time period and how many of these were also present in the preceding time period. The best way
to identify the individuals will be to identify which order they belong to (i.e. Ephemeroptera,
Plecoptera, Megaloptera, Tricoptera, Odonata and Coleoptera). I will provide petri dishes that you can
sort them into. Within each petri dish, you can then determine how many different individuals that you
have. You might want to give each “species” an identifying code name (for example Trichoptera
species A). You will want to compare your species with the other groups to make sure that everyone
has the same individual listed as Trichoptera species A. Once all of the individuals are sorted and
recorded, all of the species counts (from all 14 samplers) will need to be recorded on the data sheet as;
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Continuing species – present last week as well as this week.
Immigrant species – not present last week but present this week.
Extinct species – present last week but not present this week.
Species present for weekx = continuing species + the immigrant species.
Make comparisons only between weekx and weekx-1 (e.g week7 and week7-1); a species may immigrate
and become extinct more than once in the series. For the first week, all the species are immigrants and
there are no extinct species.
For the Lab Write-up
A. Read both your textbook and the reserve readings on island biogeography. The equilibrium
theory predicts that the number of species on an island will not change over time. Once the equilibrium
between immigration and extinction has been reached, the value of S hat should be relatively constant.
The nonequilibrium theory predicts that the value of S hat will increase as a function of time until it
equals the number of species from the mainland. According to this alternate hypothesis, the
depauperate number of species on islands is because they have not had sufficient time since their
formation to accumulate species via immigration. That is, islands have not reached an equilibrium
state. Given enough time, islands will eventually accumulate all of the species from the mainland.
(Okay, so what’s your hypothesis?)
B. The equilibrium theory predicts that although the value of S hat will be relatively constant
over time, the actual species that make up S hat will change: that is, immigration and extinction are in
a dynamic equilibrium. The nonequilibrium theory predicts that as species arrive on the island, they
will remain on the island: the increase in S hat over time depends upon this. (This suggests another
hypothesis.)
A. Use the following procedures to help you address (test) your hypotheses. Construct separate graphs
for each set of upstream and downstream samplers.
1. Make a colonization curve by plotting the number of species present against time (on the horizontal
axis). (See Figures 12.10 in Krohne or Figures 1-3 in Simberloff and Wilson, 1969). Can you
make an estimate of the equilibrium number of species from the curve? If so, what is S hat?
2. Figure immigration and extinction rates for each interval. Since the samples are one week apart, the
rates will be simply the numbers immigrating and going extinct per week.
3. Plot immigration and extinction rates on the same graph (see figure on page 1 of these
instructions). For the horizontal axis, use the number of species present at the beginning of the
week (that is, the number at the end of the preceding week). For example; plot rates for week 2
against species present for week 1. For week 1, species present will be 0, the number going extinct
will be 0, and the number immigrating will be however many species were found. Draw a best-fit
straight line for immigration and one for extinction. (You can use a best fit linear regression for
this.)
4. Do the lines cross? (Extrapolate if necessary). Estimate S hat.
5. Although you plotted immigration and extinction curves as straight lines, do you think some
other shape might be more likely biologically? Why? (Think about how emigration and
immigration rates change as the number of species present increases.)
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These two questions relate to your hypotheses.
6. Suppose a Hester-Dendy sampler was left out 14 weeks rather than 7 weeks. What extinction
and immigration rates would you expect? Would there be twice as many species?
7. Which would have more species, a small island or a large island? Does the surface area of the
sampler matter? Would a doubling of the size of the sampler result in a doubling of species
number?
8. Which theory concerning the colonization of islands is best supported by your data, the nonequilibrium theory or the equilibrium theory?
Bonus - I didn’t have you count the number of individuals of each species. However, based on
the samplers you sorted through, was there a species (and what was it) that had such a large
representation that it might have influenced the colonization of the samplers by other
species?
C. Your Report.
1. No introduction.
2. No methods
3. Results - a. Your colonization curve- what is S hat?
b. A graph of your immigration and extinction rates. What is S hat?
c. A table with Simpson’s index for each week’s sampler.
4. Discussion – Address all questions in bold type.
If you cite a reference, provide a bibliography. (And how could you not cite a reference?)
References in the Library:
MacArthur, Robert H. and Edward O. Wilson. 1963. An equilibrium theory of insular zoogeography.
Evolution 17(4) 373-387.
Simberloff, Daniel S. and Edward O. Wilson. 1969. Experimental zoogeography of islands: the
colonization of empty islands. Ecology 50(2) 278-289.
These papers are the original sources for the information in your book (pp 481-487). Both studies
tested the assertions of the equilibrium hypothesis stated in A. and B. above.
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BIO 300 Ecology
Fall 2005
Week
Week 1
Cont.
Imm.
Week 2
Ext.
Cont.
Imm.
Week 3
Ext.
Cont
Imm.
5
Week 4
Ext
Cont.
Imm.
Week 5
Ext.
Cont.
Imm.
Week 6
Ext.
Cont.
Imm.
Week 7
Ext.
Cont.
Imm.
Ext.
Species
Totals
Species Present
(Cont. + Immigrants)
Immigration and
Extinction Rates
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X
X
X
X
X
X
X
X
X
X
X
X
X
X
BIO 300 Ecology
Fall 2005
Data Collection Sheet for Island Biogeography
Sample Week # ______________
SPECIES
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Which set are your samplers from? Upstream or Downstream?
#
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