Why Are Most of the World's Species Tropical?

Why are most of the world’s species tropical? Provide a detailed analysis of the latitudinal gradient in species richness.
Latitudinal gradients in species diversity have been recognized for over a century (Wallace 1878). The cause of these gradients has been the subject of debate since then. This has produced several theories and hypotheses about what factors are dependent in the regulation of diversity (Pianka 1966). Nowadays theories think about many factors such as competition, disturbance (natural and human disturbance) fragmentation, invasion and many others. It is difficult to test the separate causes and to test them individually and also to make hypotheses including all of them. The main theory at this time is the hypothesis of latitudinal gradients of diversity.   (Stevens 1989) He produced a rule called ‘rapoport’s rule’ which states that “latitudinal ranges of plants and animals are generally smaller at low than at high latitudes. This rule was used to explain why there is higher species diversity in the tropics than at the poles because narrower ranges in the tropics would facilitate more species to coexist. He later expanded the rule to included altitudinal gradients also.
The temperature gradient has been known for many years with temperature being as low as -40C to -70C in the poles and normally over 25C all year round at the tropics. This high temperature does not only affect the heat, it also increases the speed of evaporation which then produces a more humid area and also more precipitation. This means more energy is put into these communities by the sun (the source of earths energy) and this is what many people think could make the tropics much more suitable for species richness. If the plants get more sunlight they can grow bigger and for more time over the year. The arctic is a very harsh environment. In the tundra it is very cold: the average winter temperature is -25° C, and in the summer it is rarely over 10° C. The summertime only lasts...