AP Environmental Science: Island Biogeography Theory

AP Environmental Science – Island Biogeography Theory

AP Environmental Science: Island Biogeography Theory Study Guide

What This Is

Island Biogeography Theory explains how the size and distance of an island from the mainland affect its biodiversity. Developed by ecologists Robert MacArthur and E.O. Wilson in the 1960s, this theory predicts that larger islands closer to the mainland will have more species than smaller, more isolated islands. It matters on the AP exam because it’s a foundational concept in ecology, often tested in questions about species richness, conservation, and habitat fragmentation. Real-world example: After the 1980 eruption of Mount St. Helens, ecologists studied how species recolonized the "island-like" patches of surviving habitat, demonstrating the theory in action.

Key Terms & Concepts

• Island Biogeography Theory: A model predicting species richness on islands based on immigration rates (from the mainland) and extinction rates (on the island).
• Species richness: The number of different species in a given area.
• Immigration rate: The number of new species arriving on an island per unit time. Higher on near islands (closer to the mainland) and lower on far islands.
• Extinction rate: The number of species going extinct on an island per unit time. Higher on small islands (fewer resources, smaller populations) and lower on large islands.
• Equilibrium species number (Ŝ): The point where immigration and extinction rates balance, determining the island’s species richness.
• Target effect: Larger islands are easier for immigrants to "hit" (like a bigger target), increasing immigration rates.
• Rescue effect: Populations on near islands are "rescued" from extinction by frequent immigrants from the mainland, lowering extinction rates.
• Habitat fragmentation: When large habitats are broken into smaller, isolated patches (e.g., deforestation), creating "islands" that follow island biogeography principles.
• SLOSS debate: "Single Large or Several Small"—a conservation debate over whether one large protected area or multiple smaller ones better preserves biodiversity (ties to island biogeography).
• Edge effects: Changes in abiotic/biotic conditions at the boundary of a habitat fragment (e.g., more wind, invasive species), which can reduce effective island size.

Step-by-Step / Process Flow

How to analyze an island biogeography scenario on the AP exam:

1. Identify the "island":

2. Is it a literal island (e.g., Hawaii), a habitat fragment (e.g., a forest patch in farmland), or a protected area (e.g., a national park)?

3. Determine size and distance:

4. Size: Larger = more species (lower extinction, more niches).

5. Distance: Closer to mainland = more species (higher immigration).

6. Compare immigration vs. extinction rates:

7. Draw a graph with immigration rate (decreasing curve) and extinction rate (increasing curve) on the y-axis, and number of species on the x-axis. The intersection = Ŝ (equilibrium species number).

8. Apply to conservation:

9. If asked about designing a reserve, argue for:

   
   
   • Larger size (supports more species).
   • Closer to other reserves/mainland (higher immigration).
   • Corridors (connecting fragments to reduce isolation).

10. Account for edge effects:

11. Smaller fragments have more edge relative to core habitat, which can reduce effective size (e.g., invasive species outcompete natives at edges).

Common Mistakes

• Mistake: Assuming all islands have the same species richness regardless of size/distance.
  Correction: Larger, closer islands have higher species richness due to lower extinction and higher immigration rates.

• Mistake: Confusing immigration (new species arriving) with emigration (species leaving).
  Correction: Immigration increases species richness; emigration is less relevant in this theory.

• Mistake: Ignoring edge effects when evaluating habitat fragments.
  Correction: Edge effects reduce the effective size of a fragment, increasing extinction rates (e.g., a 100-ha forest patch may function like a 50-ha island due to edges).

• Mistake: Thinking the theory only applies to oceanic islands.
  Correction: It applies to any isolated habitat (e.g., mountaintops, urban parks, caves).

• Mistake: Forgetting the SLOSS debate in conservation questions.
  Correction: One large reserve is usually better, but corridors between smaller reserves can mimic a larger area.

AP Exam Insights

• Frequently tested:
• Graph interpretation: You’ll see a graph of immigration/extinction rates and must identify Ŝ or predict how changes (e.g., island size, distance) shift the curves.
• FRQs often ask: "Design a reserve to maximize biodiversity"—use island biogeography principles (size, distance, corridors).

• Multiple-choice traps:

  
  
  • Confusing immigration (arrival of new species) with speciation (evolution of new species on the island).
  • Assuming small islands have more species because they’re "easier to colonize" (wrong—small islands have higher extinction rates).

• Tricky distinction:

• Island size vs. distance: Size affects extinction; distance affects immigration. A large but distant island may have fewer species than a small but near island.

Quick Check Questions

1. Multiple Choice:
   Which of the following would most likely increase species richness on an island?
   A) Increasing the island’s distance from the mainland
   B) Decreasing the island’s size
   C) Creating a wildlife corridor to the mainland
   D) Introducing an invasive predator
   Answer: C) Creating a wildlife corridor to the mainland (increases immigration rates).
   Why: Corridors reduce isolation, mimicking a "nearer" island.

2. Short FRQ:
   "Explain how habitat fragmentation can reduce biodiversity, using island biogeography theory."
   Answer:

3. Fragmentation creates smaller, isolated "islands" of habitat.
4. Smaller fragments have higher extinction rates (fewer resources, smaller populations).
5. Greater distance between fragments lowers immigration rates (species can’t easily recolonize).

6. Edge effects further reduce effective habitat size, increasing extinction.

7. Multiple Choice:
   A conservation group proposes protecting two small forest patches instead of one large patch of equal total area. Which principle of island biogeography suggests this is a poor strategy?
   A) Target effect
   B) Rescue effect
   C) SLOSS debate
   D) Edge effects
   Answer: D) Edge effects.
   Why: Two small patches have more edge relative to core habitat, reducing effective size and increasing extinction rates.

Last-Minute Cram Sheet

1. MacArthur & Wilson (1960s): Developed island biogeography theory.
2. Larger islands = more species (lower extinction, more niches).
3. Closer islands = more species (higher immigration).
4. Ŝ = equilibrium species number (immigration = extinction).
5. Edge effects reduce effective island size (⚠️ more edge = worse for biodiversity).
6. SLOSS debate: One large reserve > several small (but corridors help!).
7. Target effect: Larger islands are easier to "hit" (higher immigration).
8. Rescue effect: Near islands get "rescued" by immigrants (lower extinction).
9. Habitat fragments = "islands" (e.g., urban parks, mountaintops).
10. ⚠️ Don’t confuse immigration (new species arriving) with speciation (new species evolving)!