California Biodiversity Council
c/o University of California
Agriculture and Natural Resources
Program Support Unit
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Davis, California 95616
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Scientific Definitions of Biodiversity"A definition of biodiversity that is altogether simple, comprehensive, and fully operational ... is unlikely to be found." (Noss, 1990) Listed below are several different definitions used by resource managers and ecologists. Together, they should allow you to develop an understanding of the broad concept of biodiversity.
- U.S. Congress Office of Technology Assessment, "Technologies to Maintain Biological Diversity," 1987:
"Biological diversity is the variety and variability among living organisms and the ecological complexes in which they occur. Diversity can be defined as the number of different items and their relative frequency. For biological diversity, these items are organized at many levels, ranging from complete ecosystems to the chemical structures that are the molecular basis of heredity. Thus, the term encompasses different ecosystems, species, genes, and their relative abundance."
- Jones and Stokes Associates' "Sliding Toward Extinction: The State of California's Natural Heritage," 1987:
"Natural diversity, as used in this report, is synonymous with biological diversity... To the scientist, natural diversity has a variety of meanings. These include: 1) the number of different native species and individuals in a habitat or geographical area; 2) the variety of different habitats within an area; 3) the variety of interactions that occur between different species in a habitat; and 4) the range of genetic variation among individuals within a species."
- D.B. Jensen, M. Torn, and J. Harte, "In Our Own Hands: A Strategy for Conserving Biological Diversity in California," 1990:
"Biological diversity, simply stated, is the diversity of life...As defined in the proposed US Congressional Biodiversity Act, HR1268 (1990), "biological diversity means the full range of variety and variability within and among living organisms and the ecological complexes in which they occur, and encompasses ecosystem or community diversity, species diversity, and genetic diversity."
Genetic diversity is the combination of different genes found within a population of a single species, and the pattern of variation found within different populations of the same species. Coastal populations of Douglas fir are genetically different from Sierran populations. Genetic adaptations to local conditions such as the summer fog along the coast or hot summer days in the Sierra result in genetic differences between the two populations of the same species.
Species diversity is the variety and abundance of different types of organisms which inhabit an area. A ten square mile area of Modoc County contains different species than does a similar sized area in San Bernardino County.
Ecosystem diversity encompasses the variety of habitats that occur within a region, or the mosaic of patches found within a landscape. A familiar example is the variety of habitats and environmental parameters that constitute the San Francisco Bay-Delta ecosystem: grasslands, wetlands, rivers, estuaries, fresh and salt water."
- Keystone Center, "Final Consensus Report of the Keystone Policy Dialogue on Biological Diversity on Federal Lands," 1991:
"In the simplest of terms, biological diversity is the variety of life and its processes; and it includes the variety of living organisms, the genetic differences among them, and the communities and ecosystems in which they occur.
Because biological diversity is so complex, and much of it is hidden from our view, unknown, or both, it is necessary to establish means of addressing its distinct and measurable parts. Most basic of these is genetic variation. Genetic variation within and between populations of species affects their physical characteristics, viability, productivity, resilience to stress, and adaptability to change.
A second, more easily recognized aspect of biological diversity is distinct species. Some species, such as American elk, rainbow trout, and ponderosa pine are plentiful. Others such as the red cockaded woodpecker, Siler's pincushion cactus, or grizzly bear, have populations that are much reduced or may even face extinction. Conserving biological diversity includes perpetuating native species in numbers and distributions that provide a high likelihood of continued existence.
Associations of species are a third element of biological diversity. These associations are often called biological communities, usually recognized as distinct stands, patches, or sites, such as old-growth forests, riparian areas, or wetlands. Communities form the biotic parts of ecosystems. The variety of species in an ecosystem is a function of its structural and functional characteristics and the diversity of its ecological processes, and the physical environment.
Finally, at large geographic scales -- from watersheds to the entire biosphere -- biological diversity includes variety in the kinds of ecosystems, their patterns, and linkages across regional landscapes. It is from these large, regional landscapes, such as the Southern Appalachian Highlands, Sierra Nevada, and Northern Continental Divide, that people must derive sustainable yields of resources while perpetuating multiple intact examples of biologically diverse ecosystems.
