Biological diversity is usually considered at three different levels: genetic diversity, species diversity and ecosystem diversity. Genetic diversity refers to the variety of genetic information contained in all of the individual plants, animals and microorganisms.
Genetic diversity occurs within and between populations of species as well as between species. Species diversity refers to the variety of living species. Alpha-diversity is the species diversity within a single site. Beta-diversity refers to species diversity between habitats (thus reflecting changes in species assemblages along environmental gradients).
Ecosystem diversity relates to the variety of habitats, biotic communities, and ecological processes, as well as the tremendous diversity present within ecosystems in terms of habitat differences and the variety of ecological processes.