We have mentioned that number of species are important, and hinted that the abundance of species is important as well. We will talk about abundances later, but for now we need to discuss the ways that ecologists look at species diversity. Generally, ecologist recognize three levels of diversity. Alpha diversity is the number of species that live in a homogenous habitat. Of course the size of the habitat determines the number of species because of the species-area relationship, but the primary focus in alpha diversity is on the number of species that are adapted for a particular habitat.
Gamma diversity is the number of species that live in a heterogeneous region.
By region we mean a broad area that has similar climatic and topographical features but may have different habitats. Within the region organisms are adapted for the general conditions, but within different habitats they may have specialized for exploiting different resources. The actual species may be different in the habitats, so species turnover is important. Beta diversity is the species turnover in a heterogeneous region. It is very difficult to measure beta diversity, but it can be estimated by simply dividing gamma diversity by alpha diversity.