General Ecology · Community Ecology
Biogeography and Island Biogeography
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In 30 seconds
Biogeography Study of the distribution of organisms in space and time Full entry → is the study of where species live, now and in the past, and why. Patterns such as the Latitudinal diversity gradient Species richness increases toward the tropics Full entry → (more species near the tropics) and the Species-area relationship Larger areas contain more species (S = cAz) Full entry → (larger areas hold more species) summarize this distribution. MacArthur and Wilson Authors of the equilibrium theory (1967) Full entry →'s Island biogeography Theory of species richness on islands Full entry → theory explains island species richness as a balance between Immigration Arrival of new species (or individuals) Full entry → (higher on near and large islands) and Extinction Disappearance of species from an island Full entry → (higher on small islands), yielding an Equilibrium species number Richness where immigration = extinction Full entry →. Because habitat fragments behave like islands, the theory informs reserve design, though it omits evolution, history, and species identity.
Why this matters
Island biogeography underpins much of conservation planning. It explains why small, isolated reserves tend to lose species and why large, connected reserves and wildlife corridors are often favored. It informs how fragmentation of forests, wetlands, and grasslands by roads, agriculture, and development reduces the species those patches can hold. Design and management of protected areas, however, are governed by law and by local, regional, and Indigenous land, wildlife, and data-sovereignty rules, which vary by jurisdiction and must be respected in any real planning process.
The college version
1. Biogeography and Species Distribution
Biogeography studies the distribution of organisms across space and time, integrating ecology, evolution, geology, and climate. A species' Geographic range The area a species occupies Full entry → is the area it occupies, and Endemism Being found only in one particular region Full entry → is the condition of being found only in one particular place (an endemic species is restricted to a specific region, such as an island or mountain range). Distribution patterns reflect dispersal ability, environmental tolerance, historical barriers, and evolutionary origin.
2. Large-Scale Diversity Patterns
Two robust patterns organize species numbers. The latitudinal diversity gradient is the observation that species richness generally increases from the poles toward the tropics, for reasons that likely include greater solar energy and productivity, larger tropical area, longer evolutionary time without glaciation, and higher speciation rates. The species-area relationship is the finding that larger areas contain more species, often described by the power law S = cAz, where S is species richness and A is area. A Species-area curve Plot of richness against area Full entry → plots this relationship, rising steeply at first and then flattening.
3. Island Biogeography Theory
MacArthur and Wilson (1967) proposed that the number of species on an island reflects a dynamic equilibrium between immigration and extinction. The immigration rate of new species is high when few species are present and declines as the island fills up; it is also higher for islands near a mainland source. The extinction rate rises as more species crowd together and is higher on small islands (fewer individuals, less habitat). Where the two rates cross is the equilibrium species number, a turnover point at which species continue to replace one another even though the total stays roughly constant. Island size Physical area of an island Full entry → (larger = lower extinction, more habitat) and distance from source populations (farther = lower immigration) are therefore the two master variables.
How it works
- Species colonize an island from a mainland source at a rate set by distance.
- Species on the island go extinct at a rate set by island size and crowding.
- Immigration and extinction rates cross at an equilibrium species number.
- The island's diversity hovers near that equilibrium while individual species turn over.
- Larger islands and nearer sources raise the equilibrium; smaller, more isolated islands lower it.
- Habitat fragments follow the same logic, informing reserve size and connectivity decisions.
Common confusions
| Do not confuse | With | Difference |
|---|---|---|
| Geographic range | Habitat | Range is the total area a species occupies; habitat is the specific environment it uses |
| Endemism | Rarity | Endemic means restricted to one place; rare means few individuals (can be widespread but sparse) |
| Species-area relationship | Island equilibrium | Area–richness is a pattern; the equilibrium explains it via immigration and extinction |
| Immigration | Extinction | Immigration adds species; extinction removes them |
| Island size | Isolation | Size lowers extinction; isolation lowers immigration |
| Habitat island | Oceanic island | Habitat island is an isolated habitat patch on land; an oceanic island is surrounded by water |
| Equilibrium species number | Total ever-present species | Equilibrium is the steady-state count with turnover, not a fixed list |
| Fragmentation | Habitat loss | Loss removes total area; fragmentation splits remaining area into smaller, more isolated patches |
Memory aids
Remember "SIDE" for the master variables: Size (bigger = more species), Isolation (closer = more species), and the balance of Death (extinction) vs. Entry (immigration). Or picture a bathtub: immigration is the faucet, extinction is the drain, and the water level is the equilibrium species number — a big tub (island) with a strong faucet (near source) holds the most water.
Quick review
Topic Recap
- Biogeography studies where species live and why; geographic range and endemism describe distribution.
- The latitudinal diversity gradient and the species-area relationship (S = cAz) are general patterns.
- MacArthur and Wilson's island biogeography explains species richness as an equilibrium between immigration and extinction.
- Island size lowers extinction; isolation lowers immigration; together they set the equilibrium species number.
- Habitat fragments behave as islands, informing reserve design toward larger, more connected areas.
- The theory's limits include ignoring evolution, history, species identity, and interactions.
Knowledge Check
- What is the latitudinal diversity gradient, and in which direction does species richness change?
- Write the species-area relationship and define each variable.
- In island biogeography theory, which two processes determine species richness, and what do the two curves look like?
- How do island size and distance from the mainland each affect the equilibrium species number?
- Why is a small, isolated forest fragment expected to hold fewer species than a large, connected forest, and what is one limitation of using island theory to predict this?
Answers and Rationales
- The latitudinal diversity gradient is the pattern that species richness increases from the poles toward the tropics — more species near the equator, fewer at high latitudes.
