17. Following figure shows McArthur and Wilson’s equilibrium model of biota on a single island

    In this figure, terms A, B, C and D in order are
    (1) extinction. immigration, equilibrium number of species, size of species pool.
    (2) immigration. extinction, equilibrium number of species, size of species pool.
    (3) extinction. immigration, size of species pool, equilibrium number of species.
    (4) immigration, extinction, size of species pool, equilibrium number of species.

The Core of the Equilibrium Theory

The equilibrium theory, first published in the 1960s, proposes that the number of species on an island reflects a balance between two opposing processes:

• Immigration: The arrival of new species from a source pool (usually the mainland or a larger island).

• Extinction: The loss of species already present on the island.

As the number of species on the island increases, the immigration rate decreases (because fewer new species remain to colonize the island), while the extinction rate increases (due to greater competition and limited resources). The intersection of these two curves marks the equilibrium number of species.

Decoding the Graph: What Do A, B, C, and D Represent?

Let’s match each term to its place in the classic graph:

• A: The curve that starts high when there are few species and declines as the number of species increases. This is the immigration curve.

• B: The curve that starts low when there are few species and rises as the number of species increases. This is the extinction curve.

• C: The point where the immigration and extinction curves intersect. This is the equilibrium number of species—the predicted number of species the island will support at steady state.

• D: The far right of the x-axis, representing the total number of species available to colonize the island from the source pool. This is the size of the species pool.

Why This Order Is Correct

According to MacArthur and Wilson’s theory:

• The immigration rate is highest when the island is empty and declines as more species arrive.

• The extinction rate is lowest when there are few species and rises as more species compete for limited resources.

• The point where immigration equals extinction is the equilibrium number of species.

• The species pool is the maximum possible number of species that could colonize the island, usually found on the mainland.

Matching the Options

Given the options:

1. Extinction, immigration, equilibrium number of species, size of species pool

2. Immigration, extinction, equilibrium number of species, size of species pool

3. Extinction, immigration, size of species pool, equilibrium number of species

4. Immigration, extinction, size of species pool, equilibrium number of species

The correct order, based on the classic interpretation, is:

(2) Immigration, extinction, equilibrium number of species, size of species pool

Conclusion

MacArthur and Wilson’s equilibrium model of island biogeography is best interpreted in the order: immigration, extinction, equilibrium number of species, and size of species pool. This framework remains a cornerstone of ecological theory, helping scientists predict and manage biodiversity on islands and habitat fragments worldwide.