In the equilibrium model of island biogeography, once an island has reached equilibrium,

We are now moving from a discussion of genetics, populations, and species to communities and ecosystems. The next few lectures will describe concepts of major importance to conservation in terms of the effects of habitat fragmentation and maintenance of species diversity.

A great deal of conservation research has been done on islands, because they are small, replicated units of area, isolated from other habitat. They are very useful for species, community, and ecosystem studies.

Early observations of biogeography involved the examination of the geography of biodiversity around the globe. This was followed by recognition of the species-area relationship - as area increases, the number of species present (diversity) also increases. This can be represented by one of two graphs, depending on the axes used:

1) a concave, upward slope (# of species vs. area)

or

2) a straight, upward sloping line (log(# of species) vs. log(area)).

If we use the second form of the graph, we find that the equation describing the line is

log (S) = log (c) + z log(A)

where z represents the slope.

What factors influence z?

- Climate, e.g. latitudinal gradient factors

- High average r across the community or group of species

- Habitat complexity

- Isolation, e.g. distance from the mainland

- Type of species represented, e.g. mammals vs. birds

Data collected by Harris for mountaintop islands in the Great Basin show that mammals have a higher z (steeper slope on the species-area graph) than birds.

Equilibrium Theory of Island Biogeography (ETIB)

The ETIB describes the theoretical relationship between immigration and extinction of species to islands, depending on their size and distance from the mainland or other species source.

Consider the degree of isolation of the area under study:

Isolate (oceanic and continental islands) vs. Sample (e.g. Amazon)

Oceanic islands are usually created by volcanic activity.

Continental islands are formed when the water level rises (e.g. glaciers melt).

How do species access these islands over time?

1) On oceanic islands, the number of species present increases over time until it reaches the level of the nearest mainland (theoretically the source of the species which immigrate to the island).

2) On continental islands, the number of species present decreases over time. Species richness "relaxes" to a new equilibrium depending on the degree of isolation and the size of the island.

According to ETIB, the number of species present on an island is determined by a balance between immigration and extinction. Generally, as the number of species present increases, the immigration rate decreases and the extinction rate increases.

There are two general relationships to remember:

1) Immigration is higher on near islands than on distant islands (in relation to the mainland), hence the equilibrium number of species present will be greater on near islands.

2) Extinction is higher on small islands than on larger islands, hence the equilibrium number of species present will be greater on large islands.

Therefore,

The number of species on near, large islands > The number of species on distant, small islands

Work by Simberloff and Wilson on mangrove islands in Florida has validated the ETIB:

They killed all of the organisms on various sizes of mangrove islands and different distances from the "mainland" source of species and measured recolonization rates. They found that near, large islands experienced faster recolonization than distant, small islands.

Much of ETIB, which was founded on the study of true islands, can be extended to islands in fragmented habitat. Island biogeography has become an essential component of conservation biology, particularly in the analysis of preserve design, which will be covered in the next lecture.

Islands usually have unique and interesting flora and fauna (but not Merryweather), like the giant tortoises of the Galapagos and the Komodo dragons of Komodo. They tend to have a lot of endemic species, which makes sense if we consider that islands are isolated so anything that evolves on an island probably doesn't occur elsewhere, unless it disperses off the island.

Komodo dragons. 

Islands are interesting from a biogeography standpoint because anything that lives on an island had to get there from somewhere else, or evolve from something that came from somewhere else. Everything living on islands has relatives in other places. Lucky relatives, having a nice vacation connection like that.

We can also think about islands in a broader context than just oceanic islands. There are other types of islands too. Mountain peaks can be islands in a sense, because they offer a specific alpine habitat at the top but are surrounded by lower elevation land. Species that are adapted to life at the top might not be able to survive in the surrounding lowlands. When this happens, the mountain peaks are called "sky islands." Earth is an island, Mars is an island, and all other planets are islands, whether or not they have beaches and palm trees.

Brain Snack

Sky islands have more mammals than anywhere else in the United States, and are where northern species ranges meet southern species ranges. Read or listen to a short newstory about them here.

Two scientists, Robert MacArthur and E. O. Wilson, developed a theory to describe island biodiversity. Their theory is called the island equilibrium model. The island equilibrium model describes the number of species on an island based on the immigration and extinction rates of species on that island. Species have to get to the island from somewhere else, which is the immigration part, and species go extinct from the island as they run out of resources.

To understand their theory, take a look at the graph above. The axis on the bottom of the graph is the number of species. If we follow the blue line, it shows that as the number of species increases, the rate of immigration begins to decrease. That's because as more and more species arrive, the chances grow that that species is already present.

Consider this…if you went to Mars with your dog and cat, Mars would go from a population of zero species to a population of three species. If your cousin came next, maybe she'd bring her cat and a goldfish. So now, when you include the goldfish, Mars has four different species on it. Going from zero to three species is a much bigger leap than going from three to four is.

Another factor to consider as a new place is colonized is that the rate of extinction increases. That's because (1) there's just more things to go extinct and (2) there's more competition. You can see this illustrated in the orange line of the graph.

The idea of this model is that the number of species will settle around where these two lines meet. Once the island (or Mars) has a certain number of species, it stabilizes in this middle zone.

This model predicts that ALL places have the same number of species, right? WRONG. Since each island, continent, planet, or refrigerator (you really might want to clean that thing out…) has different rates of immigration and rates of extinction, these lines cross in different places.

In MacArthur and Wilson's model, two things affect immigration and extinction rates. The first is how close the island is to the mainland (or Earth, if we are talking about Mars). The "mainland" is just the source of new immigrants to the island. Assuming all new species have to immigrate to the island from the mainland, closer islands will have more species on them than far islands, just because closer islands are easier to reach. Extinction is lower on islands close to the mainland because of the likelihood of immigration. There is more of a chance that new immigrants will arrive and keep a species in existence on that island. The graph below shows how an island closer to the mainland would have higher immigration rates, and therefore maintain a higher number of species. The taller the blue line is, the farther to the right the two lines cross. 

The second thing that affects immigration and extinction rates is the size of the island. For immigration, think of the island as a target, for birds flying above or geckoes riding seagrass rafts on the water (it happens, really!). A bigger target is easier to hit than a small one, and a big island is more likely to have species land on it by chance than a small one is. Larger islands have more space than smaller islands, so there are likely to be more resources available for species to use. The opposite is true for smaller islands. Therefore extinction rates are larger on small islands. This can be seen in the graph below.

If we graph the rates of immigration and extinction, we can see the number of species on an island varies:

The equilibrium part of the island equilibrium model refers to the number of species. The island will reach equilibrium when extinction rates equal immigration rates. That is the A, B, C, and D in the graph above, which are different depending on size and distance. 

The island equilibrium model is a great way to think about the things that influence species diversity on islands. However, it does not take into account everything that can happen. Disturbances such as hurricanes affect life on islands, and the model does not take into account evolution or interactions between species, such as competition.

What is the equilibrium theory of island biogeography?

What is the island equilibrium model?

When an island is at equilibrium for species richness?

How does island equilibrium occur?