Island biogeography with simulations

Overview

Biogeography explores global patterns in diversity. Two major factors that influence the number of species that are found in a given place are the amount of available habitat and connections among communities. The Equilibrium Theory of Island Biogeography explores these concepts by focusing on diversity on islands. Students will use a simulation to develop and explain this theory.

Introduction

One of the first global patterns of diversity that early naturalists observed was the relationship between area and the number of species that a space holds. This relationship was easy to describe on islands where species richness of multiple groups of animals or plants (taxa) had been described. For example, Figure 1 demonstrates this species-area relationship. For example, Cuba is the largest island in the Caribbean, and Redonda is one of the smallest. If you look at the number of species (amphibians and reptiles, classically studied as a group by herpetologists) found on each island (y-axis), Cuba has far more species than Saba does. Ecologists noted that species richness did not just increase with size; it tended to increase at a regular rate. In general, we find that an area that is 10 times larger has about twice as many species. Note this relationship appears linear in the figure due to the use of log scales on both axes!

While studying patterns of species richness on islands, two ecologists, Robert H. MacArthur and Edward O. Wilson, noted some exceptions to the rule. For example, some large islands had fewer species than expected due to their size, while some small islands tended to be more species-rich than expected. To explain these global patterns they proposed the Equilibrium Theory of Island Biogeography. It focuses on how size and distance from a "mainland", or source of species, influences island richness. To understand the Island Theory of Biogeography, let's first consider Figure 2.

Imagine there are two islands located off the coast of the mainland. Although the two islands are about the same size, the second island is located much farther away than the first island. If you are a bird that lives on the mainland, which island are you most likely to end up on? The answer is generally the first island. This means immigration (or colonization) is influenced by the distance of an island from the mainland (a source of colonists). Therefore, islands that are closer to the mainland are more likely to receive immigrants than islands that are further away. 

Once a species manages to reach and colonize an island, the rate of extinction is largely influenced by size of the island. This is because smaller islands tends to hold smaller populations (which are more likely to experience extinction due to stochastic effects like genetic drift!). Larger habitat size reduces the probability of extinction of the colonized species due to chance events. Smaller islands are also likely to holder fewer populations in general because they have less resources and less diversity of resources. Larger islands have larger and more habitat areas, which typically leads to more differences in habitat, or habitat heterogeneity. Higher heterogeneity means that there are more opportunities for a variety of species to find their suitable niches. Habitat heterogeneity also helps increase the number of species to successfully colonize after immigration. 

We could plot both immigration and extinction relationships on a single image, like is done in Figure 3. Not the y-axis is the number of species.

This basic graph makes a lot of assumptions but also offer a lot of insight. In this graph, immigration rates (blue lines) depend on proximity to mainland. Immigration rates also decline with species richness. That's because its' easiest to immigrate to an island when it is empty because all the resources on the island are available. As the islands get more full, colonizing the island becomes more difficult.

Extinction rates (orange lines), as noted above, depend on island size. We also see that extinction rates tend to increase with the number of species. This should make sense: If there are no species on an island, extinction is impossible, but as more and more species arrive (and compete) extinction becomes more likely. 

Figure 3 shows the basic Equilibrium Theory of Island Biogeography. It suggests that islands will reach an equilibrium, or stable number of species when immigration and extinction rates are equal! Note this model could be modified in multiple ways. Size of an island, for example, likely also impacts immigration rate (larger islands are easier to hit), and islands that are close together may also share individuals (the Rescue Effect), but even this simple conceptualization has proven useful for understanding global species richness patterns.

One interesting point to note is that an equilibrium number of species is reached when immigration and extinction rates are equal, not when those processes stop! This means islands may consistently be changing species composition but should maintain fairly consistent levels of species richness. As odd as this sounds, early tests of the Equilibrium Theory of Island Biogeography supported these assumptions. When mangrove islands off the coast of Florida were fully cleared of their invertebrate (insect and arachnid) communities and allowed to recolonize, islands eventually stabilized with communities of about the same richness as they had before disturbance. 

The island biogeography model has crucial applications for wildlife management, because wildlife reserves or patches of habitat can be considered “islands” of habitat in “an ocean” of an inhabitable area. 

