Q.Why do we find a decrease in biodiversity distribution, if we move from the tropics towards the poles?

Step 1. The chapter states the latitudinal gradient directly: diversity increases moving from the poles to the equator, reaching its maximum in the tropics, between roughly 23.5°N and 23.5°S.

Step 2. This is illustrated with bird-species counts: Colombia, near the equator, has nearly 1,400 bird species, while New York at 41°N has only 105, and Greenland at 71°N has just 56 — a steady fall as latitude (distance from the equator) increases.

Step 3. The chapter explains why: tropical regions have warm, congenial habitats; environmental conditions favourable for both speciation and for supporting large numbers and varieties of organisms; temperatures in the 25-35°C range in which most metabolic activity runs efficiently; average rainfall over 200mm a year; and climate, seasons, temperature, humidity and photoperiod that stay stable rather than swinging seasonally. …

Explain the latitudinal-gradient pattern using the chapter's climatic reasons for tropical richness and its …

• Describing the pattern (diversity falls poleward) without explaining the underlying climatic reasons the chapter gives. …