U2.1 Biodiversity and Island Biogeography
Master AP Environmental Science biodiversity: species, genetic, and ecosystem diversity, the species-area relationship, island biogeography theory, and habitat fragmentation.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on U2.1 Biodiversity and Island Biogeography, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Why do larger islands and larger nature reserves shelter more species than tiny ones? Why does chopping a forest into pieces quietly drive species extinct even when total forest area seems large? This lesson answers those questions by connecting three levels of biodiversity to the classic theory of island biogeography.
You will learn to distinguish species, genetic, and ecosystem diversity, apply the species-area relationship, and use MacArthur and Wilson's model to predict how island size and distance control species richness. Finally, you will see how habitat fragmentation turns continuous landscapes into isolated 'habitat islands' — a concept the AP exam loves to test with graphs, scenarios, and free-response questions.
You will learn to distinguish species, genetic, and ecosystem diversity, apply the species-area relationship, and use MacArthur and Wilson's model to predict how island size and distance control species richness. Finally, you will see how habitat fragmentation turns continuous landscapes into isolated 'habitat islands' — a concept the AP exam loves to test with graphs, scenarios, and free-response questions.
Three Levels of Biodiversity
Biodiversity describes the variety of life, but the AP exam expects you to separate it into three distinct levels, because conservation strategies differ for each.
Species diversity measures the number of different species (richness) and how evenly individuals are distributed among them (evenness). A rainforest with thousands of species has high species diversity; a monoculture cornfield has very low species diversity.
Genetic diversity is the variety of genes within a single species. High genetic diversity gives a population more raw material for natural selection, so it can adapt to disease, climate shifts, or new predators. Small, inbred populations lose genetic diversity and become vulnerable to a single pathogen — think of the risk to genetically uniform crops.
Ecosystem diversity is the variety of habitats, communities, and ecological processes in a region. A landscape with wetlands, forests, grasslands, and rivers has high ecosystem diversity, supporting more niches and more overall life.
A key exam idea: higher biodiversity at all three levels generally increases resilience — the ability of a system to recover from disturbance — and resistance, the ability to withstand disturbance without changing.
Species diversity measures the number of different species (richness) and how evenly individuals are distributed among them (evenness). A rainforest with thousands of species has high species diversity; a monoculture cornfield has very low species diversity.
Genetic diversity is the variety of genes within a single species. High genetic diversity gives a population more raw material for natural selection, so it can adapt to disease, climate shifts, or new predators. Small, inbred populations lose genetic diversity and become vulnerable to a single pathogen — think of the risk to genetically uniform crops.
Ecosystem diversity is the variety of habitats, communities, and ecological processes in a region. A landscape with wetlands, forests, grasslands, and rivers has high ecosystem diversity, supporting more niches and more overall life.
| Level | Unit measured | Example of loss |
|---|---|---|
| Species | Number/evenness of species | A frog species goes extinct |
| Genetic | Alleles within a species | Inbreeding in cheetahs |
| Ecosystem | Habitat/community types | Draining a marsh |
The Species-Area Relationship
The species-area relationship is one of ecology's most reliable patterns: as the area of a habitat increases, the number of species it contains increases. Larger areas contain more microhabitats, larger populations, and greater resources, so they support more species.
The relationship is not linear. On a plot of species number versus area, the curve rises steeply at first and then flattens — each additional unit of area adds fewer new species. This is often expressed as , where is species number, is area, and and are constants. On a log-log plot this becomes a straight line, which is why AP graphs sometimes use logarithmic axes.
The practical takeaway the exam wants: doubling a protected area does NOT double the number of species — but destroying half a habitat can eliminate a disproportionate share of species living only in the lost portion. This directly supports arguments for large, connected conservation reserves.
A common misconception is that species number depends only on area. In reality, isolation, habitat quality, and disturbance also matter. But when other factors are held constant, area is the dominant predictor, which is exactly what island biogeography theory builds on.
The relationship is not linear. On a plot of species number versus area, the curve rises steeply at first and then flattens — each additional unit of area adds fewer new species. This is often expressed as , where is species number, is area, and and are constants. On a log-log plot this becomes a straight line, which is why AP graphs sometimes use logarithmic axes.
The practical takeaway the exam wants: doubling a protected area does NOT double the number of species — but destroying half a habitat can eliminate a disproportionate share of species living only in the lost portion. This directly supports arguments for large, connected conservation reserves.
A common misconception is that species number depends only on area. In reality, isolation, habitat quality, and disturbance also matter. But when other factors are held constant, area is the dominant predictor, which is exactly what island biogeography theory builds on.
