生物短文

生物短文

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.

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

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

Biodiversity can be divided into hierarchical categories that describe quite different aspects of living systems that scientists measure in different ways.

Genetic diversity（遗传多样性）
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 population (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 to domesticated species and populations held in zoos or botanic gardens, but increasingly the techniques are being applied to wild species.

"Each species is the repository of an immense amount of genetic information," writes E. O. Wilson in Biodiversity (1988. Washington, DC: National Academy Press). "The number of genes range from about 1,000 in bacteria and 10,000 in some fungi to 700,000 or more in many flowering plants and a few animals. A typical mammal such as the house mouse has about 100,000 genes. ...If stretched out fully, the DNA would be roughly one meter long. But this molecule is invisible to the naked eye. ...The full information contained therein, if translated into ordinary-size letter of printed text, would just about fill all 15 editions of the Encyclopedia Britannica published since 1768."

Species diversity（物种多样性）

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 greater taxonomic diversity than an island with three species of birds but no lizards.

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

Similarly, many more species live on land than in the sea, but terrestrial species are more closely related to each other than ocean species are, so diversity is higher in marine ecosystems than a strict count of species would suggest.

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

New species are still being discovered--even new birds and mammals. On average, about three new species of birds are found each year, and as recently as 1990, a new species of monkey was discovered. Other vertebrate groups are still far from being completely described: an estimated 40 percent of freshwater fishes in South America have not 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 are elusive.

Nevertheless, as long as a consistent set of criteria is used to define communities and ecosystems, their number and distribution can be measured.

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

Besides ecosystem diversity, many other expressions of biodiversity 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 policy goals, it is often important to examine not only compositional diversity -- genes, species, and ecosystems -- but also diversity in ecosystem structure and function.

For individuals and populations, these interactions include such mechanisms as: predation（捕食）, competition（竞争）, parasitism（寄生）, and mutualism（共生） while communities change through the process of succession. In yet another type of interaction, species influence 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） (the breakdown of organic materials by organisms in the environment), or participation in（参与） biogeochemical cycles (the movement of nutrients, water, and other chemical elements through living organisms and the physical environment).

Cultural diversity（文化多样性）
Human cultural diversity 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 the problems of survival in particular environments.

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

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