How does island size specifically influence the extinction rate of species?

Larger islands have lower extinction rates because they offer greater habitat diversity and more abundant resources. This supports larger population sizes, which are less likely to be wiped out by random environmental changes or disease.

What is the difference between the 'Distance Effect' and the 'Area Effect' in island biogeography?

The Distance Effect refers to how an island's proximity to the mainland determines the rate of immigration (closer = higher rate). The Area Effect refers to how an island's size determines the rate of extinction (larger = lower rate).

Why are island specialist species more vulnerable to invasive generalists?

Specialists have evolved to exploit very specific, limited resources in a stable environment without much competition. Generalists can utilize a wide range of resources and often outcompete specialists when the environment changes or when new species are introduced.

What is a common error when interpreting the relationship between island distance and extinction?

The common error is assuming that distance from the mainland affects extinction rates. In the standard MacArthur-Wilson model, distance only influences the immigration rate, while island size is the factor that influences extinction.

Why is it incorrect to say that the number of species on an island is 'fixed' once equilibrium is reached?

The equilibrium is dynamic, not static. While the total number of species (richness) may stay the same, individual species are constantly going extinct and being replaced by new colonizing species through a process called turnover.

What mistake do students make regarding 'evolution' in the context of the Theory of Island Biogeography?

Students often focus on how species evolve (speciation) rather than the ecological balance of immigration and extinction. The theory primarily predicts species richness (the number of species) rather than the genetic changes within those species.

Define 'Species Richness' in the context of island ecology.

Species richness is the total number of different species present in a community. In island biogeography, it represents the balance point where the rate of immigration equals the rate of extinction.

What is 'Colonization' in island biogeography?

Colonization is the successful arrival, survival, and establishment of a species in a new area where it was not previously present. It depends on dispersal mechanisms like wind, water currents, or animal transport.

What is the 'Species-Area Curve' formula and what does it represent?

The formula is $S = cA^z$. It represents the mathematical relationship where the number of species ($S$) increases as the area ($A$) of the island increases, typically at a predictable rate defined by the constant $z$.

How does the concept of 'Habitat Fragmentation' relate to island biogeography?

Habitat fragmentation creates 'islands' of natural land surrounded by human-altered landscapes (like cities or farms). The same principles of size and isolation apply to these fragments, helping conservationists predict biodiversity loss in protected areas.

Island Biogeography | College Board AP Environmental Science

Revision Notes

College Board

Environmental Science

Unit 2: The Living World: Biodiversity

Island Biogeography

Summary

Island biogeography is a field of ecology that examines the factors affecting the species richness and community structure of isolated natural communities. It primarily focuses on how the physical characteristics of an island—specifically its size and its distance from a mainland source—dictate the rates of colonization and extinction, ultimately determining the equilibrium number of species present.

1. Definition & Core Concepts

Island Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time, specifically within isolated environments such as oceanic islands or fragmented habitats.

The Mainland-Island Model assumes that a large, stable 'mainland' serves as a source of species that periodically migrate to and colonize smaller, isolated 'islands'.

Species Richness on an island is a dynamic equilibrium reached when the rate of new species arriving (colonization) equals the rate of established species disappearing (extinction).

Isolation refers to the degree of separation between the island and the source population, which acts as a primary filter for which species can successfully reach and establish themselves.

Graph showing the Theory of Island Biogeography where the intersection of the immigration rate curve and extinction rate curve determines the equilibrium number of species (S*).

2. Underlying Principles

The Area Effect states that larger islands support more species because they offer a wider variety of habitats and resources, which reduces the probability of extinction by supporting larger population sizes.

The Distance Effect posits that islands closer to a mainland source have higher immigration rates because they are easier for dispersing organisms to find and reach by wind, water, or flight.

Colonization is the process by which a species spreads to a new area; on islands, this is often limited by the dispersal ability of the organism and the physical barriers of the surrounding environment.