Island Biogeography Theory
Robert MacArthur and E.O. Wilson proposed that the number of species on an island reflects a dynamic balance between two rates: the immigration of new species arriving and the extinction of species already present. Species number holds at equilibrium where these rates cross, even as species identities keep changing (turnover).
Two island traits shift this balance:
Size. Larger islands support larger populations, which go extinct less often, and present bigger targets for arriving colonists. So larger islands hold more species. This is the 'bigger is better' rule.
Distance from the mainland (source). Islands close to a mainland receive colonists easily, raising immigration; distant islands receive few arrivals. So closer islands hold more species than far ones.
Ranking is a favorite exam task: a large, near island holds the most species; a small, far island holds the fewest. Note that 'island' need not be surrounded by water — mountaintops, lakes, and forest patches all function as islands for organisms that cannot cross the surrounding matrix.
Two island traits shift this balance:
Size. Larger islands support larger populations, which go extinct less often, and present bigger targets for arriving colonists. So larger islands hold more species. This is the 'bigger is better' rule.
Distance from the mainland (source). Islands close to a mainland receive colonists easily, raising immigration; distant islands receive few arrivals. So closer islands hold more species than far ones.
| Factor | Effect on immigration | Effect on extinction | Result |
|---|---|---|---|
| Large island | Higher | Lower | More species |
| Small island | Lower | Higher | Fewer species |
| Near mainland | Higher | — | More species |
| Far from mainland | Lower | — | Fewer species |
Habitat Fragmentation and Habitat Islands
When roads, farms, or cities divide a once-continuous habitat, the remaining patches behave like islands surrounded by inhospitable terrain. This is habitat fragmentation, and applying island biogeography to it is a central AP skill.
Because each fragment is smaller than the original, it supports smaller populations that are more prone to local extinction and loss of genetic diversity. Because fragments are separated by developed land, immigration between them drops — organisms that avoid open ground cannot recolonize. The result mirrors small, distant islands: fewer species over time.
Fragmentation also increases the edge effect. Edges have different light, temperature, wind, and predation than interiors, so fragments have proportionally more edge and less core habitat. Interior-dependent species decline while edge and invasive species may increase.
A key solution the exam expects you to know is the habitat corridor (wildlife corridor): a strip of protected land connecting fragments. Corridors raise effective immigration, allow gene flow, and let animals migrate seasonally, partly restoring the connectivity lost to fragmentation. This is why conservation planning favors reserves that are large, close together, and linked, rather than many tiny scattered patches.
Because each fragment is smaller than the original, it supports smaller populations that are more prone to local extinction and loss of genetic diversity. Because fragments are separated by developed land, immigration between them drops — organisms that avoid open ground cannot recolonize. The result mirrors small, distant islands: fewer species over time.
Fragmentation also increases the edge effect. Edges have different light, temperature, wind, and predation than interiors, so fragments have proportionally more edge and less core habitat. Interior-dependent species decline while edge and invasive species may increase.
A key solution the exam expects you to know is the habitat corridor (wildlife corridor): a strip of protected land connecting fragments. Corridors raise effective immigration, allow gene flow, and let animals migrate seasonally, partly restoring the connectivity lost to fragmentation. This is why conservation planning favors reserves that are large, close together, and linked, rather than many tiny scattered patches.
Key terms
- Species diversity.
- The number of different species in an area (richness) combined with how evenly individuals are spread among those species (evenness).
- Genetic diversity.
- The variety of genes and alleles within a single species, which increases its ability to adapt to environmental change.
- Ecosystem diversity.
- The variety of habitats, communities, and ecological processes present within a region or landscape.
- Species-area relationship.
- The pattern that larger areas support more species, following a rising, decelerating curve described by .
- Island biogeography theory.
- MacArthur and Wilson's model in which island species number equals the equilibrium between immigration and extinction rates, controlled by island size and distance.
- Habitat fragmentation.
- The breaking of continuous habitat into smaller, isolated patches that function as habitat islands, reducing populations and connectivity.
- Edge effect.
- The changed environmental conditions and species composition found at the boundary between two habitats, more prevalent in small fragments.
- Habitat corridor.
- A protected strip of land linking habitat fragments that allows movement, gene flow, and recolonization between patches.
Worked example
Four islands lie off a mainland coast. Island A is large and near shore; Island B is large and far offshore; Island C is small and near shore; Island D is small and far offshore. Rank the islands from most to fewest expected species, and explain using immigration and extinction rates.