- S = cAz, where S is species richness, A is habitat area, and c and z are fitted constants (c scales richness; z is the slope in log space, roughly 0.2–0.35). Larger area predicts more species.
- Immigration and extinction. The immigration curve declines as species number rises (fewer new species remain to arrive), and the extinction curve rises with crowding. Their intersection is the equilibrium species number.
- Larger size lowers the extinction rate, raising the equilibrium; greater distance lowers the immigration rate, lowering the equilibrium. Size and isolation push the equilibrium in opposite directions.
- A small, isolated fragment has lower immigration (fewer arrivals) and higher extinction (smaller populations, less habitat), so its equilibrium species number is lower. A limitation is that the equilibrium model ignores species identity, interactions, evolution, and history, so it predicts richness only approximately.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a row of islands of different sizes at different distances from the mainland. A big island close to shore gets a steady stream of new plants and animals drifting or flying in from the mainland, so it ends up with many species. A tiny island far out at sea gets few newcomers and easily loses the ones it has, so it stays species-poor. This is the heart of island biogeography: the number of species on an island is a running balance between new species arriving (immigration) and existing species disappearing (extinction).
The comparison stops being exact because islands are not empty containers that simply fill up to a fixed number. Which species arrive and survive depends on their abilities to travel, on chance, on the island's evolutionary history, and on interactions among species once they are there. Real islands also gain species by evolving new ones in place (endemism), not only by immigration. Still, the size-and-distance logic transfers directly to conservation: a patch of forest surrounded by farms is, for many of its species, an "island," and the same rules about size and isolation shape what can survive there.
Simple Example
A small woodland reserve surrounded by highways and suburbs supports fewer bird species than a large, connected forest nearby. Even though both are "forest," the small isolated patch acts like a distant, tiny island: fewer arrivals, more local extinctions, and a lower equilibrium number of species.
Worked example
- Measure the relationship between area and species. The species-area relationship is often written
S = cAz
where S is species richness (number of species, dimensionless count), A is habitat area (e.g., km²), and c and z are fitted constants: c scales richness (it varies with the taxon and region), and z is the slope in log space, typically about 0.2–0.35 for islands. Taking logarithms gives logS = logc + z logA, a straight line with slope z.
- Build the equilibrium curves. Plot immigration rate (new species per unit time) against the number of species already present: it declines because new arrivals are increasingly likely to be species already there. Plot extinction rate against species number: it rises with crowding. Both curves are model constructs summarizing expected rates, not directly counted values.
- Shift the curves with size and isolation. Increasing island size lowers the extinction curve (more habitat, larger populations), raising the equilibrium species number. Increasing distance from the mainland lowers the immigration curve, lowering the equilibrium number.
- Find the equilibrium. The equilibrium species number Ŝ is the species count where immigration and extinction rates are equal. At Ŝ the total stays roughly constant even though the identity of species keeps changing (turnover).
- Apply to habitat islands. Treat fragmented habitat patches as islands: a small, isolated patch is expected to hold fewer species at equilibrium than a large, connected one. Reserve design uses this to argue that, other things equal, larger and more connected reserves sustain more species.
Assumptions, limits, and uncertainty. The equilibrium model is a simplification: it treats species as equivalent, ignores evolution (in situ speciation and adaptation), ignores the identity and interactions of species, and assumes immigration and extinction reach a steady balance. Real islands also carry a historical and evolutionary signature — age, past connections, and speciation — that the pure equilibrium model does not capture, one of the core limitations of island biogeography. The species-area relationship is an empirical generalization with scatter, not a universal law, and its constants vary among taxa and regions.
Key takeaways
- High yield: Biogeography = where species live (now and historically) and why.
- High yield: The latitudinal diversity gradient: more species near the tropics, fewer toward the poles.
- High yield: Species-area relationship S = cAz; larger area → more species.
- High yield: MacArthur–Wilson equilibrium: species number balances immigration and extinction.
- High yield: Immigration is higher near sources and on islands with few species; extinction is higher on small islands.
- High yield: Island size (lowers extinction) and isolation (lowers immigration) determine equilibrium richness.
- Endemic species are restricted to one region and are often the most vulnerable.
- Habitat fragments act as islands, so reserve design favors larger, more connected areas.
- The equilibrium model ignores evolution, history, and species identity — key limitations.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Define biogeography and explain how geographic range and endemism describe where species live.
- Describe the latitudinal diversity gradient and the species-area relationship.
- Explain MacArthur and Wilson's equilibrium theory of island biogeography, including the roles of island size, isolation, immigration, and extinction.
- Apply island biogeography to habitat islands and reserve design, and identify its limitations.
Key vocabulary
- Biogeography
- Study of the distribution of organisms in space and time
- Geographic range
- The area a species occupies
- Endemism
- Being found only in one particular region
- Latitudinal diversity gradient
- Species richness increases toward the tropics
- Species-area relationship
- Larger areas contain more species (S = cAz)
- Species-area curve
- Plot of richness against area
- Habitat area
- The amount of suitable habitat available
- Isolation
- Distance/separation from other habitat or sources
- Island biogeography
- Theory of species richness on islands
- MacArthur and Wilson
- Authors of the equilibrium theory (1967)
- Immigration
- Arrival of new species (or individuals)
- Extinction
- Disappearance of species from an island
- Equilibrium species number
- Richness where immigration = extinction
- Island size
- Physical area of an island
- Distance from source populations
- How far an island is from a colonist source
- Habitat islands
- Isolated habitat patches (e.g., forest fragments)
- Fragmentation application
- Using island theory for broken-up habitats
- Reserve design concepts
- Principles (size, connectivity) for protected areas
- Historical and evolutionary context
- Age, past connections, and speciation history
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