Extinction rates are generally higher on smaller islands due to limited resources and smaller population sizes, which are more susceptible to random environmental fluctuations and genetic drift.

3. Methods & Techniques

4. Key Distinctions

It is vital to distinguish between Colonization (the arrival and establishment of a species) and Speciation (the evolutionary process where new species arise from an ancestor due to isolation).

Feature	Large Islands	Small Islands
Extinction Rate	Lower (more resources)	Higher (fewer resources)
Habitat Diversity	High (more niches)	Low (fewer niches)
Population Stability	Higher	Lower
Feature	Near Islands	Far Islands
---	---	---
Immigration Rate	Higher (easier to find)	Lower (harder to reach)
Species Richness	Generally Higher	Generally Lower
Genetic Flow	More frequent	Rare (leads to endemism)

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Island Biogeography

Summary

1. Definition & Core Concepts

The Mainland-Island Model assumes that a large, stable 'mainland' serves as a source of species that periodically migrate to and colonize smaller, isolated 'islands'.

Species Richness on an island is a dynamic equilibrium reached when the rate of new species arriving (colonization) equals the rate of established species disappearing (extinction).

Isolation refers to the degree of separation between the island and the source population, which acts as a primary filter for which species can successfully reach and establish themselves.

Graph showing the Theory of Island Biogeography where the intersection of the immigration rate curve and extinction rate curve determines the equilibrium number of species (S*).

2. Underlying Principles

The Distance Effect posits that islands closer to a mainland source have higher immigration rates because they are easier for dispersing organisms to find and reach by wind, water, or flight.

Extinction rates are generally higher on smaller islands due to limited resources and smaller population sizes, which are more susceptible to random environmental fluctuations and genetic drift.

3. Methods & Techniques

To predict species richness, ecologists analyze the Species-Area Relationship, often expressed by the power law formula $S = cA^z$ , where $S$ is the number of species, $A$ is the area, and $c$ and $z$ are constants specific to the taxonomic group and island type.

Researchers use comparative studies between islands of similar size but different distances from the mainland to isolate the impact of colonization rates on biodiversity.

Longitudinal monitoring of newly formed islands (such as those created by volcanic activity) allows scientists to observe the chronological sequence of primary succession and colonization in real-time.

The principles are applied to Conservation Biology by treating isolated nature reserves as 'habitat islands' within a 'sea' of human-developed land to determine the minimum viable area for protecting biodiversity.

4. Key Distinctions

Feature	Large Islands	Small Islands
Extinction Rate	Lower (more resources)	Higher (fewer resources)
Habitat Diversity	High (more niches)	Low (fewer niches)
Population Stability	Higher	Lower
Feature	Near Islands	Far Islands
---	---	---
Immigration Rate	Higher (easier to find)	Lower (harder to reach)
Species Richness	Generally Higher	Generally Lower
Genetic Flow	More frequent	Rare (leads to endemism)

5. Exam Strategy & Tips

Always check if a question is asking about the rate of immigration (influenced by distance) or the rate of extinction (influenced by island size).

When analyzing graphs, remember that the equilibrium point $S^*$ shifts to the right (more species) for islands that are both large and near the mainland.

Identify Specialist species as the typical residents of stable island environments; they are highly adapted to specific niches but are the first to suffer when invasive generalists are introduced.

Look for keywords like 'fragmentation' or 'wildlife corridors'; these are modern applications of island biogeography where human-made barriers create artificial islands.

6. Common Pitfalls & Misconceptions

A common mistake is assuming that 'distance' affects extinction; in the basic model, distance only affects the rate at which new species arrive, while size affects how long they survive.

Students often confuse Biogeography with Evolution; while isolation leads to evolution, the Theory of Island Biogeography is primarily concerned with the number of species, not how they change genetically over time.

Do not assume that the number of species on an island is constant; it is a dynamic equilibrium, meaning the specific species present may change even if the total count remains relatively stable.