Quantity of plant and animal species found in a given environment. Sometimes habitat diversity (the variety of places where organisms live) and genetic diversity (the variety of traits expressed within a species) are also considered types of biodiversity. The estimated 3–30 million species on Earth are divided unequally among the world's habitats, with 50–90% of the world's species living in tropical regions. The more diverse a habitat, the better chance it has of surviving a change or threat to it, because it is more likely to be able to make a balancing adjustment. Habitats with little biodiversity (e.g., Arctic tundra) are more vulnerable to change. The 1992 Earth Summit resulted in a treaty for the preservation of biodiversity.