Start with the two rules of island biogeography. Size affects extinction: larger islands support larger populations that resist extinction, so large beats small. Distance affects immigration: islands nearer the mainland receive more colonists, so near beats far.
Combine the effects. Island A is large AND near — high immigration, low extinction — so it holds the most species. Island D is small AND far — low immigration, high extinction — so it holds the fewest.
The middle two require judgment because one is large-far and the other is small-near. In the standard MacArthur–Wilson framework both size and distance matter, but size (through reduced extinction) is generally the stronger driver, so the large-far island typically edges out the small-near one.
Final ranking: A (large, near) > B (large, far) > C (small, near) > D (small, far).
On the exam, always justify by naming the mechanism: 'Larger area lowers extinction rate; shorter distance raises immigration rate; species number is the equilibrium between the two.' Simply stating the ranking without the mechanism loses points on free-response questions.
Combine the effects. Island A is large AND near — high immigration, low extinction — so it holds the most species. Island D is small AND far — low immigration, high extinction — so it holds the fewest.
The middle two require judgment because one is large-far and the other is small-near. In the standard MacArthur–Wilson framework both size and distance matter, but size (through reduced extinction) is generally the stronger driver, so the large-far island typically edges out the small-near one.
Final ranking: A (large, near) > B (large, far) > C (small, near) > D (small, far).
On the exam, always justify by naming the mechanism: 'Larger area lowers extinction rate; shorter distance raises immigration rate; species number is the equilibrium between the two.' Simply stating the ranking without the mechanism loses points on free-response questions.
Practice questions
According to island biogeography theory, which island would be expected to support the greatest number of species?
- A small island located far from the mainland
- A large island located far from the mainland
- A small island located close to the mainland
- A large island located close to the mainland
Answer: A large island located close to the mainland
Large size reduces extinction rates because populations are bigger and more stable, while proximity to the mainland increases immigration of new colonists. The island maximizing both high immigration and low extinction — large and close — supports the most species at equilibrium.
A continuous 1,000-hectare forest is split by a new highway into two 500-hectare patches. Explain two reasons why the total number of species is likely to decline over time, and identify one conservation strategy that could reduce this loss.
Answer: Smaller patches support smaller populations more prone to local extinction and reduced genetic diversity; the highway lowers immigration/recolonization between patches; a wildlife corridor could restore connectivity.
This applies island biogeography to fragmentation. Each patch acts like a smaller, more isolated island: smaller area raises extinction risk and shrinks genetic diversity, while the road barrier cuts immigration so locally extinct species cannot be replaced. Increased edge effect also degrades interior habitat. A habitat corridor reconnects the patches, raising effective immigration and gene flow, which is the standard mitigation the exam expects.
Distinguish genetic diversity from species diversity and explain why a population with low genetic diversity is more vulnerable to extinction.
Answer: Species diversity is the variety of species in an area; genetic diversity is the variety of alleles within one species. Low genetic diversity leaves a population less able to adapt to new threats like disease.
Species diversity operates across species while genetic diversity operates within a single species. When genetic diversity is low, there is little variation for natural selection to act on, so a new pathogen, parasite, or climate shift can wipe out the whole population because no individuals carry resistant alleles. This is why small, inbred populations and genetically uniform crops are at high extinction or collapse risk.
FAQ
- Does an 'island' in island biogeography have to be surrounded by water?
- No. Any patch of suitable habitat surrounded by unsuitable terrain acts as an island. Mountaintops, lakes, cave systems, and isolated forest fragments all function as habitat islands for species that cannot cross the surrounding environment.
- What is the difference between the species-area relationship and island biogeography theory?
- The species-area relationship simply states that larger areas hold more species. Island biogeography theory explains the mechanism behind species number — a balance between immigration and extinction rates — and adds the effect of distance/isolation, not just area.
- Why is habitat fragmentation worse than losing the same total area in one block?
- Fragmentation creates several small, isolated patches instead of one large connected one. Small patches have smaller, more extinction-prone populations, more edge and less core habitat, and reduced immigration between patches, so they lose species faster than a single large reserve of equal total area.
- How is this tested on the AP exam?
- Expect to rank islands or reserves by expected species number using size and distance, interpret species-area graphs, explain fragmentation effects, and propose solutions like wildlife corridors. Free-response answers must name mechanisms (immigration, extinction, edge effect), not just state conclusions.
Learn this with a teacher, not a page
The Crimsora tutor teaches U2.1 Biodiversity and Island Biogeography live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.