AP Environmental Science

## Learning Outcomes

- Understand how the **theory of island biogeography** explains colonization on islands
- Identify why **near islands** receive more colonists than **far islands**
- Explain how **immigration rate** changes as species richness increases
- Distinguish between **species richness** and **species turnover**
- Predict which islands have the greatest biodiversity using **distance** and **area**
- Apply island colonization ideas to **habitat fragments** on continents
- Use island biogeography reasoning in AP-style multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand island colonization by species, with a focus on the following syllabus points:

- Know that the **theory of island biogeography** explains species richness as a balance between **immigration** and **extinction**
- Identify **Robert H. MacArthur** and **Edward O. Wilson** as the scientists associated with the model
- Explain why **near islands** have higher colonization rates than **far islands**
- Understand that immigration is highest when an island has few species and declines as species accumulate
- Distinguish the effect of **distance from mainland** on immigration from the effect of **island area** on extinction
- Predict that **large-near islands** usually have the highest species richness and **small-far islands** the lowest
- Understand that **equilibrium species richness** means immigration rate equals extinction rate
- Know that **species turnover** can occur even when total species number stays near equilibrium

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why do islands closer to the mainland usually have more species than islands farther away?
2. What happens to immigration rate as the number of species already living on an island increases?
3. Which island would usually have the highest biodiversity: large-near, small-near, large-far, or small-far?
4. What is meant by equilibrium species richness?
5. Why does island biogeography matter for habitat fragments on continents?

## Introduction

## Introduction

Island biogeography is one of the most testable biodiversity topics in AP Environmental Science because it connects a simple model to many real situations. It explains how species colonize isolated habitats and why some islands, or island-like habitat patches, support more species than others.

For the exam, you need more than the basic statement that “near islands have more species.” You need to understand **why** colonization works that way, **how** immigration changes over time, and **what** the model predicts when distance and area are combined. You also need to avoid a common mistake: treating equilibrium as if nothing changes. In reality, islands are dynamic systems.

> **Key Term: theory of island biogeography**
> A model explaining species richness on an island as the balance between immigration of new species and extinction of species already present.

### What counts as an island in AP Environmental Science?

In this topic, an island does not have to be a piece of land surrounded by ocean. In ecology, an island can be any suitable habitat separated from other suitable habitat by an area organisms cannot easily cross.

That broader meaning matters on the exam. A true ocean island fits the model, but so do:

- forest patches surrounded by roads or farms
- isolated mountaintops
- wetlands cut off by development
- prairie remnants surrounded by suburbs

So when APES asks about island biogeography, think **isolated habitat patch**, not just “land in the sea.”

> **Key Term: colonization**
> The arrival and successful establishment of a species in a new habitat; in this topic, colonization and immigration are often used interchangeably.

The source of colonists is usually called the **mainland source pool**. This means the larger population, or set of populations, from which dispersers come. Seeds may blow in, birds may fly in, insects may be carried by wind, and some animals may raft or swim across short distances. The more isolated an island is, the fewer of these arrivals will succeed.

A common student error is to think colonization just means the first arrival ever. On APES, it usually means the ongoing arrival of species over time. Colonization is a rate, not just a one-time event.

### How immigration works and why it declines

The MacArthur-Wilson model starts with immigration. Imagine a brand-new island with zero species. At that moment, almost any species arriving from the mainland would be “new” to the island. Immigration rate is therefore high.

As more species become established, that rate falls. Why? Because fewer arriving organisms belong to species not already present. If a bird species is already established, another bird of the same species landing there does not add to species richness. Immigration is still happening in a literal sense, but the rate of **new species added** decreases.

> **Key Term: immigration rate**
> The rate at which new species arrive and establish on an island.

This downward trend is essential. Students often memorize “near islands have higher immigration” but miss the second part: immigration rate also depends on how many species are already there.

A useful way to reason through it:

1. Few species on island
2. Many empty niches or unoccupied roles
3. Most arrivals are new additions
4. Immigration rate is high

Then later:

1. Many species already present
2. Fewer unoccupied opportunities remain
3. More arrivals belong to species already there
4. Immigration rate falls

Near islands have a higher immigration curve than far islands because they are easier to reach. A bird is more likely to find a nearby island. Seeds are more likely to arrive. Storms and currents move more organisms successfully across shorter distances.

Large islands can also receive more colonists because they are bigger targets. But in the classic model, **distance** mainly shifts immigration, while **area** mainly shifts extinction. That distinction is a favorite exam trap.

> **Test Tip:** If a question asks why one island gets more colonists, mention **distance from the mainland** first. If it asks why one island loses fewer species, mention **area** first.

### Extinction, equilibrium, and species turnover

Immigration alone does not determine biodiversity. Extinction matters too. As species richness rises, extinction rate tends to rise. More species means more competition, smaller average populations for some species, and greater chance that one population will drop to zero after disturbance, disease, or random bad luck.

Small islands are especially vulnerable. They support smaller populations, fewer habitat types, and less buffering against chance events like storms or drought. A small island may lose an entire species from just one bad season.

Large islands usually have lower extinction rates because they provide:

- more total habitat
- more varied habitat types
- larger populations
- more chances for recolonization within the island itself

Where the immigration curve and extinction curve cross, the island reaches equilibrium species richness.

