AP Environmental Science

## Learning Outcomes

- Define **island biogeography** in AP Environmental Science terms
- Explain why **species richness** changes with island size and isolation
- Understand how **immigration** and **extinction** determine island species numbers
- Distinguish **species richness** from broader biodiversity or species diversity
- Apply the idea to real islands and isolated habitat patches
- Predict which islands will have the highest and lowest species richness in exam questions

## AP Environmental Science Syllabus

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

- Know that island biogeography is the study of how island size and isolation affect the number of species an island can support
- Identify **species richness** as the main measure used in this topic
- Explain that the number of species on an island depends on the balance between **immigration** and **extinction**
- Understand that larger islands usually support more species than smaller islands
- Understand that islands closer to a mainland or other **source population** usually support more species than more isolated islands
- Recognise that AP Environmental Science can apply the idea to true islands and to isolated habitat patches on land
- Predict that **large, near islands** have the greatest species richness, while **small, far islands** have the lowest
- Understand that equilibrium means a stable total number of species, even though the identities of species can still change over time

## 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. What is island biogeography?
2. Why do larger islands usually have more species than smaller islands?
3. Why do islands near the mainland usually have more species than far islands?
4. What is the difference between species richness and species diversity?
5. What does it mean to say an island has an equilibrium number of species?

## Introduction

Island biogeography is a core AP Environmental Science idea because it links biodiversity patterns to clear environmental causes. Instead of treating species numbers as random, the theory explains them using a small set of ecological forces: how easily new species arrive and how likely existing species are to disappear.

For the exam, you need a definition that is simple but exact. You also need to be able to move beyond the definition and explain the reasoning behind it. A student who only memorises “big islands have more species” may miss questions that ask why that pattern happens or how it applies to fragmented habitats on land.

> **Key Term: island biogeography**
> The study of how island size and isolation affect the number of species an island or isolated habitat can support.

## Introduction to the definition

### What AP Environmental Science means by an "island"

In everyday language, an island is land surrounded by water. In AP Environmental Science, that is only the starting point. An island can also mean any suitable habitat surrounded by unsuitable habitat.

That matters because the theory is used more widely than just for oceanic islands. A mountaintop can act like an island if the species living there cannot easily cross the lowlands. A forest patch surrounded by roads, farms, or urban development can also act like an island. An isolated wetland between built-up areas can as well.

This broader meaning helps explain why island biogeography matters for conservation. Habitat fragmentation often creates “habitat islands” on land, and the same patterns appear there: smaller and more isolated patches usually support fewer species.

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

The definition still stays focused on two main variables:

- **Island area** or size
- **Isolation** or distance from a source of colonists

If a question asks for the definition, keep it centred on those variables and on species number.

### Species richness is the main measure

Island biogeography usually focuses on the number of different species present, not on how evenly individuals are spread across those species.

That is why **species richness** is the key measurement here. A large island with 40 species is richer than a small island with 12 species, even if the small island has a more even spread of individuals among its species.

Students often confuse species richness with species diversity. Diversity is broader. It can include richness, but may also include evenness. On AP Environmental Science questions about island biogeography, richness is the safer and more precise term unless the question specifically asks about biodiversity more generally.

> **Key Term: species richness**
> The number of different species present in a given area.
>
> **Test Tip:** If a multiple-choice question asks what island biogeography predicts, look for an answer about the **number of species**, not the total number of organisms.

## The immigration-extinction model

### Why the number of species is not fixed

The theory of island biogeography, associated with **Robert H. MacArthur** and **Edward O. Wilson**, argues that the number of species on an island tends toward a balance between new species arriving and existing species disappearing. Their 1967 work gave this idea its classic form.

An island does not have a species number written into nature forever. Instead, the number shifts over time as species colonise the island and as local populations die out. The important idea is that these two processes can balance each other.

- **Immigration** adds species
- **Extinction** removes species

At first, when an island has few species, immigration can raise species richness quickly because many possible colonists have not yet arrived. But as more species establish, there are fewer “new” species left to immigrate. So the immigration rate falls.

At the same time, when more species are packed into the same island, competition, chance events, and small population sizes make extinction more likely. So the extinction rate rises.

Where the two rates balance, the island tends toward an equilibrium number of species.

> **Key Term: immigration**
> The arrival of new species from a mainland or other source population to an island or isolated habitat.
>
> **Key Term: extinction**
> The local disappearance of a species from an island or isolated habitat.
>
> **Key Term: equilibrium number of species**
> The stable number of species reached when the rate of immigration equals the rate of extinction.

This equilibrium is dynamic, not static. That means the total number of species can stay similar even if the actual species present change over time.

For example, suppose an island tends to support about 20 species. One bird species may disappear after a storm, while another bird species later colonises from the mainland. The total may remain near 20 even though the membership changes.

### Why immigration falls and extinction rises

Students often memorise the intersection idea without understanding the logic. The logic matters.

