Getting Started
Islands are natural laboratories for ecology and evolution. An island, in an ecological context, is any area of suitable habitat surrounded by an inhospitable expanse. This chapter explores the processes that determine which species live on these islands and in what numbers, a field known as island biogeography. We will examine how an island's physical characteristics shape its biological community and how the unique pressures of island life drive evolutionary change.
What You Should Be Able to Do
After completing this section, you should be able to:
Explain how an island's size and its distance from a mainland source of species influence its overall biodiversity.
Describe how the balance between species immigration and extinction rates determines the number of species an island can support.
Connect the limited resources on islands to the evolutionary trend of species becoming highly specialized.
Analyze how the introduction of invasive species can threaten the survival of native island specialists.
Key Concepts & Mechanisms
The number of species found on an island is not random. It is the result of a dynamic process governed by the island's size, its isolation, and the constant interplay of new arrivals and local extinctions.
Inputs & Preconditions
For an island ecosystem to develop, two primary factors are required: a source of colonists and a place for them to live.
Source of Species: There must be a "mainland" or a larger, species-rich area that can provide a pool of potential colonizing organisms. These organisms must be capable of dispersing across the barrier separating the mainland from the island, whether by flying, swimming, or rafting on debris.
Island Characteristics: The physical nature of the island itself sets the stage.
Size: Larger islands generally have a greater variety of habitats and can support larger population sizes for each species. More resources and space mean that species are less likely to go extinct.
Distance from Mainland: Islands closer to the mainland are easier targets for colonizing organisms. The shorter the distance, the higher the probability that a dispersing plant or animal will successfully arrive.
Key Steps / Mechanism: The Equilibrium Model
The Theory of Island Biogeography proposes that the number of species on an island is a dynamic equilibrium controlled by two opposing forces: immigration and extinction.
Immigration: This is the rate at which new species arrive and successfully establish themselves on the island. The immigration rate is highest on a barren island because every new arrival represents a new species. As the number of species on the island increases, the rate of new immigration slows down, because arriving organisms are more likely to belong to a species that is already present. Immigration rates are higher for islands near the mainland (near islands) and lower for islands far from the mainland (far islands).
Extinction: This is the rate at which species already on the island disappear. The extinction rate is low when there are few species. As the number of species increases, so does the competition for limited resources, leading to a higher rate of extinction. Extinction rates are higher on small islands (which have fewer resources and smaller populations) and lower on large islands.
Equilibrium: The number of species on an island stabilizes when the rate of immigration equals the rate of extinction. This point of intersection determines the island's equilibrium number of species. It is important to note that this is a dynamic equilibrium; the total number of species may remain stable, but the actual composition of species can change over time as new species arrive and existing ones die out.
Outputs & Impacts
The processes of colonization and extinction on islands have profound ecological and evolutionary consequences.
Predictable Species Richness: The model allows us to predict that large islands near the mainland will have the highest number of species, while small islands far from the mainland will have the lowest.
Evolution of Specialists: Resources on an island (food, water, shelter) are finite. To minimize competition, many island species evolve to become specialist species, which have a very narrow ecological niche (the specific role and requirements a species has in its ecosystem). They may adapt to eat only one type of food or live in one specific type of habitat. This reduces competition and allows more species to coexist.
Vulnerability to Extinction: This specialization, while advantageous in a stable environment, makes island species extremely vulnerable. If their single food source disappears or their habitat is altered, they have no alternatives. Furthermore, they have often evolved in the absence of major predators or competitors. The introduction of an invasive generalist species—one with a broad niche that can eat many things and live in many habitats—can be catastrophic. The generalist can outcompete the native specialist for resources, leading to a rapid decline in the specialist's population and, potentially, its extinction.
Key Models & Diagrams
The relationship between island size, distance, immigration, and extinction determines the equilibrium number of species.
| Island Characteristic | Influence on Immigration Rate | Influence on Extinction Rate | Resulting Species Richness |
|---|---|---|---|
| Large Size | No direct effect on arrival probability. | Decreases extinction rate (more resources, more habitats, larger populations). | Higher |
| Small Size | No direct effect on arrival probability. | Increases extinction rate (fewer resources, intense competition, smaller populations). | Lower |
| Near Mainland | Increases immigration rate (shorter distance to cross). | No direct effect on resource availability. | Higher |
| Far from Mainland | Decreases immigration rate (longer, more difficult distance to cross). | No direct effect on resource availability. | Lower |
Conclusion: A large, near island will have the highest species richness, while a small, far island will have the lowest.
