The region near the middle of the Earth between the Tropic of Cancer and Tropic of Capricon is considered as the Tropic region. This region receives the most amount of solar energy which leads to better productivity and higher species diversity. Also, the seasonal variations in such regions are low and environment mainly remains the same supporting high species richness and promoting niche specialization. Species-area relationship show a rectangular hyperbolic function. Thus, species diversity decreases as we move away from the equator towards poles.
Also as we move from low altitude to higher altidue, the species diversity decreases.