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Basic Levels of Biodiversity
There are three basic levels of biodiversity: genetic diversity, species diversity, and ecosystem diversity. Genetic diversity refers to variation within and variation among populations of species. It is measured in terms of difference between genes or between DNA or amino acid sequences. All species require a diversity of genes spread among their populations if they are to retain their ability to adapt to changing environments. Genetic diversity also is necessary for food production, forestry, and new pharmaceuticals. It is the selection and manipulation of genetic variability in crop plants that provides the raw material for agriculture. Because genetic variation is the material from which new species evolve, it is also key to biodiversity over the long haul.
The species is the basic unit of classification commonly used by biologists; it can be defined as a group of individuals that are capable of breeding with each other (see Botany for Gardeners). While for many purposes species are the basic unit of biodiversity, it is important to note that they are made up of populations. The local breeding population is the fundamental unit on which natural selection and evolution operates. Individuals within a population contain genetic information, and it is this genetic diversity that determines the population's evolutionary fate and therefore underlies species diversity.
Species do not exist in isolation but rather in a wide array of ecological groupings. On the largest scale, the earth's natural vegetation is divided into large areas of forest, grassland, and desert called plant provinces, or biomes. Each plant province is a plant community on a grand scale containing many smaller communities within its borders. For instance, the great sea of grass that once covered the central United States and south-central Canada included both tall- and shortgrass prairies as well as riverside communities. In the same way, different groups of plants predominate in "cold" and "hot" deserts of the West. What is more, within one of the warm deserts, such as the Sonoran Desert, the assemblages of plants growing in relatively moist areas like arroyos are different from those found in dry, open, exposed expanses.