In general, diversity decreases from the equator toward the poles. Habitats that have experienced historical changes are undersaturated because of inadequate time for species to colonize and adapt.
High productivity increases the number of species can be supported, allows more specialization.
Small, isolated ephemeral, or physically extreme habitats have lower colonization rates, higher extinction rates than large, continuous, permanent, physically equable habitats. In a constant environment, species can specialize on predictable resources.