19.3 Island Biogeography
Key Takeaways
MacArthur and Wilson treated species number as a balance between immigration of new species and extinction of residents.
Larger islands hold more species because extinction rates are lower and habitats are more varied.
Islands closer to a mainland receive more immigrants and hold more species than distant islands of the same size.
Equilibrium is dynamic: species identity can change while the number stays similar, so equilibrium does not mean extinction has stopped.
A large near island should hold more species than a small far island, all else equal, and habitat fragments on land can behave like islands.
19.3 Island Biogeography
A balance of two rates
Island biogeography explains why islands differ in species number. Robert MacArthur and Edward O. Wilson treated that number as a balance between two rates. Immigration adds species not yet established. Extinction removes species already there. Items usually ask which island should hold more species, or what equilibrium does not mean. Answer from the rates. Do not invent a species total for a real island.
On an empty island near a mainland pool, almost every mainland species is still absent, so immigration of new species starts high. As more species establish, fewer mainland species remain missing, and immigration of new species falls. Extinction runs the other way. It starts low when few species are present and rises as the island fills, because populations are smaller and competition is sharper.
Draw immigration as a falling curve and extinction as a rising curve. They cross. That crossing is the equilibrium number of species: new arrivals about match losses. The theory expects richness to hover near that crossing. It does not require one named list.
Equilibrium is dynamic
Equilibrium does not mean extinction has stopped. Species keep arriving and species keep disappearing, so the names turn over while the total can stay similar. A later survey can find a different set and a similar count, and that result fits the model. A storm or a failed breeding season can change the roster without showing that the balance of rates has failed. The stable feature is the number, not the membership.
Area and distance
The two island properties shift different curves.
Area mainly lowers extinction, and it adds habitat variety. A larger island supports larger populations, which are less likely to vanish in a bad year, and it offers more kinds of places for different species. The extinction curve sits lower, so the crossing moves toward more species. Larger islands hold more species because extinction rates are lower and habitats are more varied. The mainland pool can be the same. The large island simply loses residents more slowly and houses more kinds of habitat.
Distance mainly changes immigration. A near island is easier to reach, so more individuals of more species arrive and the immigration curve sits higher. A distant island of the same size receives fewer immigrants, so its immigration curve sits lower and crosses extinction at fewer species. Closer islands hold more species than distant islands of the same size. Distance affects immigration more directly than it changes the island's area. A far island is not smaller merely because it is far. Near versus far compares islands of one size. Large versus small compares islands at one distance.
| Island feature | Rate it changes most directly | Result, all else equal |
|---|---|---|
| Larger area | Lower extinction and more habitat variety | More species than a smaller island |
| Smaller area | Higher extinction and fewer habitat types | Fewer species than a larger island |
| Closer to the mainland | Higher immigration | More species than a farther island of the same size |
| Farther from the mainland | Lower immigration | Fewer species than a nearer island of the same size |
Note
Species number balances immigration of new species against extinction of residents. The equilibrium is dynamic: names can change while the number stays similar. Distance changes immigration. It does not, by itself, change area. Equilibrium does not mean extinction has stopped.
Large and near versus small and far
Compare two islands and change nothing else. Island A is large and near the mainland. Island B is small and far from the mainland. Area favors A, because extinction is lower and habitats are more varied. Distance also favors A, because immigrants arrive more readily. Both effects point the same way. All else equal, the large near island should hold more species than the small far island. The comparison is qualitative. It does not assign a head count to any real coastline.
When the effects oppose
A large far island versus a small near island is a different problem. Area favors the large island. Distance favors the near island. The effects oppose, and the model does not say which one wins without more information. The clean prediction is the comparison in which size and distance agree.
At the crossing, immigration of new species equals extinction of residents. Richness knocked below that point tends to climb back, because immigration exceeds extinction. Richness pushed above it tends to fall back. That return is a balance of rates, not a promise that any named species is safe. Turnover can continue while the number sits near the crossing.
Fragments on land
A habitat fragment can function as an island for species that will not cross the surroundings. A woodlot in farmland, a forest isolated on a peak, a lake in dry country, or a reserve ringed by cities can play that role. The surrounding land is the barrier.
The same predictions carry over. A larger fragment loses species more slowly because populations are larger and habitats inside it are more varied. A fragment close to a source of colonists gains species faster than a distant fragment of the same size. Distance still acts on immigration. It does not rewrite the area inside the boundary. A small woodlot is small because the boundary encloses a small area. The kilometers to the next forest explain scarce arrivals. They do not shrink the hectares inside the fence.
A large reserve near other suitable habitat should hold more species than a small reserve far from any source, all else equal. Real landscapes also differ in hunting, pollution, and the species present at the start. The exam version isolates area and distance unless the stem gives another fact. A fragment on a continent is not free of extinction. Equilibrium still includes loss, and the roster can change while the count stays similar.
What is true of equilibrium in the MacArthur and Wilson island model?
Greater distance increases an island's area and therefore its immigration
Area is the only variable, so distance cannot change how many species an island holds
Extinction stops, and the list of resident species stays frozen
Immigration of new species balances extinction, and species identity can change while the number stays similar
All else equal, which island should hold more species?
A far island, because distance lowers extinction without changing immigration
A small island, because a high extinction rate raises the species count
A large island near the mainland, rather than a small island far from the mainland
A small island far from the mainland
Which statement uses island biogeography correctly for a forest fragment?
The fragment can behave like an island, and distance from colonists affects immigration more directly than it changes the fragment's area
Once the fragment is at equilibrium, resident species no longer go extinct
A far fragment receives more immigrants than a near fragment of the same size
A smaller fragment holds more species because its extinction rate is higher
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