What factors explain the difference in biodiversity between the tundra and the tropical rain forest?

Tundra is flat, barren-like land whereas a tropical rainforest is a vast, more complex environment. Due to this, tundra provides less habitats for organisms to live in than the rainforest. This means that a greater variety of species are able to co-exist in the rainforest than in tundra, so species diversity is greater.