Describing biodiversity

Biodiversity is the variety of living systems. Genetic diversity describes differences in inherited information among individuals and populations of a species. Species diversity includes species richness, the number of species, and evenness, how evenly individuals are distributed among those species. Ecosystem diversity concerns the variety of interacting communities and physical environments across a region. Counting many individuals does not necessarily demonstrate high diversity.

A plantation containing thousands of nearly identical trees can have lower genetic and species diversity than a smaller native woodland.

Comparisons require consistent identification and comparable sampling effort. Surveying a place in different seasons may produce different lists because flowering, migration and detectability change. Compare the same season, area and method before interpreting a change as a biological loss. Classification makes such comparisons possible by attaching stable meanings to names. Organisms are arranged in nested groups: domain, kingdom, phylum, class, order, family, genus and species. Each narrower group belongs within a broader group.

Scientific species names combine a capitalised genus with a lower-case specific epithet, usually printed in italics. International naming conventions allow researchers speaking different languages to discuss the same organism.

Classification hypotheses use morphology, reproduction and molecular sequences. Similar DNA sequences can support shared ancestry, but comparisons must use corresponding sequences and enough evidence. Shared inherited structures are generally more useful than superficial resemblance caused by similar environments. A branching relationship diagram represents hypotheses of common ancestry: a recent shared branching point indicates closer relationship, while the order of tips across the page does not. Classification can change after new specimens or sequence evidence are debated, checked and published.

Revision is a normal response to evidence, not proof that scientific naming is arbitrary.

A morphological species definition groups organisms by diagnostic structures; it works with fossils but may confuse cryptic species or separate different sexes. A reproductive definition groups natural populations capable of producing fertile offspring and reproductively isolated from others. It is difficult to apply to fossils, asexual organisms and geographically separated populations. Genetic definitions use sequence similarity or distinct genetic lineages, but no single universal percentage separates every species. Combining evidence is usually stronger than relying on appearance alone.