The variability among living organisms on the earth, including the variability within and between species and within and between ecosystems.
Biological diversity, often shortened to biodiversity, is the variation of life at all levels of biological organization, referring not only to the sum total of life forms across an area, but also to the range of differences between those forms. Biodiversity runs the gamut from the genetic diversity in a single population to the variety of ecosystems across the globe.
Greater biodiversity in ecosystems, species, and individuals leads to greater stability. For example, species with high genetic diversity and many populations that are adapted to a wide variety of conditions are more likely to be able to weather disturbances, disease, and climate change. Greater biodiversity also enriches us with more varieties of foods and medicines.
The measurement of biodiversity is complex and has a qualitative as well as a quantitative aspect. If a species is genetically unique — if, for instance, it's far out on a remote arm of the evolutionary tree, like the distinctive, peculiar platypus — its biodiversity value is greater than that of a species clustered with many similar species because it preserves a unique part of the evolutionary history of the planet. This means that biodiversity can't be defined merely as the aggregate total of genes, species, or habitats, but must also be understood as a measure of the variety of their differences.
That said, the easiest shorthand way to describe biodiversity is often through species counts. Current estimates of global species diversity vary between 2 million and 100 million species, with a popular estimate of somewhere near 13 to 14 million. The majority of them are arthropods. But very little is known about most species. Only roughly 1.5 million species have been described, and only 40,000 to 50,000 species have had their conservation status assessed. Roughly a third of these are believed to be at some risk of extinction.
Diversity is concentrated in certain areas and is highest in the tropics, in a band around the equator, declining progressively toward the polar regions. Specific places with high overall diversity or high levels of endemism — arrays of species found nowhere else — are often called hotspots and include parts of the southwestern United States and Mexico, Brazil, California, and South Africa, as well as Hawaii, Madagascar, New Zealand, and other islands across the world. Of course, every intact ecosystem — hotspot or not — is important to preserve, not only because they all provide services, such as clean water and climate moderation, but also because each contains a unique ecological composition and priceless evolutionary information.
The Earth's biodiversity is the result of 4 billion years of evolution — change in the inherited traits of a population of organisms from one generation to the next. Up until about 600 million years ago, life consisted of single-celled organisms.
The history of biodiversity during the Phanerozoic era (the past 540 million years) begins with the rapid growth of the Cambrian explosion — the period in which most phyla of multicellular organisms appeared. Over the next 400 million years, global diversity showed little overall trend and was marked by periodic, massive losses of diversity classified as mass extinction events. The largest of these occurred about 250 million years ago and is often called the P-Tr or Permian-Triassic extinction event; various mechanisms, ranging from increased volcanic eruptions to a drastic decrease in the air's oxygen, are thought to have contributed to the P-Tr, which killed about 96 percent of all marine species and an estimated 70 percent of land species. Recovery from this “Great Dying” didn't even begin for 4 to 6 million years, during which only a small number of resilient species roamed the earth. The most recent mass extinction, the K-T event, happened 65 million years ago and ended the reign of the dinosaurs.
The causes of previous mass extinctions aren't fully understood, but extinctions tend to occur when long-term stresses like climate change are compounded by sudden shocks. Four of the previous five mass extinction events were probably caused by greenhouse gas emissions and associated global warming; only the K-T event appears to have been a non-warming impact.
Most biologists agree that our era comprises a new, sixth mass extinction, the Holocene extinction event, caused by ongoing human impacts on the biosphere — primarily numerous forms of habitat destruction and fragmentation. Global warming will soon trump other threats as the leading cause of extinction. Scientists such as E.O. Wilson argue that the current extinction rate, which is between 100 and 10,000 times the “background” rate (and projected to rise), will eliminate most species on Earth within the next century.
Beyond its intrinsic value, biodiversity is necessary to human survival. Ecosystem diversity is crucial to ecosystem integrity, which in turn enables our life support, giving us a livable climate, breathable air, and drinkable water. Food-crop diversity and pollinating insects and bats allow agriculture to support our populations; when disease strikes a food crop, only diversity can save the system from collapse. Plant and animal diversity provide building blocks for medicine, both current and potential; almost half of the pharmaceuticals used in the United States today are manufactured using natural compounds, many of which cannot be synthesized. They also provide critical industrial products used to build our homes and businesses, from wood and rubber to the fuels that underpin our economies — even coal and oil are the products of ancient plant matter and preserved zooplankton remains.
Biodiversity plays a central mythic and symbolic role in our language, religion, literature, art, and music, making it a key component of human culture with benefits to society that have not been quantified but are clearly vast. From our earliest prehistory, people have never lived in a world with low biodiversity. We've always been dependent on a varied and rich natural environment for both our physical survival and our psychological and spiritual health. As extinctions multiply, and cannot be undone, we tread further and further into unexplored terrain — a journey from which there is no return.