There is higher amount of solar energy available in the tropics because of the following reasons.
(i) The rays of sun are more concentrated in the tropics.
(ii) The sun's rays have less atmosphere to pass through, so energy lost in absorption and reflection by the atmosphere is less.
(iii) Tropical rainforest areas absorb there radiations due to presence of dense vegetation. Availability of more solar energy in tropics contribute higher productivity which in turn is likely to contribute greater maximum biodiversity.