> **Key Term: equilibrium species richness**
> The number of species at which immigration rate equals extinction rate.

This does **not** mean the island becomes frozen in time. It means the total number of species tends to stay around a similar level. The identities of those species may still change.

> **Key Term: species turnover**
> The replacement of some species by others over time while the total number of species remains near equilibrium.

This is one of the most common misconceptions. Students often think equilibrium means no change. But if one species goes locally extinct and another colonizes, the total may stay the same even though the community composition changes.

Imagine an island with 20 species at equilibrium. A beetle species disappears after a storm, dropping the island to 19. Soon after, a new bird species colonizes, bringing the total back to 20. Species richness is stable, but species composition has changed.

> **Test Tip:** If an answer choice says equilibrium means “the same species remain permanently,” it is wrong. Equilibrium refers to **number of species**, not fixed membership.

### Near vs far, large vs small: making predictions

AP questions often ask you to compare four island types:

- large-near
- small-near
- large-far
- small-far

The safest overall prediction is:

- **large-near** = highest species richness
- **small-far** = lowest species richness

Why does large-near come out highest? Because it combines the best of both sides of the model:

- high immigration, since it is close
- low extinction, since it is large

Why does small-far come out lowest?

- low immigration, since it is far
- high extinction, since it is small

The middle two can sometimes be less straightforward, because one has better immigration and the other has better extinction. But for APES, the main must-know pattern is the top and bottom extremes.

A quick reasoning table helps:

| Island type | Immigration | Extinction | Expected richness |
| ----------- | ----------: | ---------: | ----------------: |
| Large-near  |        high |        low |           highest |
| Small-near  |        high |     higher |            medium |
| Large-far   |       lower |        low |            medium |
| Small-far   |         low |       high |            lowest |

Notice the important distinction:

- **near vs far** mainly changes colonization
- **large vs small** mainly changes extinction

Students often mix these up and say large islands have more species because they are closer, or near islands have more species because extinction is lower. Those statements can sound plausible but miss the main mechanism.

A classic AP-style question might ask:

“Which island has the highest biodiversity?”
If the options include a **large island close to the mainland**, that is usually the correct answer.

### Colonization in habitat fragments and why the model matters

Island biogeography is not just about ocean islands. It is heavily used to understand biodiversity loss on land when habitats are broken into fragments.

> **Key Term: habitat fragmentation**
> The breaking of a continuous habitat into smaller, isolated patches.

A forest cut by roads or farmland acts like a set of islands. Each patch is surrounded by unsuitable habitat for many species. Colonization between patches becomes harder, especially for species that cannot cross open land easily. Small patches also have higher extinction risk.

This is why fragmentation is such a serious threat to terrestrial biodiversity:

1. Habitat patches become smaller
2. Populations become smaller
3. Extinction risk rises
4. Movement between patches becomes harder
5. Recolonization slows
6. Biodiversity falls

Fragmentation also increases **edge effects**, which can reduce the quality of interior habitat. That further lowers suitability for some species, especially specialists.

Island biogeography therefore supports several conservation ideas:

- protect larger habitat areas where possible
- reduce isolation between patches
- maintain movement routes or corridors
- avoid breaking one large habitat into many small fragments

The theory does not solve every conservation problem, but it gives a strong framework for predicting where biodiversity is most at risk.

### Evidence and case studies students should know

One useful example is Darwin’s observations of finches in the Galápagos. The islands are isolated, and different islands supported distinct finch species. For APES, the key point is not detailed evolution theory, but that isolated islands can have unique colonization histories and distinct communities.

A stronger example for this specific subtopic is the mangrove island recolonization experiment by **Daniel Simberloff** and **E.O. Wilson** in the Florida Keys during the 1960s. Small mangrove islets were fumigated to remove arthropods, then observed as species recolonized them.

The findings matched island biogeography predictions:

- species returned fairly quickly
- total species number moved back toward its earlier level
- exact species identities were not always the same

That is powerful evidence for **dynamic equilibrium** and **turnover**.

Another example is Krakatau after the **1883** eruption. The islands were devastated and then gradually recolonized by plants and animals. This showed how colonization builds communities over time after isolation and disturbance.

These examples matter because they show the theory is not just abstract. It has been tested in both controlled and natural settings.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The theory of island biogeography explains species richness as a balance between immigration and extinction
- An ecological island can be a true island or any isolated habitat patch surrounded by unsuitable habitat
- Colonization and immigration are treated as the same idea in this APES topic
- Near islands have higher immigration rates because they are easier for species to reach from the mainland source pool
- Immigration rate is highest when an island has few species and declines as more species establish
- Extinction rate tends to increase as species richness increases
- Large islands usually have lower extinction rates because they support larger populations and more habitats
- Equilibrium species richness is the point where immigration rate equals extinction rate
- Equilibrium does not mean a fixed community because species turnover can still occur
- Large-near islands usually have the highest species richness
- Small-far islands usually have the lowest species richness
- Habitat fragments on continents can behave like islands and often lose biodiversity when they become small and isolated

## Key Terms and Concepts

- theory of island biogeography
- colonization
- immigration rate
- equilibrium species richness
- species turnover
- habitat fragmentation


Island Biogeography - Island colonization by species