Biodiversity（生物多样性） is the totality of genes（基因）,species（物种）, and ecosystems （生态系统）in a region.

Thewealth of life on earth today is the product of hundreds of millions of yearsof evolutionary（进化的） history. Over the courseof time, human cultures have emerged and adapted to the local environment,discovering, using, and altering local biotic resources（生物资源）.

Manyareas that now seem "natural" bear the marks of millennia of humanhabitation, crop cultivation（耕作）,and resource harvesting（收获）.The domestication（驯养）and breeding（饲养） of local varieties ofcrops and livestock have further shaped biodiversity.

Biodiversitycan be divided into hierarchical categories that describe quite differentaspects of living systems that scientists measure in different ways.

• Genetic diversity（遗传多样性）

Genetic diversity refers to the variation ofgenes within species.

This covers distinct populations of the same species (such as thethousands of traditional rice varieties in India) or genetic variation within apopulation (which is very high among Indian rhinos（犀牛）, for example, and very low among cheetahs（印度豹）).

Until recently, measurements of genetic diversity were applied mainly todomesticated species and populations held in zoos or botanic gardens, butincreasingly the techniques are being applied to wild species.

"Each species is the repository of an immense amount of geneticinformation," writes E. O.Wilson in Biodiversity (1988. Washington, DC: National AcademyPress). "The number of genes range from about 1,000 in bacteria and 10,000in some fungi to 700,000 or more in many flowering plants and a few animals. Atypical mammal such as the house mouse has about 100,000 genes. ...If stretchedout fully, the DNA would be roughly one meter long. But this molecule isinvisible to the naked eye. ...The full information contained therein, iftranslated into ordinary-size letter of printed text, would just about fill all15 editions of the Encyclopedia Britannica published since 1768."

Species diversity（物种多样性）

Species diversity refers to the variety ofspecies within a region.

Such diversity can be measured in many ways, and scientists have notsettled 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, "taxonomicdiversity," also considers the relationship of species to each other.

For example, an island with two species of birds and one species of lizardhas greater taxonomic diversity than an island with three species of birds butno lizards.

Thus, even though there may be more species of beetles on earth than allother species combined, they do not account for the greater part of speciesdiversity because they are so closely related.

Similarly, many more species live on land than in the sea, but terrestrialspecies are more closely related to each other than ocean species are, sodiversity is higher in marine ecosystems than a strict count of species wouldsuggest.

Estimates of global species diversity havevaried from 2 million to 100 million species, with a best estimate of somewherenear 10 million, and only 1.4 million have actually been named.

New species are still beingdiscovered--even new birds and mammals. On average, about three new species ofbirds are found each year, and as recently as 1990, a new species of monkey wasdiscovered. Other vertebrate groups are still far from being completelydescribed: an estimated 40 percent of freshwater fishes in South America havenot yet been classified.

• Ecosystem diversity（生态系统多样性）

Ecosystem diversity is harder to measure than species or genetic diversity because the"boundaries" of communities（群落） -- associations of species -- and ecosystems areelusive.

Nevertheless, as long as a consistent set of criteria isused to define communities and ecosystems, their number and distribution can bemeasured.

Until now, such schemes have been applied mainly atnational and sub-national levels, though some coarse global classificationshave been made.

Besides ecosystem diversity, many other expressions ofbiodiversity can be important. These include:

• the relative abundance of species,
• the age structure of populations,
• the pattern of communities in a region,
• changes in community composition and structure over time, and
• ecological processes as predation, parasitism, and mutualism.

More generally, to meet specific management or policygoals, it is often important to examine not only compositional diversity --genes, species, and ecosystems -- but also diversity in ecosystem structure andfunction.

For individuals and populations, these interactions include suchmechanisms as: predation（捕食）,competition（竞争）, parasitism（寄生）, and mutualism（共生） while communities changethrough the process of succession. In yet another type of interaction, speciesinfluence their physical environment--whether through primary production（初级生产者） (the transformation of solar energy to biomass through photosynthesis（光合作用）), decomposition（分解） (thebreakdown of organic materials by organisms in the environment), or participation in（参与） biogeochemical cycles (the movement of nutrients, water, and otherchemical elements through living organisms and the physical environment).

• Cultural diversity（文化多样性）

Human culturaldiversity could also be considered part of biodiversity.

Like genetic or species diversity, some attributes of human cultures (say,nomadism or shifting cultivation) represent "solutions" to theproblems of survival in particular environments.

And, like other aspects of biodiversity, cultural diversity helps peopleadapt to changing conditions.

Cultural diversity is manifested by diversity in language, religiousbeliefs, land-management practices, art, music, social structure, cropselection, diet, and any number of other attributes of human society.