Biodiversity or biological diversity refers to the breadth of species represented within ecosystems, or even on Earth as a whole. Biodiversity may be defined at three levels: Genetic diversity refers to the variety of genes found within a population or between populations of the same species; species diversity, which may also be described as species richness—how many different species are there within a habitat; and ecosystem diversity, which is an attempt to keep track of the loss of different types of habitats. This, in turn, gives scientists a sense of what types of species are going extinct at any given time.