Species diversity decreases from equator towards poles because
(i) Equator (tropic) regions have less seasonal or more constant environments, which promote niche speciation and thereby more species diversity.
(ii) These regions have more solar energy which contributes to more productivity and greater diversity e.g. The tropics (between 23.5 °N to 23.5 °S) harbours more species than temperate and polar regions. Columbia situated near equator has about 1400 species of birds, while New York (at 41 °N) has about 105 species and Greenland (at 71°) only 56 species.
