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.
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