Key Components & Evidence
Theory of Island Biogeography: The central model, developed by Robert MacArthur and E.O. Wilson, explaining that species richness on an island is a balance between immigration and extinction rates.
Specialist Species: Organisms with a narrow ecological niche. For example, the Hawaiian honeycreepers evolved dozens of species from a common ancestor, with each developing a unique beak shape to feed on specific flowers or insects.
Generalist Species: Organisms with a broad ecological niche. Rats and wild pigs are classic examples of generalist invaders on islands; they can eat a wide variety of foods and thrive in many habitats, often outcompeting native specialists.
Invasive Species: A non-native species that is introduced to an ecosystem and causes ecological or economic harm. The brown tree snake, introduced to Guam, is responsible for the extinction of most of the island's native forest bird species.
Adaptive Radiation: An evolutionary process where a single ancestral species diversifies rapidly to fill a variety of empty ecological niches. Darwin's finches in the Galápagos Islands are the quintessential example, having evolved different beak sizes and shapes to exploit different food sources.
Habitat Islands: The principles of island biogeography apply to any isolated patch of habitat. This includes national parks surrounded by urban development, forests fragmented by agriculture, or alpine zones on mountaintops separated by low-elevation valleys.
The Galápagos Islands: A volcanic archipelago in the Pacific Ocean famous for its unique species. Its isolation and varied environments provided the perfect conditions for adaptive radiation and were crucial to Charles Darwin's development of the theory of evolution by natural selection.
Resource Partitioning: An evolutionary outcome where species with similar needs evolve to use resources in different ways to avoid competition. On an island, one bird species might evolve to feed on insects on tree trunks, while another from the same ancestor evolves to feed on insects on the ground.
Skill Snapshots
Causation
Cause: An island is geographically small. → Effect: It supports smaller populations, which are more vulnerable to random events and have a higher rate of extinction.
Cause: An island is located far from the mainland. → Effect: The rate of new species immigrating to the island is low, limiting the potential for biodiversity.
Cause: Limited resources and competition exist on an island. → Effect: Species evolve specialized traits (e.g., specific diets) to minimize competition, a process known as resource partitioning.
Comparison
Specialist species have a narrow range of tolerance and specific needs, whereas generalist species have a broad range of tolerance and can utilize many different resources.
Large islands tend to have lower extinction rates and support higher species richness compared to small islands.
Islands near a mainland experience higher rates of immigration and thus tend to have greater species richness than islands far from a mainland.
Change and Continuity Over Time (CCOT)
Baseline: A newly formed, barren volcanic island has zero terrestrial species and high potential for colonization.
Change 1: Over time, pioneer species arrive from the mainland. The rate of immigration is high, and the rate of extinction is low.
Change 2: As more species arrive, competition increases, and evolutionary pressures drive some species toward specialization to occupy unique niches.
Continuity: The fundamental forces of isolation and limited resources continue to shape the island's community, maintaining a dynamic equilibrium where extinction and immigration rates are balanced.
Common Misconceptions & Clarifications
Misconception: The term "island" only refers to a piece of land surrounded by water.
Clarification: In ecology, an "island" can be any patch of habitat isolated from similar habitats. A city park, a mountain summit, or a lake are all considered habitat islands and are subject to the same biogeographical principles.
Misconception: Once an island reaches its equilibrium number of species, its species composition is stable.
Clarification: The equilibrium is dynamic. The number of species may stay relatively constant, but the identity of those species changes. This is called species turnover, where one species may go extinct while a new one immigrates.
Misconception: All species that arrive on an island will become specialists.
Clarification: While there is a strong evolutionary pressure toward specialization over the long term, the initial colonizers of an island are often generalists, as they are better equipped to survive the journey and establish themselves in a new, unpredictable environment.
One-Paragraph Summary
Island biogeography is the study of how species are distributed across isolated habitats. The theory posits that the number of species on an island represents a dynamic equilibrium between the rate of immigration from a mainland source and the rate of local extinction. Immigration is higher for islands that are closer to the mainland, while extinction is lower for islands that are larger. These conditions of isolation and limited resources create intense selective pressures, often causing species to evolve into specialists with narrow ecological niches to avoid competition. This specialization, however, makes island species highly vulnerable to extinction, especially when invasive generalist species are introduced and disrupt the fragile ecological balance.