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

Explanation

(i) Temperature, precipitation, distance from the equator (latitudinal gradient), altitude from sea level (altitude gradient) are some of the factors that determine biodiversity distribution patterns.
(ii) The most important of biodiversity is latitudinal gradient in diversity. This means of there is an increasing diversity from the poles to equator.
(iii) Diversity increases as one moves towards the temperature zones and reaches the maximum at the tropics. Thus, tropics harbour more biodiversity than temperature or polar regions, especially between the latitudes of `23.5^(@)N and 23.5^(@)S`. Harsh conditions exist in temperature areas during the cold seasons while very harsh conditions prevail for most of the year in polar regions.