The gradual reduction in biomass and species numbers that occurs with increasing latitude. There are several theories to explain why life is more abundant in the tropics than in cooler regions. The simplest explanation is that the greater surface area of the planet at the equator, compared to the poles, provides more space in which species can evolve. Another theory suggests that the relative environmental stability of the tropics enables species to specialize to a greater extent, so that more can be packed into any given ecosystem. Further, the greater input of solar energy in the tropics increases available resources, resulting in greater biomass and population sizes compared to colder regions.