This hierarchy of the parts and processes of biological diversity is admittedly artificial, and it has a distinct human context. However, it provides a focus for a concept that is infinitely varied and dynamic and that must be addressed in light of the full spectrum of human needs and aspirations."
- World Resources Institute, World Conservation Union, and United Nations Environment Programme, "Global Biodiversity Strategy," 1992:
"Biodiversity is the totality of genes, species, and ecosystems in a region... Biodiversity can be divided into three hierarchical categories -- genes, species, and ecosystems -- that describe quite different aspects of living systems and that scientists measure in different ways.
Genetic diversity refers to the variation of genes within species. This covers distinct populations of the same species (such as the thousands of traditional rice varieties in India) or genetic variation within a populations (high among Indian rhinos, and very low among cheetahs)...
Species diversity refers to the variety of species within a region. Such diversity can be measured in many ways, and scientists have not settled on a single best method. The number of species in a region -- its species "richness" -- is one often- used measure, but a more precise measurement, "taxonomic diversity", also considers the relationship of species to each other. For example, an island with two species of birds and one species of lizard has a greater taxonomic diversity than an island with three species of birds but no lizards...
Ecosystem diversity is harder to measure than species or genetic diversity because the "boundaries" of communities -- associations of species -- and ecosystems are elusive. Nevertheless, as long as a consistent set of criteria is used to define communities and ecosystems, their numbers and distribution can be measured..."
- Edward Grumbine, "Ghost Bears: Exploring the Biodiversity Crisis," 1993:
"There is much more to biodiversity than the numbers of species and kinds of ecosystems. Ecologist Jerry Franklin portrays ecosystems as having three primary attributes: composition, structure, and function.
Ecosystem components are the inhabiting species in all their variety and richness. Many different species, gene-pool abundance, and unique populations are what most people think of when they hear the term "biodiversity". But there is much more to consider.
Ecosystem structure refers to the physical patterns of life forms from the individual physiognomy of a thick-barked Douglas-fir to the vertical layers of vegetation from delicate herbs to tree canopies within a single forest stand. An ecosystem dominated by old, tall trees has a different structure than one comprised of short, quaking aspen. And there is more structure in a multilayered forest (herbs, shrubs, young trees, canopy trees) than in a single sagebrush grassland, prairie, or salt marsh...
Ecosystem functions are hard to see in action. "You can't hug a biogeochemical cycle," says one ecologist. But without the part of the carbon cycle where small invertebrates, fungi, and microorganisms work to break down wood fiber, the downed logs in an ancient forest would never decay. Natural disturbances also play a role. Wildfires release nutrients to the soil, weed out weak trees, and reset the successional clock. The energy of falling water creates spawning beds for salmon even while it carves a mountain's bones. Plants breathe oxygen into the atmosphere. Ecological processes create landscapes and diverse environmental conditions out of life itself.
Ecosystem components, structures, and functions are all interdependent. To understand biodiversity, one has to think like a mountain and consider not only the biotic elements of plants, animals, and other living beings, but also the patterns and processes that shape volcanoes and forests."
- Reed Noss, "Indicators for Monitoring Biodiversity: A Hierarchial Approach," Conservation Biology 4(4):355-364. 1990:
"Biodiversity is not simply the number of genes, species, ecosystems, or any other group of things in a defined area...A definition of biodiversity that is altogether simple, comprehensive, and fully operational (i.e. responsive to real- life management and regulatory questions) is unlikely to be found. More useful than a definition, perhaps, would be a characterization of biodiversity that identifies the major components at several levels of organization.
...(C)omposition, structure, and function...determine, and in fact constitute, the biodiversity of an area. Composition has to do with the identity and variety of elements in a collection, and includes species lists and measures of species diversity and genetic diversity. Structure is the physical organization or pattern of a system, from habitat complexity as measured within communities to the pattern of patches and other elements at a landscape scale. Function involves ecological and evolutionary processes, including gene flow, disturbances, and nutrient cycling."