Biology for AP Courses · Ecology and the Biosphere
Biogeography
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Biogeography The study of species' geographic distributions and their causes. Full entry → is the study of the geographic distribution of species and ecosystems — where organisms live and why they live there and not elsewhere. It combines ecology (the environmental demands of species) with Earth history (Continental drift The slow movement of continents over geological time. Full entry →, climate change, and the rise and fall of land bridges). This topic introduces the major patterns of distribution, such as the Latitudinal diversity gradient The pattern of higher species richness toward the equator. Full entry → and island effects, and the historical and ecological processes — including Vicariance Splitting of a population by a new barrier. Full entry → and Dispersal Movement of organisms across a barrier to a new area. Full entry → — that create them.
Why this matters
Biogeography explains some of biology's most striking patterns: why Australia has marsupials but few native placental mammals, why Madagascar's species are found nowhere else, and why islands so often lose species when people arrive. It is also the science behind conservation planning — identifying Endemic Found naturally in only one region. Full entry → hotspots, predicting where invasive species will spread, and designing protected areas. For AP students, biogeographic evidence (matching fossils across continents, unique island species) is a core line of evidence for evolution and plate tectonics.
The college version
Core Concepts
The Global Pattern: Species Richness and Latitude
The best-known pattern in biogeography is the latitudinal diversity gradient: Species richness The number of species in a given area. Full entry → — the number of species in an area — generally increases toward the equator and decreases toward the poles. Tropical rainforests hold far more species than temperate or polar regions. Proposed explanations (commonly taught and still debated) include greater solar energy and productivity in the tropics, longer periods of evolutionary time without glaciation, and higher speciation rates. No single explanation accounts for the pattern, a reminder that ecological patterns usually have multiple causes.
Dispersal and Vicariance: How Species Get Around
Two historical processes explain why related species live where they do. Dispersal is the movement of organisms across an existing barrier — a bird blown to an island, seeds rafted across an ocean. Vicariance is the splitting of a once-continuous population by a new barrier — a mountain range rising, or sea level rising to flood a land bridge. Both processes leave signatures: dispersal tends to produce species on the far side of a barrier that resemble the source population, while vicariance divides a single lineage into separated populations that then diverge. Continental drift is the grandest vicariance event: the breakup of Pangaea separated lineages that now occupy different continents.
The Geography of Evolution: Wallace's Line and Endemism
Alfred Russel Wallace noticed that the islands of Bali and Lombok, though only about 35 km apart, have strikingly different faunas: Asian species on the western side, Australian species on the eastern side. The boundary, Wallace's Line The faunal boundary between Asian and Australian regions. Full entry →, marks where deep water kept the two regions separate even when sea levels fell — species could not cross, and each side evolved its own fauna. Such boundaries explain endemism: species found naturally in only one place. Madagascar's lemurs, Australia's marsupials, and the Galápagos' giant tortoises are classic endemic examples, each the product of long isolation and independent evolution.
Island Biogeography: The Equilibrium Model
Islands are natural laboratories for biogeography. Robert MacArthur and E. O. Wilson's Equilibrium theory of island biogeography Model balancing immigration and extinction on islands. Full entry → (a landmark, commonly taught model) proposes that the number of species on an island reflects a balance between two rates: immigration of new species from the mainland and extinction of species already present. Large islands have lower extinction rates than small ones because they offer more resources and habitat, and islands near the mainland have higher immigration rates than distant ones. The model predicts the Species–area relationship The tendency of larger areas to hold more species. Full entry →: larger islands hold more species. It also frames modern conservation: habitat fragments behave like islands, so fragment size and distance matter for how many species they can retain.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Dispersal | Vicariance | Dispersal moves organisms across a barrier; vicariance splits a population with a new barrier. |
| Endemic species | Native species | Endemic means found nowhere else naturally; native just means naturally present (it may occur elsewhere). |
| Species richness | Species diversity | Richness is a count of species; diversity also accounts for relative abundance. |
| Wallace's Line | A country border | It is a biogeographic boundary between Asian and Australian faunas, not a political line. |
| Island size | Island distance | Both matter: size drives extinction rate; distance drives immigration rate. |
| Continental drift | Dispersal | Drift moves the land beneath the organisms (vicariance); dispersal moves organisms across land or sea. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Biogeography is the study of "why animals live where they do on the map." Some move to new places on their own, like birds blown to an island — that is dispersal. Others get separated when land splits apart or mountains rise — that is vicariance. And islands, like the Galápagos, are nature's experiment bowls: the farther and smaller they are, the fewer species live there.
Worked example
Scenario — predicting an island's fauna. Two islands lie off the same coast: one large and 10 km offshore, another small and 400 km offshore. The equilibrium model predicts the large, near island will accumulate species quickly (high immigration) and lose them slowly (low extinction), so its richness will be high; the small, far island will have low immigration and high extinction, so its richness will be low and its turnover high. A field survey confirms the pattern. Now a national park creates a corridor connecting two fragments of habitat: this effectively enlarges the "island" and raises immigration, which the model predicts will slow extinctions. The same logic Wallace applied to Bali and Lombok is used today to design protected areas.
Key takeaways
- Species richness generally peaks near the equator — the latitudinal diversity gradient (commonly taught; causes still debated).
- Dispersal = organisms cross a barrier; vicariance = a barrier splits a population.
- Continental drift (the breakup of Pangaea) is the largest vicariance event in Earth history.
- Wallace's Line separates Asian and Australian faunas between Bali and Lombok.
- Endemic species are found in one place only — Madagascar lemurs, Australian marsupials, and Galápagos tortoises are classic examples.
- Island biogeography balances immigration against extinction: larger and nearer islands hold more species.
- Habitat fragments behave like islands, which is why the equilibrium model informs conservation design.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
What is the latitudinal diversity gradient, and why is it debated?
Show answer
Species richness is higher near the equator and declines toward the poles; proposed causes include tropical productivity and energy, longer evolutionary time, and speciation rates — multiple causes are likely.
Distinguish dispersal from vicariance, giving an example of each.
Show answer
Dispersal: organisms cross a barrier (birds colonizing an island). Vicariance: a barrier splits a population (a rising mountain range dividing a lineage).
What does Wallace's Line separate, and what created it?
Show answer
Asian and Australian faunas; deep water that remained between the regions even during low sea levels, preventing crossing.
What two rates does the equilibrium theory of island biogeography balance, and how does island size affect them?
Show answer
Immigration and extinction; larger islands have lower extinction rates because they offer more resources and habitat.
Why do habitat fragments behave like islands?
Show answer
Because surrounding altered land acts like ocean: fragments have reduced immigration and higher extinction, just like small distant islands.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Biogeography
- The study of species' geographic distributions and their causes.
- Species richness
- The number of species in a given area.
- Latitudinal diversity gradient
- The pattern of higher species richness toward the equator.
- Dispersal
- Movement of organisms across a barrier to a new area.
- Vicariance
- Splitting of a population by a new barrier.
- Continental drift
- The slow movement of continents over geological time.
- Endemic
- Found naturally in only one region.
- Wallace's Line
- The faunal boundary between Asian and Australian regions.
- Equilibrium theory of island biogeography
- Model balancing immigration and extinction on islands.
- Species–area relationship
- The tendency of larger areas to hold more species.
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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