What trend is observed in the species–area relationship for islands?

• A. The increase in number of species with increasing land area slows for larger islands.
• B. Species number increases linearly with area.
• C. Species number decreases with island area.
• D. The area of an island has no effect on species richness.

Answer: (D)

On islands, the number of species tends to rise as area increases, but the increase slows down for larger islands. Bigger areas provide more habitat types and larger populations, which supports more species and reduces extinction risk. Yet the gain in species is not proportional to area; doubling the area yields a smaller-than-double increase in species because the curve is concave. This pattern is captured by the species–area relationship S ≈ cA^z, with z between about 0.2 and 0.4 for islands, meaning that S grows with area but at a diminishing rate as area gets larger. So the best description is that species richness increases with land area, but the rate of increase decelerates for larger islands.