Immigration falls as species richness rises because:

1. Early on, many mainland species are absent from the island, so there are many possible colonists.
2. Later, more of those possible colonists are already present.
3. Therefore, each additional arrival is less likely to be a truly new species.

Extinction rises as species richness rises because:

1. More species on the island means more competition for space, food, nesting sites, or other resources.
2. Populations may be smaller when more species share the same area.
3. Small populations are easier to wipe out through disease, storms, drought, or random bad luck.
4. Therefore, extinction risk increases.

That is why the theory is about process, not just pattern. The visible pattern is species number. The mechanism is immigration versus extinction.

> **Test Tip:** If an FRQ says “justify your prediction,” do not stop at “large islands have more species.” Add the process: “because larger islands have lower extinction rates and often more habitat types.”

## Why size and isolation matter

### Why larger islands usually support more species

Larger islands generally have more species richness for several linked reasons.

First, they contain more total habitat. More area can support more individuals, and larger populations are less likely to fall to zero after a disturbance. A storm, disease outbreak, or drought that wipes out a tiny population on a small island may only reduce a larger population on a big island.

Second, larger islands often contain more varied habitats. A small island might have only one or two environmental conditions. A larger island may include different elevations, moisture levels, soils, shoreline types, or vegetation communities. More habitat types create more niches, so more species can establish.

Third, large islands are bigger “targets” for dispersing organisms. A bird, insect, or wind-blown seed is more likely to encounter a bigger island than a tiny one. So size can support richness partly through lower extinction and partly through easier colonisation.

A common AP shortcut is:

- Bigger island → lower extinction rate → more species

That shortcut is correct, but you should also remember habitat variety.

### Why nearer islands usually support more species

Nearer islands usually have greater species richness because they receive immigrants more easily.

A mainland, or another occupied island, acts as a **source population**. The closer an island is to that source, the easier it is for seeds, insects, birds, and other organisms to reach it. Wind can carry some propagules. Ocean currents can move others. Flying animals can arrive directly. But distance reduces the odds.

A far island is harder to find and harder to reach. Fewer colonists arrive, so immigration rates are lower. With fewer arrivals to replace local extinctions, the total number of species tends to be lower.

This gives the second AP shortcut:

- Near island → higher immigration rate → more species
  > **Key Term: source population**
  > A mainland or nearby island population from which new species can colonise an island.

Put the two shortcuts together and you get the standard ranking:

- **Large and near** = highest species richness
- **Small and far** = lowest species richness

The two mixed cases, large-far and small-near, depend on the diagram or context. Large-far benefits from low extinction. Small-near benefits from high immigration. Exam questions may ask you to compare them carefully.

## Applying the definition in AP questions

### Using the idea on real islands and habitat islands

AP questions may use actual islands such as the Galápagos, but they can also use forest fragments, lakes, mountaintops, or patches of grassland between roads and urban development.

The logic stays the same.

A forest patch surrounded by farmland behaves like an island because many forest species cannot move easily across the non-forest land. If that forest patch is small, interior species may disappear more easily. If it is isolated, fewer replacement colonists arrive.

This is why island biogeography helps in preserve design. Larger protected areas are usually better than smaller ones, and connected patches are usually better than isolated ones because they reduce extinction risk and improve immigration and gene flow.

Do not overstate this, though. The definition itself remains about species richness, island size, and isolation. Preserve design is an application of the theory, not the definition.

### Common mistakes students make

One common mistake is saying island biogeography is only about oceanic islands. AP Environmental Science uses the broader meaning of isolated habitat patches.

A second mistake is treating equilibrium as permanent sameness. Equilibrium means stable **number**, not stable **identity**.

A third mistake is confusing species richness with total population size. An island may hold many individuals of just a few species. That does not mean high species richness.

A fourth mistake is saying large islands have more species simply because they are bigger, without explaining extinction risk and habitat variety.

A final mistake is assuming far islands must always be “worse” in every way. They often have fewer species overall, but they may still develop high endemism over long periods because isolation can promote separate evolutionary paths. For a definition question, though, stay focused on richness patterns.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Island biogeography studies how island size and isolation affect species richness
- In AP Environmental Science, an island can mean a real island or any isolated habitat patch
- Species richness means the number of different species present in an area
- The theory of island biogeography explains species number using immigration and extinction
- Immigration adds species to an island from a mainland or other source population
- Extinction removes species from an island when local populations disappear
- The equilibrium number of species is reached when immigration and extinction rates balance
- Larger islands usually support more species because they have more habitat and lower extinction rates
- Nearer islands usually support more species because they receive more immigrants
- Large, near islands usually have the greatest species richness
- Small, far islands usually have the lowest species richness
- Habitat fragmentation can create land-based “islands” to which this theory also applies

## Key Terms and Concepts

- island biogeography
- habitat fragmentation
- species richness
- immigration
- extinction
- equilibrium number of species
- source population


Island Biogeography - Island biogeography definition