Species Distribution and Biogeography

To investigate the effects of island size and distance on species distribution, form groups of three. One member of the group will simulate colonization by throwing balls into the groups of cups representing the islands. Another will simulate extinction by rolling dice to randomly remove balls from the cups.

The third group member will record the data. Before starting the experiment, take time to familiarize yourself with the four island stations. There are two large islands with 12 cups and two small islands with six cups.

Two of the stations will have tape lines that are two meters away from the cups. These are the far islands. The other two stations will have tape lines that are one meter away from the cups.

These are the near islands. Thus, the stations are far large and near large and far small and near small. In this experiment, one alternate hypothesis might be that the larger islands with more cups will support a greater number of species than the smaller islands.

The null hypothesis might be that all islands will support this same number of species regardless of size. Another alternate hypothesis might be that islands closer to the mainland will have a higher colonization rate than islands farther from the mainland. The null hypothesis might be that all islands will have the same colonization rate regardless of their distance from the mainland.

Additionally, in all trials we may hypothesize that the colonization rate will be expected to decrease as more species inhabit an island whereas the extinction rate will increase as more species are found on the island. Remember, the point at which these rates meet is the dynamic equilibrium for that island. Meaning that the total number of species will remain relatively constant although the species composition may change.

The null hypothesis might be that the colonization rate and extinction rates are expected to stay the same across all numbers of species on the island. To begin the experiment, first choose one of the four stations as a starting place. Here, the students chose the near large island trial.

From the line of tape at this location, the colonization simulator must throw the ball to the island and try to get the ball into a cup. One round consists of five throws. Each throw representing a colonization attempt.

If the ball lands in a cup that does not already have a ball in it, this is a successful colonization attempt and the ball should be left in the cup. The data collector now places a tally on the colonizers box for the round in the recording sheet found at the station. If the ball does not land in a cup, retrieve it.

This is an unsuccessful colonization attempt. If a ball lands in a cup that already has a ball in it, retrieve it. This is also an unsuccessful colonization attempt.

Add the number of species at the beginning of the round with the tally of colonizers. Record this number in the column entitled number of species after colonization in the recording sheet found at the station. The extinction simulator should now roll a di.

If the trial is occurring at a large island site, choose one of the two colored dice at random with eyes closed. Roll the di. Remove the ball if there is one present from the cup that is of the same color as the di and labeled with the number rolled by that di.

Count the removal as an extinction in the station recording table. In a trial at the small island station, follow the same procedure except that it is only necessary to match the number rolled by the di to the cup. If there is no ball in the cup, no extinction is recorded.

For both the large and small islands, repeat the extinction roll for a total of two rolls if there are three to eight species present on the island and three rolls if there are nine to 12 species on the island. If there are fewer than three species on the island, do not roll the di again. This is the end of one round.

After this, count the total number of species currently in the cups at this station. And then tally this number into the number of species at beginning of round column on the station recording sheet for the next round. Groups should switch stations after five rounds at one table to control for the throwing ability of the colonization simulators.

Repeat the trials and rotation through the stations until 20 rounds have been recorded in total for all groups combined at all four stations. Count the tally marks for the number of species at the beginning of the round, colonizers, total number of species after colonization, and extinctions for each round at each island station. Enter this data in an Excel or a Google Docs spreadsheet made available to the entire class.

First, look at your data recording sheet. How many times were there zero species at the beginning of a round? How many times was there only one species at the beginning of a round?

Repeat this calculation for zero through 12 species present at the beginning of a round. For example, there were three rounds with six species present at the beginning of the round. Then, tally these values in the corresponding rows of the rounds with this many species column in the appropriate colonization rate calculation table.

In the colonization attempts column, multiply these values by five to represent the total number of ping pong balls thrown. Look at your data recording sheet once more and calculate the total number of colonizers for each value in the number of species at beginning of round column. For example, for the seven rounds with eight species present at the beginning of the round, there were 19 successful colonizers in total.

Record these values in the successful colonization attempts column of the appropriate colonization rate calculation table. Divide the successful colonization attempts by the colonization attempts for each initial species value and record this in the colonization rate column of the appropriate colonization rate calculation table. Repeat the procedure for determining the colonization rate for the near small, far large and far small trials, recording in the appropriately named table.

To calculate the extinction rate for each of the trials, return to the data recording sheet for the near large island trial. Again, using the data recording sheet, count the number of times that the number of species after colonization matched each number from one to 12 as previously demonstrated. Record these values in the rounds with this many species column in the appropriate extinction rate calculation table.

To calculate extinction attempts, multiply the value in the rounds with this many species cells by one if the species after colonization value is between zero and two by two if the value is between three and eight and by three if the value is nine or above. These values correspond with the number of times the di was rolled during the extinction simulation. Then, using the data contained in the data recording sheet, count the number of extinctions that occurred for each value in the number of species after colonization column.

For example, in the four rounds with eight species after colonization, there were four extinctions in total. Record these values in the extinctions column of the appropriate extinction rate calculation table. Divide the number of extinctions by the extinction attempts and record this value in the extinction rate column of the appropriate extinction rate calculation table.

Repeat these steps to determine the extinction rate for the near small, far large, and far small trials recording calculations in the appropriately named table. After this, plot the colonization and extinction rates versus the number of species for each of the island stations. Draw the best fit lines for the colonization and extinction rates in each of the four graphs.

Compare the colonization rate between the near and far islands. Which distance had a greater dynamic equilibrium of species? Why do you think this might be?

Next, compare the extinction rates between the large and small islands. Which size had a greater dynamic equilibrium number of species? Observe the slops of the colonization rates in the plots.

Were the slops of the colonization rates positive or negative? Now look at the slops of the extinction rates. Were these slops positive or negative?

Why do you think this might be the case for each rate?