a. Temperate areas have undergone frequent glaciations in the past killing most of the species . However, no such disturbances occurred in the tropics where species continued to flourish and evolve undisturbed for millions of years.
b. Tropical environments are relatively more constant promoting niche specialization and lead to greater species diversity.
c. Availability of more solar energy in the tropics contributes to higher productivity and in turn may contribute to greater diversity indirectly.