The Genesis of the Theory

MacArthur and Wilson’s theory was first articulated in a landmark 1963 paper and later expanded into their influential 1967 book. They proposed that islands—whether literal islands or isolated patches of habitat—are ideal natural laboratories for studying the processes that regulate biodiversity. Their model used simple, elegant graphs to illustrate how immigration and extinction rates change as the number of species on an island increases.

The Dynamic Equilibrium Concept

The key prediction of the equilibrium theory is that the number of species on an island tends toward a constant value over time—a concept known as dynamic equilibrium. This equilibrium is not a static state; rather, it is maintained by the ongoing processes of new species arriving (colonization) and existing species disappearing (extinction). At equilibrium, the rate at which new species colonize the island is balanced by the rate at which species go extinct.

How the Balance Works

• When an island is empty: Immigration rates are high because every new arrival is a new species, and extinction rates are low because there is little competition.

• As more species arrive: Immigration rates drop (fewer new species remain to colonize), and extinction rates rise (more competition and limited resources).

• At equilibrium: The number of species stabilizes because the rate of new arrivals matches the rate of extinctions.

Experimental Validation in the 1960s

During the 1960s, experiments were conducted to test the predictions of MacArthur and Wilson’s theory. One of the most famous involved the defaunation (removal of animal life) of small mangrove islands in the Florida Keys. Researchers observed how species recolonized these islands over time. The results showed that:

• The number of species initially increased rapidly as colonization occurred.

• Over time, the number of species stabilized, even though the identities of the species continued to change.

• This supported the idea of a dynamic equilibrium—not a fixed set of species, but a stable number maintained by balanced colonization and extinction rates.

Recent studies using fossil and DNA evidence have further confirmed that island biodiversity tends to reach and maintain this equilibrium over long periods, validating MacArthur and Wilson’s original prediction127.

What the Experiments Revealed

The main findings of these classic studies indicate that, over a long period of time, the overall rate of colonization will be balanced by the rate of extinction. This means that while the specific species present on an island may change, the total number of species remains relatively constant, provided the island’s size and isolation do not change significantly.

Why Not the Other Options?

• The rate of extinction and colonization are not equal to each other: This is incorrect at equilibrium; the theory specifically predicts that they become equal at the equilibrium point.

• The colonization rates gradually exceed extinction rates: This only occurs in the early stages of colonization, not over the long term.

• Rate of colonization will continue to increase while extinction rates will decline: Both rates change in response to the number of species present, but they do not move in one direction indefinitely.

The Lasting Impact of the Theory

MacArthur and Wilson’s equilibrium theory has had profound implications for ecology and conservation. It is used to guide the design of nature reserves, predict the impacts of habitat fragmentation, and understand the dynamics of biodiversity in isolated systems.

Conclusion

The equilibrium model of island biogeography, validated by experiments since the 1960s, demonstrates that over a long period of time, the overall rate of colonization will be balanced by the rate of extinction. This dynamic equilibrium shapes the richness and resilience of island ecosystems and remains a foundational principle in ecological science.

Correct statement:
(3) the overall rate of colonization will be balanced by the rate of extinction