In which natural zone the diversity of flora and fauna is greatest? And in which one is the smallest?

In terms of the diversity of animal and plant species, the richest natural zone is the tropical forest, which is located in the equatorial and subequatorial climatic zones. About two thirds of all described species of animals and plants are concentrated in this zone. The reason for this is the corresponding natural conditions (large amount of precipitation, constant positive temperature, etc.).
For the poorest is the zone of arctic deserts, which is located on the extreme parts of the planet. Here plants are completely absent and only a few dozen species of animals live.