Localities at lower latitudes generally have more species than localities at higher latitudes. Understanding the global distribution of biodiversity is one of the most significant objectives for ecologists and biogeographers. Beyond purely scientific goals and satisfying curiosity, this understanding is essential for applied issues of major concern to humankind, such as the spread of invasive species, the control of diseases and their vectors, and the likely effects of global climate change on the maintenance of biodiversity (Gaston 2000). Tropical areas play a prominent role in the understanding of the distribution of biodiversity, as their rates of habitat degradation and biodiversity loss are exceptionally high.
There are three general hypothesis for this:
1) Spatial/Area hypotheses:depend solely on the spatial and areal characteristics of the tropics. Spatial/area hypotheses all depend on the fact that tropical areas are warmer and wetter to explain species richness.
2) Historical/Evolutionary hypotheses: Low species richness at higher latitudes is a consequence of an insufficient time period available for species to colonize or recolonize areas because of historical perturbations such as glaciation.Additionally, higher evolutionary rates in the tropics have been attributed to higher ambient temperatures, higher mutation rates, shorter generation time and/or faster physiological processes.
3) Biotic hypothesis: This hypothesis claims ecological species interactions such as competition, predation, mutualism, and parasitism are stronger in the tropics and these interactions promote species coexistence and specialization of species, leading to greater speciation in the tropics.
Most information taken from Lecture and Wikipedia