In this exercise, we will simulate the island biogeography model using an online tool. The goal of this exercise is to investigate how island size and distance between an island and the mainland influence the equilibrium between immigration and extinction.

Methods

You will be using the Island Biogeography simulator found here: http://virtualbiologylab.org/ModelsHTML5/IslandBiogeography/IslandBiogeography.html

You will do a series of simulation trials by manipulating different model parameters and assessing the outcomes, described below. 

Getting Started

Before you get started, read through the Tutorial tab to familiarize yourself the different parameters of the model you'll be working with. You also can review the concepts in the Background Information tab, if you wish. 

Simulation Trials

You will need: 

• a timer (the timer on your phone works fine)

• a copy of the spreadsheet above to enter/visualize your data

First choose a habitat type (Tropical, Subtropical, Temperate, Tundra, or Desert) and record it in your spreadsheet. Then choose a taxon (animal group type) and record it also.

Trial 1- No Manipulation

Keep both islands as they are (same size, same distance). Set the speed to maximum (8x) and begin a timer for two minutes and let the simulation run for that length of time. At the end of the two minutes, hit the pause button, click on the the "To Data" tab and record in your lab spreadsheet: Current # of Species , Average # of Species, and Top 2 Most Abundant Species*. Now hit “Clear Islands” and repeat the procedures so you get a total of three trials.

 *If more than one species has the same number at the end of your simulation, list these too; e.g., if Species 3 and 4 both have the same top number of individuals, list both of them. This applies to the entire lab.

Trial 2- Distance

Now hit “Clear Islands” and return to the settings page. Move Island 2 away from the mainland to a distance of your choice.

Record the distance you chose where indicated in your spreadsheet (green cell).

Then run the simulation for another two minute. Then record your results in the Trial 2 box in your spreadsheet. Now hit “Clear Islands” and repeat the procedures so you get a total of three trials.

Trial 3- Size

Hit “Clear Islands” and return to the settings page. Move Island 2 back to its original position close to the mainland. Then shrink the diameter of Island 1 to a size of your choice.

Record the diameter you chose in your spreadsheet where indicated.

Then run the simulation for another two minutes. Record your results in the Trial 3 box in your spreadsheet. Now hit “Clear Islands” and repeat the procedures so you get a total of three trials.

 Trial 4- Size Plus Distance

Now hit “Clear Islands” and return to the settings page. Now alter the size and distance of either or both islands. 

Record the diameter(s) and distance(s) you chose in your spreadsheet.

Then run the simulation for another two minutes. Record your results in the Trial 4 box in your spreadsheet. Now hit “Clear Islands” and repeat the procedures so you get a total of three trials.

Trial 5- Migration and Mortality

Hit “Clear Islands” and return to the model settings page. Keep your island sizes and distances the same from Part 4. Now change the Migration and Mortality rates for both islands to a setting of your choice. You decide whether you make them higher or lower for your islands.

Record the Migration and Mortality Rates you chose in your spreadsheet.

Then run the simulation for another two minutes. Record your results in your spreadsheet. Now hit “Clear Islands” and repeat the procedures so you get a total of three trials.

Review Questions

1. How can you be sure that each of your trial islands reached an equilibrium between immigration and extinction? What conditions do you think would need to be met?

2. For each of your manipulations, compare the number of species (current) on the island at the end of each of your two minute trials. Were they similar? How did they compare to the average number of species?

3. Now, for each of your manipulations, compare the identity of your top two species at the end of each of your two minute trials. Were they always the same? Why or why not?

4. Now compare the number of species you found on the islands when you changed distance and size of island two separately. Which do you think played a bigger role in terms of species colonization: size or distance? Why do you think that? 

5. How did changing migration and mortality rates impact the number of species you found on the island at the of your trials? How do these parameters related to the basic components (immigration, extinction) of the Equilibrium Theory of Island Biogeography? How would changing these parameters impact the curves we see if Figure 3 (above)?

6. If you were to run the entire simulation series again using a different taxon (animal group, plural = taxa), do you think your results would be different? Do you think some taxa might be better at dispersing than others?

Extensions

Re-run the above trials while varying species or habitat type. Do these changes influence your results? What do your results suggest about the model?

Still interested?