Why are the Tropics more productive than Temperate regions?

Tropics have plenty of rains & sunlight without any extremes of temperature,which are the basic tenets for primary production. Temperature extremes in temperate regions is a limiting factor for production.Areas covered with ice & snow just cannot compete with tropics in terms of primary production.But there are exceptions in temperate regions which do show good production,like the grasslands of Ukraine,& some regions of Spain & France.More productive ecosystems means that there are huge varieties of plants which in turn can support many varieties of animals,whose fecal matter gets distributed by the animals as they move about in places,which in turn fertilizes the soil,which leads to more production-due to rains & sunlight.So more the varieties of plants,more are the varieties of animals available & this leads to supporting more trophic levels.Reference:Ecology by Eugene.P.Odum.