18. The theory of island biogeography has synthesized into theory the following concepts, except:
(1) Competition                                              (2) Immigration
(3) Equilibrium                                                          (4) Speciation

Core Concepts Synthesized in the Theory

MacArthur and Wilson’s equilibrium model is built around several key ecological concepts:

• Immigration: The arrival of new species from a source pool, typically the mainland or a larger, more diverse habitat. Immigration rates are highest when an island is empty and decline as the number of resident species increases.

• Extinction: The loss of species from the island, which becomes more likely as the number of species rises due to increased competition and limited resources.

• Equilibrium: The point where immigration and extinction rates balance, resulting in a stable number of species on the island, even though the actual species present may change over time.

• Competition: The theory recognizes that as more species accumulate, competition for resources intensifies, leading to higher extinction rates1589.

The Excluded Concept: Speciation

While the theory of island biogeography elegantly synthesizes immigration, extinction, equilibrium, and competition, speciation is not a central component of the original model. The primary focus is on the dynamic balance between species arriving and going extinct, rather than on the evolution of new species on the island itself.

Although later extensions and related research have considered the role of speciation—especially on large or remote islands—MacArthur and Wilson’s original equilibrium theory did not integrate speciation as a driving force in determining equilibrium species richness17. The model assumes a fixed species pool from which immigrants arrive, and does not account for the creation of new species through evolutionary processes.

Summary Table

Conclusion

Speciation is the concept not synthesized into the original theory of island biogeography by MacArthur and Wilson. The model focuses on immigration, extinction, competition, and the resulting equilibrium, but not on the evolutionary process of new species forming on islands.

Correct answer: (4) Speciation