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Island
Biogeography

Explore how island size and distance from the mainland determine species richness through the balance of immigration and extinction.

AP Environmental Science

What Is Island Biogeography?

Island biogeography is the study of the distribution of species on islands and island-like habitats. Developed by ecologists Robert MacArthur and E.O. Wilson in 1967, this theory explains why some islands support many species while others support few.

Key Terms to Know

Immigration
The arrival of new species to an island from a mainland or other source.
Extinction
The local disappearance of a species from an island due to limited resources or competition.
Species Richness
The total number of different species present on an island at any given time.

Why Does This Matter?

Island biogeography is one of the most influential theories in conservation biology. It helps scientists and policymakers decide:

1
Nature reserve design: How large should protected areas be, and how close together?
2
Habitat fragmentation: What happens to species when continuous habitat is split into "islands"?
3
Biodiversity prediction: How many species can a given patch of habitat support?

The Equilibrium Model

The core idea: an island's species number reaches a dynamic equilibrium (\( \hat{S} \)) where the immigration rate equals the extinction rate.

Number of Species on Island Rate Immigration Extinction
\( \hat{S} \) (Equilibrium)
P

An island keeps its species count while its cast changes

A forested island, mainland and field observer accompany exact species-identity snapshots and illustrative crossing rate lines.Species richness can stay the same across two snapshots even though one species disappears and another arrives. A stable count can therefore accompany continuing turnover.

Equilibrium does not freeze the cast

Equilibrium does not freeze the cast. Species richness can stay the same across two snapshots even though one species disappears and another arrives. A stable count can therefore accompany continuing turnover.Species richness can stay the same across two snapshots even though one species disappears and another arrives. A stable count can therefore accompany continuing turnover.

Two Factors That Shift Equilibrium

Island Size
Large islands have lower extinction rates (more resources, more niches). This shifts \( \hat{S} \) to the right: more species.
Distance from Mainland
Near islands have higher immigration rates (easier to reach). This also shifts \( \hat{S} \) to the right: more species.

Effect of Island Size

Number of Species Rate Immigration Extinction (Small) Extinction (Large)
\( \hat{S} \)_{\text{small}}
\( \hat{S} \)_{\text{large}}

Effect of Distance from Mainland

Number of Species Rate Immigration (Near) Immigration (Far) Extinction
\( \hat{S} \)_{\text{far}}
\( \hat{S} \)_{\text{near}}

The Species-Area Relationship

A mathematical model links island area to species richness. The power function model states:

Taking the logarithm of both sides gives a linear form:

Oceanic vs Habitat Islands

Oceanic Islands Habitat Islands
Examples Hawaii, Galapagos, Madagascar Forest patches, lakes, mountaintops
Barrier Ocean water Roads, farmland, urban sprawl
Immigration source Mainland continent Surrounding intact habitat
Typical \( z \) value 0.25 to 0.35 0.12 to 0.18
Conservation use Marine reserves, endemic species Wildlife corridors, reserve design

How to Apply the Theory

1
Identify the "island": What is the isolated habitat? What surrounds it (the "ocean")?
2
Determine area: Larger area means lower extinction rate, therefore higher \( \hat{S} \).
3
Determine distance: Closer to source means higher immigration rate, therefore higher \( \hat{S} \).
4
Predict species richness: Use \( S = cA^z \) or compare equilibrium curves to rank islands.
5
Apply to conservation: Recommend corridors, buffer zones, or larger reserves to increase \( \hat{S} \).

Example 1: Ranking Islands

Four islands exist off a mainland coast. Rank them from most to fewest species:

Mainland Ocean A Large, Near B Small, Near C Large, Far D Small, Far

Example 2: Using \( S = cA^z \)

A biologist surveys bird species on Caribbean islands. The data yields \( c = 18 \) and \( z = 0.30 \). Predict species richness for an island of area \( A = 100 \text{ km}^2 \).

1
Write the equation: \( S = cA^z = 18 \times 100^{0.30} \)
2
Compute \( 100^{0.30} \): \( 100^{0.30} = 10^{2 \times 0.30} = 10^{0.60} \approx 3.98 \)
3
Multiply: \( S = 18 \times 3.98 \approx 72 \) bird species

Example 3: Conservation Design

A national park must choose between two reserve designs. Which will support more species?

Option A: One Large
\( 500 \text{ km}^2 \)
Option B: Four Small 125 125 125 125
Total: \( 500 \text{ km}^2 \)
vs

Habitat fragmentation, corridors and gene flow

Three qualitative forest landscapes show continuous woodland, fragmented woodland and a restored vegetated road crossing.Equal total reserve area can conceal differences in core habitat. With a one-unit edge zone, one 10-by-10 square retains 64 square units of core, while four 5-by-5 squares retain 36 in total.

Equal area does not mean equal core

Equal area does not mean equal core. With a one-unit edge depth, one 10-by-10 square contains 64 square units of core. Four 5-by-5 squares have the same total area but only 36 square units of core.With a one-unit edge depth, one 10-by-10 square contains 64 square units of core. Four 5-by-5 squares have the same total area but only 36 square units of core.

The SLOSS Debate

Ecologists have debated: should we protect a Single Large or Several Small reserves? The answer depends on context:

Single Large Wins When:
Species need large home ranges, habitat interior is critical, or edge effects are damaging.
Several Small Wins When:
Habitats are diverse across sites, disease risk is high (isolation protects), or species pools differ by location.

Common Mistakes

✕ Confusing Static with Dynamic
Equilibrium does not mean the same species stay forever. Species continuously turn over: the number stays constant, not the identity.
✕ Doubling Area = Doubling Species
Since \( z < 1 \), doubling area increases species by a factor of \( 2^z \approx 1.2 \), not 2. The relationship is sublinear.
✕ Ignoring Habitat Quality
Size and distance are not the only factors. A small island with diverse habitats (wetlands, forests, cliffs) may exceed predictions.

Summary Table

Factor Effect on Immigration Effect on Extinction Effect on \( \hat{S} \)
Larger area Slight increase (target effect) Decreases Increases
Smaller area Slight decrease Increases Decreases
Closer to source Increases No direct effect Increases
Farther from source Decreases No direct effect Decreases

Question 1 of 5

According to the theory of island biogeography, which island would have the highest species richness?

Question 2 of 5

In the equation \( S = cA^z \), what does the exponent \( z \) typically range between for oceanic islands?

Question 3 of 5

At the equilibrium point \( \hat{S} \) on MacArthur and Wilson's model, which statement is true?

Question 4 of 5

A forest is divided by a highway into two fragments. According to island biogeography, what is the most likely long-term effect on the fragments?

Question 5 of 5

If \( c = 20 \) and \( z = 0.25 \), what is the predicted number of species on an island with area \( A = 10{,}000 \text{ km}^2 \)?