Question

Islands typically have higher species extinction rates and are more isolated from sources of colonists than the mainland. The four panels below (P, Q, R, S) each plot the log of species number against the log of area, with a solid line for the mainland relationship and a dashed line for islands of different sizes.

Which one of the figures above represents the most likely relationship between species richness and area for mainland versus island sites?

• A. P
• B. Q
• C. R
• D. S

Hint:

Islands should have fewer species than a mainland area of the same size due to isolation and extinction, but the island species-area relationship usually rises more steeply with area than the mainland's.

Answer 1

The Correct Option is A

This question is about the classic island biogeography pattern: mainland areas and islands both gain species as area grows, but islands lag behind because they are harder to colonise and lose species faster. Let's check each panel against that expectation.

1. Panel P: The mainland (solid) line starts above the island (dashed) line and both climb, with the island's line climbing more steeply and catching up as area grows. This matches both parts of the expected pattern, islands hold fewer species at a given area, and the gap narrows as area increases, since bigger islands act more like scaled down mainlands.
2. Panel Q: Here the mainland line rises while the island line actually falls as area grows, and the two lines cross. There is no ecological reason for island richness to decline as island area increases, so this panel does not fit.
3. Panel R: Here the dashed island line sits above the solid mainland line across the whole range. This would mean islands host more species than equally sized mainland areas, which contradicts the higher isolation and extinction rates islands are known to have.
4. Panel S: Here the mainland line is completely flat, showing no change in richness with area at all. Since larger mainland areas reliably support more species, a flat mainland line does not match real data.

Only Panel P keeps the mainland above the island, matching lower island richness, while letting the island's steeper climb close the gap as area increases, matching how small islands are disproportionately species poor.

Let's summarize:

• Both mainland and island species richness rise with area.
• At a given area, islands hold fewer species than the mainland because they are more isolated and lose species faster.
• The island species-area slope is typically steeper than the mainland's.

So Panel P is the figure that matches the real, observed relationship.