The biodiversity of planet Earth is the total variability of life forms. Currently around 1.9 million extant species are believed to have been described, but some scientists believe 20% are synonyms, reducing the total valid described species to 1.5 million. Scientists expect there to be 5 ± 3 million extant species on Earth. Some 250,000 valid fossil species have also been named, but this is believed to be an even smaller proportion of all species that have ever lived.
Global biodiversity is affected by extinction and speciation. The background extinction rate varies among taxa but it is estimated that there is approximately one extinction per million species years. Mammal species, for example, typically persist for 1 million years. Biodiversity has grown and shrunk in earth's past due to (presumably) abiotic factors such as extinction events caused by geologically rapid changes in climate. Climate change 299 million years ago was one such event. A cooling and drying resulted in catastrophic rainforest collapse and subsequently a great loss of diversity, especially of amphibians. However, the current rate and magnitude of extinctions are much higher than background estimates. This, considered by some to be leading to the sixth mass extinction, is a result of human impacts on the environment.
Drivers that affect biodiversity
- Habitat change (see: habitat fragmentation or habitat destruction) is the most important driver currently affecting biodiversity, as some 40% of forests and ice-free habitats have been converted to cropland or pasture. Other drivers are:
- invasive species
- climate change
Biodiversity is usually plotted as the richness of a geographic area, with some reference to a temporal scale. Types of biodiversity include:
- taxonomic diversity
- species diversity
- ecological diversity
- morphological diversity
- genetic diversity
Taxonomic diversity, that is the number of species, genera, family is the most commonly assessed diversity type. A few studies have attempted to quantitatively clarify the relationship between different types of diversity. For example, Sarda Sahney a researcher at the University of Bristol has found a close link between vertebrate taxonomic and ecological diversity.
Chapman, 2005 and 2009 has attempted to compile perhaps the most comprehensive recent statistics on numbers of extant species, drawing on a range of published and unpublished sources, and has come up with a figure of approximately 1.9 million estimated described taxa, as against possibly a total of between 11 and 12 million anticipated species overall (described plus undescribed), though other reported values for the latter vary widely. It is important to note that in many cases, the values given for "Described" species are an estimate only (sometimes a mean of reported figures in the literature) since for many of the larger groups in particular, comprehensive lists of valid species names do not currently exist. For fossil species, exact or even approximate numbers are harder to find; Raup, 1986  includes data based on a compilation of 250,000 fossil species so the true number is undoubtedly somewhat higher than this. It should also be noted that the number of described species is increasing by around 18,000-19,000 extant, and approaching 2,000 fossil species each year at the present time, The number of published species names is higher than the number of described species, sometimes considerably so, on account of the publication, through time, of multiple names (synonyms) for the same accepted taxon in many cases.
Based on Chapman's (2009) report, the estimated numbers of described extant species as of 2009 can be broken down as follows:
|Major group||Described||Global estimate (described + undescribed)||Component group||Described||Global estimate||Component group (2)||Described||Global estimate|
|Mesozoa (Rhombozoa, Orthonectida)||106||-|
|Plants sens. lat.||~310,129||~390,800|
|Ferns and allies||~12,000||~15,000|
|Fungi||98,998 (incl. Lichens 17,000)||1,500,000 (incl. Lichens ~25,000)|
|Chromista [incl. brown algae, diatoms and other groups]||25,044||~200,500|
|Protoctista [i.e. residual protist groups]||~28,871||>1,000,000|
|Prokaryota [ Bacteria and Archaea, excl. Cyanophyta]||7,643||~1,000,000|
|Total (2009 data)||1,899,587||~11,327,630|
Estimates of total number of species
However the total number of species for some taxa may be much higher.
In 1982, Terry Erwin published an estimate of global species richness of 30 million, by extrapolating from the numbers of beetles found in a species of tropical tree. In one species of tree, Erwin identified 1200 beetle species, of which he estimated 163 were found only in that type of tree. Given the 50,000 described tropical tree species, Erwin suggested that there are almost 10 million beetle species in the tropics.
Global biodiversity indices
After the Convention on Biological Diversity was signed in 1992, biological conservation became a priority for the international community. There are several indicators used that describe trends in global biodiversity. However, there is no single indicator for all extant species as not all have been described and measured over time. There are different ways to measure changes in biodiversity. The Living Planet Index (LPI) is a population-based indicator that combines data from individual populations of many vertebrate species to create a single index. The Global LPI for 2012 decreased by 28%. There are also indices that separate temperate and tropical species for marine and terrestrial species. The Red List Index is based on the IUCN Red List of Threatened Species and measures changes in conservation status over time and currently includes taxa that have been completely categorized: mammals, birds, amphibians and corals. The Global Wild Bird Index is another indicator that shows trends in population of wild bird groups on a regional scale from data collected in formal surveys. Challenges to these indices due to data availability are taxonomic gaps and the length of time of each index. The Biodiversity Indicators Partnership was established in 2006 to assist biodiversity indicator development, advancement and to increase the availability of indicators.
Importance of biodiversity
Biodiversity is important for humans through ecosystem services and goods. Ecosystem services are broken down into: regulating services such as air and water purification, provisioning services (goods), such as fuel and food, cultural services and supporting services such as pollination and nutrient cycling.
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