Populations of a species often live in local areas that have various levels of connection with populations in other areas.  If a local population dies out, the area can be recolonized by migration from another area.  One of the simplest ways to study processes that lead to localized extinctions (technically called extirpations) is to use small islands.  Two scientists, Edward O. Wilson his graduate student, Daniel Simberloff, used tiny mangrove islands off the coast of Florida to study the processes of colonization (typically from the mainland) and extinction on each of these islands.  They used insecticides to kill all the arthropods on the islands and then monitored them for new species arriving and, after a species arrived, its future disappearance from the island (and later possible recolonization).  This was done to test a theory about the effects of distance of an island from the mainland and island size on rates of colonization and extinction.   

Another key tool of scientists is to think about what factors matter in a process and then build a mathematical model of how the process works.  So, in this case, the focus is on what factors influence the number of species on an island across time.  You can use past data to build your model and then make predictions for the future under a variety of scenarios.  

In this homework assignment, you will use a simplified computer model to view how this works.  

Go to the site: http://virtualbiologylab.org/ModelsHTML5/IslandBiogeography/IslandBiogeography.html

by clicking on the link within the assignment on CANVAS or copying and pasting above link.  

Read through the “Background Information” and “Tutorial” material, using the image of a house (home button) to return to the main page when needed.  

First, you are going to collect some starting (benchmark) data that you can use for comparison when you start to change things in the experimental set-up.

I.  Simulation # 1:  Click on “Run Experiments” from home page.  

Notice that your 2 islands are both 10 km from the mainland and 256 km wide.  

First, change the habitat from Tropical to Subtropical by clicking on the word “Subtropical”.

Leave the taxon as “Arthropods” – recall that arthropods include insects, spiders, mites, etc.  

Next, click on the “To Data” arrow at the bottom right.  You are going to collect data on # individuals on each island (large table at top) and average number of species per island (small table at lower right).  Do NOT write down numbers for current number of species (small table at lower right).  

You are going to run the model with these settings 5 times (to get average values).  Each time you run the model, you will let it go to a “run time” of 500.  

Directions:  Stop the model by clicking the stop button at lower right (button with 2 vertical lines).  Click “clear islands” and “reset average” (blue buttons).  Then, click the start button (large right pointing arrow at lower left).  Watch the “run time” and click the stop button when it gets to a run time = 500.  Note that you can speed up the model by clicking the button to the right of start/stop.  

After running the model once, fill in your data in the table on the next page, then click “clear islands” and “reset average” and run it again.  Run it for a total of 5 times, writing down your data each time.  Finally, calculate the averages for your five runs.   

NOTE:  The two pieces of data you will record are Total # Individuals = Island Total (last row for 2 columns of data at top) and Average # species (from small box on lower right of screen).  

To calculate an average for a column, add up the five values in that column, then divide by 5.  

Table 1:  Both islands 10 km from mainland; island size = 256 km diameter; run time 500

	Island 1 Total # individuals	Island 1 Avg. # Species	Island 2 Total # individuals	Island 2 Avg. # Species
Trial # 1
Trial # 2
Trial # 3
Trial # 4
Trial # 5
Average for each column

Note that this data shows you how much variability there is in the data for the 2 islands due purely to chance (since the relevant variables were all set to identical values).  In general, we will want to set up the two islands to be different for comparison purposes.

II.  Simulation # 2:  Click on the arrow at the bottom right called “To Design”

For this simulation, you are going to alter the distance from the mainland for one of the islands and see how this affects both total # individuals and average # species.  Using the arrows, for island # 2,  increase the “Island distance (km)) to 410 km, the maximum allowed in the simulation.   

Run this model 5 times and fill in your data in the table below.  Use the directions from simulation 1 if you have forgotten how to do any of the steps

Table 1:  Island 1, 10 km from mainland; Island 2: 410 km from mainland;  island size = 256 km diameter for both islands.   Run time = 500

	Island 1 Total # individuals	Island 1 Avg. # Species	Island 2 Total # individuals	Island 2 Avg. # Species
Trial # 1
Trial # 2
Trial # 3
Trial # 4
Trial # 5
Average for each column

 

What differences did you notice between the two islands, both as you watched the process of colonization and extinction, and based on what you see in the data?  Be specific.

_____________________________________________________________________________________

What do you think was the biggest impact of distance from the mainland (consider colonization rates and extinction rates)?  

_____________________________________________________________________________________

III.  Simulation # 3:  Click on the arrow at the bottom right called “To Design”

For this simulation, you are going to alter the size of one island and see how this affects both # individuals and average # species.  Using the arrows, set “Island distance (km)) to 110 for both islands AND set Island diameter to 64 for Island 2 (leave it at 256 for island 1).     

Run this model 5 times and fill in your data in the table below.  Use the directions from simulation 1 if you have forgotten how to do any of the steps

Table 1:  Both islands, 110 km from mainland; island size:  Island 1: 256 km²; Island 2: 64 km²

Run time = 500

	Island 1 Total # individuals	Island 1 Avg. # Species	Island 2 Total # individuals	Island 2 Avg. # Species
Trial # 1
Trial # 2
Trial # 3
Trial # 4
Trial # 5
Average for each column

 

What differences did you notice between the 2 islands, both as you watched the process of colonization and extinction, and based on what you see in the data?  Be specific.

_____________________________________________________________________________________

 

_____________________________________________________________________________________

What do you think was the biggest impact of island size (again, the focus is on colonization and extinction)?  

_____________________________________________________________________________________

IV.  Simulation # 4:  Design your own simulation to test something.  For this simulation, figure out something you want to test, set it up and run it. Don’t make it too complicated or it will be difficult to interpret your results. It is best to just alter a single variable at a time – a comparison here means something about your two islands should be different (though you can alter other factors overall too, such as taxon or habitat).

Provide a detailed explanation of what you compared and why, a table with your data, and your results and conclusions 

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