Describe the latitudinal gradient and the species-area relationship as patterns of biodiversity.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Open with the idea. Biodiversity is not spread evenly over the Earth; it shows clear patterns.
2 · Describe the latitudinal gradient. Diversity falls from the equator to the poles: Colombia about 1,400 bird species, New York 105, Greenland 56.
3 · Give the reasons.
- Long, undisturbed evolutionary time in the tropics.
- Less seasonal, predictable climate allows niche specialisation.
- More solar energy gives higher productivity.
4 · Describe the species-area relationship. Humboldt: richness rises with area up to a limit; a rectangular hyperbola; log S = log C + Z log A.
5 · Explain Z. Z = 0.1 to 0.2 within regions; 0.6 to 1.2 for whole continents.
6 · Draw the graphs. [Two graphs: S against A (a curve); log S against log A (a straight line with slope Z and intercept log C)]
7 · Link to real life. The Western Ghats, nearer the equator, hold far more species than Ladakh.
8 · Close in one line. So tropical and larger areas generally hold more species.

Scoring tip: Write the equation with each symbol explained, and give both Z ranges. The log graph is a straight line; do not draw it curved.