Island Biogeography
- Explain how island size and distance from the mainland shape species richness
- Apply the theory of island biogeography to habitat fragments on land
- Predict extinction and immigration rates for islands of differing size and isolation
Islands as natural experiments
The theory of island biogeography (MacArthur and Wilson) explains why some islands hold more species than others. The number of species on an island reaches a balance between two opposing rates: the immigration of new species arriving from a mainland source, and the extinction of species already present. Where these two rates cross, the island holds a roughly stable number of species — an equilibrium — even as which species are present keeps changing.
Size and distance are the master variables
Two features of an island set that equilibrium. Size: larger islands support more species because they offer more habitat, larger populations (which are less likely to go extinct), and more resources — the area effect. Distance from the mainland: nearer islands receive more colonizers because organisms reach them more easily, so they have higher immigration — the distance effect. Combining them, a large, near island holds the most species; a small, far island holds the fewest.
From islands to habitat fragments
The theory is not only about real islands. When roads, farms, and cities carve a continuous forest into isolated patches — habitat fragmentation — each remaining patch behaves like an island in a "sea" of developed land. Smaller, more isolated fragments lose species over time, which is why conservation biologists favor large protected areas connected by wildlife corridors: corridors raise the effective immigration rate and let populations rescue one another.
Four islands lie off one mainland. Rank them from most to fewest expected species: (W) large & near, (X) small & near, (Y) large & far, (Z) small & far.
- 1.Large area lowers extinction and adds habitat, raising species number; nearness raises immigration, also raising species number.
- 2.W is large AND near — both effects favor high richness, so W is highest.
- 3.Z is small AND far — both effects favor low richness, so Z is lowest.
- 4.Between X (small & near) and Y (large & far), the area effect is usually the stronger driver, so the large-far island Y typically edges out the small-near island X.
According to island biogeography, which island would you expect to support the GREATEST number of species?
Do not assume equilibrium means the same species stay forever. The number of species holds roughly constant, but species turnover continues — new arrivals replace others that go locally extinct. Island biogeography predicts how many, not which ones.
A highway splits a large forest into several small, isolated patches. Applying island biogeography, what is the most effective way to reduce the resulting loss of species?
AP questions often show a graph with immigration falling and extinction rising as species number increases; the equilibrium is where the two curves cross. Moving the curves — a nearer island lifts the immigration curve, a larger island lowers the extinction curve — shifts that crossing point to more species.
Answer the 2 checkpoints as you read.
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